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Abstract

Although levee-overbank areas of deepwater systems consist primarily of clays, silts, thinly bedded sands and sandstones (hereafter termed “thin beds”) are also deposited on these areas. Such deposits are composed of thin-bedded, laminated (Bouma Tb) to rippled (Bouma Tc) sands that sometimes have excellent porosity and darcy-range permeability. Thin beds are ideal stratigraphic traps because of their lateral wedging and thin interbedding of sand and mud; in fact, many “low resistivity, low contrast pay” reservoirs in the northern deep Gulf of Mexico were discovered in such deposits. As a result, several studies have evaluated whether these reservoirs are sufficiently large to warrant economic development. In such systems, production rates can be quite high initially, then decline rapidly, and finally persist at lower levels. As stand-alone reservoirs, thin beds may not be sufficiently economic in the northern Gulf of Mexico or worldwide; however, as secondary reservoirs, they can be quite important.

Levee-overbank deposits form as the finer-grained portion of individual sediment gravity flows overtop their banks and spread laterally beyond the channel margin. Through time, the proximal levee receives more sediment than the distal levee because of the rapid reduction in flow velocity as the flow overtops its banks. The end result, after a period of time, is a wedge-shaped body, with a thick proximal levee and a thinner distal overbank portion (Figure 7-1). Thin-bedded reservoirs associated with levee-overbank sediments are most prevalent in mixed-mud-sand to mud-dominated systems (Richards and Bowman, 1998; and Chapter 1).

Introduction

Although levee-overbank areas of deepwater systems consist primarily of clays, silts, thinly bedded sands and sandstones (hereafter termed “thin beds”) are also deposited on these areas. Such deposits are composed of thin-bedded, laminated (Bouma Tb) to rippled (Bouma Tc) sands that sometimes have excellent porosity and darcy-range permeability. Thin beds are ideal stratigraphic traps because of their lateral wedging and thin interbedding of sand and mud; in fact, many “low resistivity, low contrast pay” reservoirs in the northern deep Gulf of Mexico were discovered in such deposits. As a result, several studies have evaluated whether these reservoirs are sufficiently large to warrant economic development. In such systems, production rates can be quite high initially, then decline rapidly, and finally persist at lower levels. As stand-alone reservoirs, thin beds may not be sufficiently economic in the northern Gulf of Mexico or worldwide; however, as secondary reservoirs, they can be quite important.

Levee-overbank deposits form as the finer-grained portion of individual sediment gravity flows overtop their banks and spread laterally beyond the channel margin. Through time, the proximal levee receives more sediment than the distal levee because of the rapid reduction in flow velocity as the flow overtops its banks. The end result, after a period of time, is a wedge-shaped body, with a thick proximal levee and a thinner distal overbank portion (Figure 7-1). Thin-bedded reservoirs associated with levee-overbank sediments are most prevalent in mixed-mud-sand to mud-dominated systems (Richards and Bowman, 1998; and Chapter 1).

Figure 7-1.

Block diagram of a channel-levee system, illustrating the key subenvironments in the levee-overbank systems: proximal and distal levees, slides, crevasse splays, and sediment waves. Modified from Roberts and Compani (1996).

Figure 7-1.

Block diagram of a channel-levee system, illustrating the key subenvironments in the levee-overbank systems: proximal and distal levees, slides, crevasse splays, and sediment waves. Modified from Roberts and Compani (1996).

A generalized model of a leveed-channel system is shown in Figure 7-1. The components of this setting include channel fill (described in Chapter 6), a proximal levee, a distal-levee overbank, slides, crevasse splays, and sediment waves. Of these, the channel-fill (Chapter 6), proximal-levee, and splay deposits can form reservoirs.

To date, no comprehensive, predictive model for thin-bedded reservoirs has been published, unlike the cases for channel-fill and sheet-sand reservoirs (Chapter 6 and Chapter 8, respectively). This is primarily because, for the past decade, most major companies have focused on developing channel-fill reservoirs (West Africa, West of Shetlands, northern Gulf of Mexico, Kutei Basin, and the Nile) or sheet sandstones (northern Gulf of Mexico and Brazil). In addition, many thin-bedded zones have been overlooked because they appear on conventional well logs as shaley intervals. Individual beds are beneath conventional well log resolution, so that cores and/or borehole image logs are required to resolve the thin interbeds for more accurate net pay calculations. What we present here is a preliminary model for thin-bed reservoirs that is still evolving based on current level of understanding from outcrop, shallow hazards seismic surveys, and from deeper exploration/development drilling and seismic surveys.

In this chapter, we review the important attributes of thin-bedded levee-overbank reservoirs with respect to (1) their overall regional distribution, geometry, and seismic-stratigraphic characteristics, and (2) their development-scale stratigraphic and reservoir-quality heterogeneities that control their reservoir-fluid flow, as is evinced in outcrops, cores, and borehole-image logs. Finally, we review examples of thin-bedded reservoirs in three fields, focusing on their geological and geophysical characteristics, their production history, and the lessons they have demonstrated.

Regional-Scale Characteristics

The important regional-scale features of levee-overbank settings are reviewed here. First, we review seafloor images based on side-scan sonar systems and seismic amplitude; we then review the subsurface characteristics from multifold seismic and wireline logs. We summarize the important characteristics of shape, size, area, thickness, edge relationships, internal reflection associations, wireline-log patterns, and lithologic trends determined from seismic and wireline logs. Continuity issues can be addressed using high-resolution shallow-penetration seismic and limited published outcrop information.

Seafloor images

In most modern submarine-fan systems, the levee and overbank areas constitute the bulk of the sediment present on the fan’s surface. For example, on the Amazon Fan, active, sinuous channels constitute an extremely small percentage of the overall fan surface (Figure 7-2). Ocean Drilling Program cores through the fan reveal that the majority of the sediment is thin-bedded levee silt and clay. Shallow-penetration, high-resolution seismic data show that individual leveed channels have migrated, over time, in a compensatory fashion, building up a thick sequence of dominantly levee mudstones (Figure 7-2). The sediments deposited in the levee overbank depend on the local conditions present. On side-scan sonar images and various seismic-attribute extractions of the seafloor, the backscatter or attributes are fairly uniform, indicating a similar grain size across the surface and similar depositional processes. Localized areas of change in the backscatter pattern or amplitude reflect where sediments have been deposited in different subenviroments, such as crevasse splays, sediment waves, or slides.

Figure 7-2.

(a) Map showing the distribution of late Pleistocene channels of the Amazon Fan, offshore Brazil. Also included are ODP Leg 155 sites (numbers in ellipses), and two prominent mass-transport deposits (shaded gray). Approximately 90% of the fan surface consists of levee-overbank sediments, with two slides originating from the slope. (b) Schematic cross section of the channel-levee systems of the Amazon Fan. Note the predominance of muddy sediment, as determined from cores, and the offset stacking patterns in the lev-eed-channels through time. Locations of the ODP sites are shown. After Piper et al. (1997).

Figure 7-2.

(a) Map showing the distribution of late Pleistocene channels of the Amazon Fan, offshore Brazil. Also included are ODP Leg 155 sites (numbers in ellipses), and two prominent mass-transport deposits (shaded gray). Approximately 90% of the fan surface consists of levee-overbank sediments, with two slides originating from the slope. (b) Schematic cross section of the channel-levee systems of the Amazon Fan. Note the predominance of muddy sediment, as determined from cores, and the offset stacking patterns in the lev-eed-channels through time. Locations of the ODP sites are shown. After Piper et al. (1997).

A false-color image of the Mississippi Fan’s surface helps us to interpret the broad regional distribution of sediment and grain sizes (Figure 7-3). On this surface, hot colors (yellow) reflect high backscatter, which is interpreted to be sand-rich portions of the fan, whereas cooler colors (blue) represent low backscatter and muddier sediments (Figure 7-3). The image indicates that most of the sands occur in the depositional lobes at the termini of the fan channels (orange and red areas); some occur in narrow, sinuous zones of the channels; and yellow areas occur across the surface of the fan and correspond to crevasse splays and other zones within the levee-overbank area.

Figure 7-3.

False-color image derived from the GLORIA II side-scan sonar image of the Mississippi Fan surface. Youngest channel is shown by white line (labeled). Brighter colors correspond to layers that are interpreted to be sand-rich. Each depositional lobe (red and yellow colors) at the termini of the channels is interpreted to be a sand-rich area, in contrast to other portions of the fan surface. Blue areas represent finer-grained, overbank sediments. Note the linear distribution of sediment (yellow and green colors) in the overbank areas close to the youngest channel (outlined by dashed line). These areas may contain slightly coarser-grained material than does the surrounding overbank. After Wen et al. (1995). Reprinted with permission of Chapman-Hall and Neil Kenyon.

Figure 7-3.

False-color image derived from the GLORIA II side-scan sonar image of the Mississippi Fan surface. Youngest channel is shown by white line (labeled). Brighter colors correspond to layers that are interpreted to be sand-rich. Each depositional lobe (red and yellow colors) at the termini of the channels is interpreted to be a sand-rich area, in contrast to other portions of the fan surface. Blue areas represent finer-grained, overbank sediments. Note the linear distribution of sediment (yellow and green colors) in the overbank areas close to the youngest channel (outlined by dashed line). These areas may contain slightly coarser-grained material than does the surrounding overbank. After Wen et al. (1995). Reprinted with permission of Chapman-Hall and Neil Kenyon.

Seismic-amplitude extractions from 3D seismic of the seafloor (Nigerian Slope) show the details of the levee-overbank sediments and their interpreted grain sizes (Figure 7-4). In these images, the blue areas (lower amplitude) are interpreted to be muddy sediments, and the areas of higher amplitude (yellow) are interpreted to be sand prone. Coarser-grained, levee-overbank sediments are interpreted to occur on both sides of the levees. The area of the hotter colors (which may be slightly coarser-grained sediments) extends parallel to subparallel to the channel and is about two to four times the width of the channel. At bends in the channel, the sediments tend to preferentially overspill and spread across the surface by the processes of flow stripping and/or breaching of the levee, as we discuss in Chapter 4.

Figure 7-4.

Seismic-amplitude displays of channel and overbank settings, modern seafloor, Block 221, the Nigerian slope. (a) Detailed image of sinuous channel and overbank areas (Ov). Proximal levees consist of areas of higher amplitude (yellow). Possible sediment waves are also present (Sw). Channel fill consists of high amplitudes (Ha). Scours associated with slides (Sc), fault traces (Ft) and mud volcanoes (MV) are present. Sediment waves trend subperpendicular to the channel and subparallel to the trend of the levees. The sediment waves are bottom current indicators. Slides form randomly in the levee-overbank areas. (b) Detail of the same channel as in (a), but this time 40 km downdip. Channel has a slightly braided appearance. Overbank areas consist of high amplitudes, suggesting that coarser-grained sediments spilled into the area. Areas are about 3 km in width on either side of the channel. Arcuate slide scars (Sa) are present. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-4.

Seismic-amplitude displays of channel and overbank settings, modern seafloor, Block 221, the Nigerian slope. (a) Detailed image of sinuous channel and overbank areas (Ov). Proximal levees consist of areas of higher amplitude (yellow). Possible sediment waves are also present (Sw). Channel fill consists of high amplitudes (Ha). Scours associated with slides (Sc), fault traces (Ft) and mud volcanoes (MV) are present. Sediment waves trend subperpendicular to the channel and subparallel to the trend of the levees. The sediment waves are bottom current indicators. Slides form randomly in the levee-overbank areas. (b) Detail of the same channel as in (a), but this time 40 km downdip. Channel has a slightly braided appearance. Overbank areas consist of high amplitudes, suggesting that coarser-grained sediments spilled into the area. Areas are about 3 km in width on either side of the channel. Arcuate slide scars (Sa) are present. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Three other geomorphic features commonly are imaged in the overbank areas on the seafloor: crevasse splays, sediment waves, and slides. Crevasse splays are elongate, fan-shaped deposits that generally occur at the outer bend in a channel (Figures 7-5, 7-6). These deposits appear to originate where a channel begins to avulse. Instead of a single channel forming, a network of distributary channels develops, thereby forming the crevasse splays. The higher amplitude indicates that these may be sand-rich areas, in contrast to the cooler colors, which indicate finer-grainer sediments.

Figure 7-5.

An rms amplitude extraction map 170 ms below the seafloor in one intraslope miniba-sin in the Brunei continental slope. The upslope channel passes through shale ridges to an elongate, sheet deposit (depositional lobe). Finer-grained slope and overbank areas are indicated by the blue color. Note the possible coarser-grained levee sediments (orange color) next to the channel and the prominent crevasse-splay deposit (orange) next to the sinuous channel. After Demyttenaere et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-5.

An rms amplitude extraction map 170 ms below the seafloor in one intraslope miniba-sin in the Brunei continental slope. The upslope channel passes through shale ridges to an elongate, sheet deposit (depositional lobe). Finer-grained slope and overbank areas are indicated by the blue color. Note the possible coarser-grained levee sediments (orange color) next to the channel and the prominent crevasse-splay deposit (orange) next to the sinuous channel. After Demyttenaere et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-6.

Amplitude extraction maps and interpreted line drawings of upper Pleistocene (a) and (b) crevasse splays and (c) overbank splays, northern deep Gulf of Mexico. Overbank splays are similar to crevasse splays but differ in size and the absence of levee deposits associated with them. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Figure 7-6.

Amplitude extraction maps and interpreted line drawings of upper Pleistocene (a) and (b) crevasse splays and (c) overbank splays, northern deep Gulf of Mexico. Overbank splays are similar to crevasse splays but differ in size and the absence of levee deposits associated with them. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Sediment waves form in some, but not all, levee settings. Normark et al. (2002) summarized the studies of sediment waves from six different deepwater systems. These studies included 3-D seismic, multibeam bathymetry, cores, and direct measurements of turbidity currents at overflow channels. Sediment waves can form on one or both sides of the channel (Figures 7-4, 7-7). Upslope migration of sediment waves is indicated from cores and seismic data, with thicker and coarser beds deposited on the upcurrent flank of the waves. Sediment waves have a variety of orientations. Some are orthogonal to channel trend and apparently were initiated from the overflow of turbidity currents from channels. Waves that are subparal-lel to channels results from local spillover. Adjacent to sinuous channels, waves may mimic the sinuous planform. Sediment waves can develop from thicker overflow of turbidity currents, as well as be maintained with relatively thin overflow. Their typical dimensions are as much as 20 m (65 ft) in height, with wavelengths of 0.5 to 0.8 km (0.3 to 0.5 miles) (Posamentier and Kolla, 2003). Where they are cored, sediment waves can contain fine to medium sand (Migeon et al., 2000) to mud (Piper and Normark, 1983). Where coarser grained, sediment waves may have typically laminated to thin bedded sands up to several cm thick. (Piper and Normark, 1983; Migeon et al., 2000).

Figure 7-7.

(a) Dip attribute map 48 ms below the seafloor of a late Pleistocene channel-overbank system, Makassar Straits, eastern Borneo. Overbank splays and sediment waves are present. (b) and (c) Seismic profiles across the channel-levee system. Note the prominent sediment waves at the top of the levee. Locations of these seismic profiles are shown in (a). After Posamentier et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-7.

(a) Dip attribute map 48 ms below the seafloor of a late Pleistocene channel-overbank system, Makassar Straits, eastern Borneo. Overbank splays and sediment waves are present. (b) and (c) Seismic profiles across the channel-levee system. Note the prominent sediment waves at the top of the levee. Locations of these seismic profiles are shown in (a). After Posamentier et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Slides are present on the surface of many overbank areas. They form in areas of over-pressuring in the muddy sediments and respond by failure. They are observed along the margins of channels, where sediments have slid into the channel (Figure 7-8); throughout the overbank setting, where they deform in place (Figure 7-4a, Sc; Figure 7-4b, Sa); or where they have come from updip in a slope region. We review slides in greater detail in Chapter 9.

Figure 7-8.

(a) 3D perspective of a late Pleistocene channel, northeastern deep Gulf of Mexico. Image is derived from 3D seismic data and illuminated from the west. View is to the north. Florida Escarpment forms the vertical image to the east. Note the overall elevation in the channel-levee system, sediment waves in the eastern levee, oblique to the overall trend of the channels, and downfan bifurcation of the channel. Location of the enlargement in (b) is shown. (b) Enlargement of the channel in (a), illustrating the elevated levees and channel. Location of the profile in (c) is shown. (c) Seismic profile across the channel-levee system. High-amplitude reflections (channel fill) migrate to the right (east) and aggrade. At the surface, the channel is 625 m wide, and the levees have 6–7 m of depositional relief. See (b) for location of the profile. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Figure 7-8.

(a) 3D perspective of a late Pleistocene channel, northeastern deep Gulf of Mexico. Image is derived from 3D seismic data and illuminated from the west. View is to the north. Florida Escarpment forms the vertical image to the east. Note the overall elevation in the channel-levee system, sediment waves in the eastern levee, oblique to the overall trend of the channels, and downfan bifurcation of the channel. Location of the enlargement in (b) is shown. (b) Enlargement of the channel in (a), illustrating the elevated levees and channel. Location of the profile in (c) is shown. (c) Seismic profile across the channel-levee system. High-amplitude reflections (channel fill) migrate to the right (east) and aggrade. At the surface, the channel is 625 m wide, and the levees have 6–7 m of depositional relief. See (b) for location of the profile. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Determining the origin of some of the sediments in the large classification of “levee-overbank areas” can be quite difficult. Specifically, it can be difficult to determine the relative proportion of sediments derived from levee-overbanking processes, in contrast to those in unchannelized flows that may have come from upslope. In addition, the relative percentage of hemipelagic sediment interbedded with overbank deposits is often unknown.

Seismic-stratigraphic and wireline-log expressions

Levees are probably the easiest deepwater element to identify on seismic profiles, because they are elevated bathymetrically next to their adjacent channel (Figures 7-1, 7-7, 7-8).

Shape in plan view, and size

The shape of levee-overbank deposits in plan view depends on the size of adjacent channels, the volumes and durations of flows, and the size of the receiving basin. In general, the overall shape is elongate and trends roughly parallel to the channel (Figures 7-1 through 7-5, 7-7, 7-8). Levees develop bathymetric expression next to the channels, but they lose their bathymetric expression both laterally away from the channel and downfan, as the channel fill spreads into sheet sands (Figures 7-9, 7-10).

Figure 7-9.

Four seismic profiles across one intraslope minibasin, northern deep Gulf of Mexico, illustrating the downslope changes in geometries in one channel and the associated levees from: (a) an erosional channel with slight bathymetric relief on the levees, to (b) and (c) strongly aggra-dational levees with prominent bathymetric relief, to (d) erosional channels with levees that have little relief. (e) Inset map shows locations of the seismic profiles. After Badalini et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-9.

Four seismic profiles across one intraslope minibasin, northern deep Gulf of Mexico, illustrating the downslope changes in geometries in one channel and the associated levees from: (a) an erosional channel with slight bathymetric relief on the levees, to (b) and (c) strongly aggra-dational levees with prominent bathymetric relief, to (d) erosional channels with levees that have little relief. (e) Inset map shows locations of the seismic profiles. After Badalini et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-10.

(a) Seismic profile along the right (southern) side of one levee crest adjacent to a late Pleistocene sinuous channel, Makassar Straits, eastern Borneo. The profile is flattened on the top of the levee crest. Levee reflections are low amplitude and discontinuous. Overall, the levee thickness decreases downfan. Note that the thickness of the levee sediments is greater along the outer channel bend and thinner along the inner bends. (b) Azimuth dip map of the sinuous channel, showing the location of the seismic profile. After Posamentier and Kolla (2003). Reprinted with permission of the SEPM.

Figure 7-10.

(a) Seismic profile along the right (southern) side of one levee crest adjacent to a late Pleistocene sinuous channel, Makassar Straits, eastern Borneo. The profile is flattened on the top of the levee crest. Levee reflections are low amplitude and discontinuous. Overall, the levee thickness decreases downfan. Note that the thickness of the levee sediments is greater along the outer channel bend and thinner along the inner bends. (b) Azimuth dip map of the sinuous channel, showing the location of the seismic profile. After Posamentier and Kolla (2003). Reprinted with permission of the SEPM.

The area of the levee-overbank deposits is highly variable and can extend from tens of square kilometers to tens of thousands of square kilometers (Figures 7-2, 7-3). The area of the levee-overbank deposits depends on the size of the basin, the size of the channels from which the levee sediments emanate, and the volume of the flows transported through the channels. In intraslope basins, overbank sediments can extend across the entire basin (Figure 7-11) or pinch out before the edge of the intraslope basin (Figure 7-9). In some base-of-slope, unconfined basins, levee-overbank deposits can extend into the area for hundreds of kilometers across, as they do in modern fans (Figures 7-2, 7-3).

Figure 7-11.

(a) Seismic profile across an intraslope basin, offshore Angola. The sequence is composed of a mass-transport deposit (labeled “sand debrite”) at the base, consisting of low-amplitude, chaotic, mounded, and hummocky reflections, overlain by amalgamated channelized and channel-levee systems. Levee reflections are low amplitude, with variable continuity, and lap out against the side of the basin. (b) Wireline logs through a channel-levee system, Angola. Thin bedded sands are deposited near the top of the sequence associated with the leveed channel. After Sikkema and Wojcik (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-11.

(a) Seismic profile across an intraslope basin, offshore Angola. The sequence is composed of a mass-transport deposit (labeled “sand debrite”) at the base, consisting of low-amplitude, chaotic, mounded, and hummocky reflections, overlain by amalgamated channelized and channel-levee systems. Levee reflections are low amplitude, with variable continuity, and lap out against the side of the basin. (b) Wireline logs through a channel-levee system, Angola. Thin bedded sands are deposited near the top of the sequence associated with the leveed channel. After Sikkema and Wojcik (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

The thickness of levee-overbank deposits is highly variable. In modern large fans, individual levees associated with one channel deposited within one depositional sequence can be 400 to 700 m (1300 to 2250 ft) thick (e.g., the Mississippi Fan, Weimer, 1989; Indus Fan, Droz and Bellaiche, 1991; Bengal Fan, Emmel and Curray, 1985; and the Amazon, Piper et al., 1997). Levees with this thickness may be explored for in those unconfined settings where later deformation has placed them in a trapping configuration (e.g. deepwater foldbelts, Chapter 15). Exploration targets are often deepwater systems deposited in slope basins that are considerably smaller than modern fans. As a consequence, explorationists focus on levees that also generally are considerably thinner than those in modern fan systems, that is, that are no thicker than 50 to 100 m (165 to 330 ft) (Figures 7-11, 7-12) and often are thinner than that. Producing sands that are deposited within the levee are even thinner: An entire reservoir package may be 30 m (100 ft) in maximum thickness (Figure 7-13). However, where large uncon-fined systems have later been deformed, the levee deposits may be considerably thicker. The thickness of the deposits and amount of bathymetric relief of the levee decreases from updip to downdip, as well as along the inner and outer bends of the channel (Figure 7-10). The levees are more elevated and deposits are thicker along the outer bends, where sediment preferentially overbank.

Figure 7-12.

(a) Seismic profile across the Ram-Powell field, northern deep Gulf of Mexico. A high-amplitude reflection associated with the L sand reservoir is shown and is interpreted to be caused by gas content. See Figure 7-19 for location of the profile. After Clemenceau et al. (2000). (b) Detailed seismic profile illustrating the reflection associated with the L sand reservoir. A possible low-amplitude reflection associated with channel-fill sediments is to the west (left) of the levee reservoir. After Kendrick (2000). Both profiles are reprinted with permission of Gulf Coast Section SEPM Foundation.

Figure 7-12.

(a) Seismic profile across the Ram-Powell field, northern deep Gulf of Mexico. A high-amplitude reflection associated with the L sand reservoir is shown and is interpreted to be caused by gas content. See Figure 7-19 for location of the profile. After Clemenceau et al. (2000). (b) Detailed seismic profile illustrating the reflection associated with the L sand reservoir. A possible low-amplitude reflection associated with channel-fill sediments is to the west (left) of the levee reservoir. After Kendrick (2000). Both profiles are reprinted with permission of Gulf Coast Section SEPM Foundation.

Figure 7-13.

(a) Wireline-log cross section across the Ram-Powell L sand reservoir. Proximal-levee reservoirs consist of 100 ft (30 m) of 60% net:gross sands, whereas the distal levee, 3 mi (5 km) to the east (right), has a net:gross value of 27%. Note the overall upward fining and thinning in both the proximal- and distal-levee sediments. Channel-fill sands are water wet. (b) Dipmeter logs for proximal- and distal-levee facies, Ram-Powell field, showing the two styles of dip patterns. (c) Photograph (normal and UV light) of cores through proximal- and distal-levee facies, showing the higher net sand and greater dip angles in the proximal-levee facies than in the distal-levee beds. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-13.

(a) Wireline-log cross section across the Ram-Powell L sand reservoir. Proximal-levee reservoirs consist of 100 ft (30 m) of 60% net:gross sands, whereas the distal levee, 3 mi (5 km) to the east (right), has a net:gross value of 27%. Note the overall upward fining and thinning in both the proximal- and distal-levee sediments. Channel-fill sands are water wet. (b) Dipmeter logs for proximal- and distal-levee facies, Ram-Powell field, showing the two styles of dip patterns. (c) Photograph (normal and UV light) of cores through proximal- and distal-levee facies, showing the higher net sand and greater dip angles in the proximal-levee facies than in the distal-levee beds. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Elevation and thickness of levees

Skene et al. (2002) demonstrated consistent and quantifiable relationships for levee thickness in six modern submarine fans. They noted that thickness in levee sediments decreases exponentially along the levee, perpendicular to the channel: eky , where y (m [meters]) is the distance across the levee from the channel axis, (m) is the y intercept of the regression equation, and k(m-1) is the slope f of the regression equation and represents a spatial decay constant. In addition, the thickness of the levee sediments in the upper reaches of the channel decreases exponentially downfan: lc lc(0)e-x where x (m) is the distance downchannel and lc(0) Gail- we will need to discuss these fonts, is the thickness of sediment at the levee crest, at x = 0. From these relationships, Skene et al. (2002) noted that levee width and length are proportional to channel width and length. They concluded that the dimensions of the channel and levee sediments are intimately related, limiting the range of potential channel and levee morphologies. However, these authors concluded that, qualitatively, the channel morphology does not relate to the levee thickness relationships.

Recognizing the effects of compaction on levee sediments is important when one is reconstructing the thickness of buried levee-overbank systems. Because levee-overbank sediments have a higher percentage of mud than do channel-fill sediments, levees are more susceptible than channels are to compaction. With increasing burial, levees will compact more than sand-filled channels, to the point at which the levees will appear to be equal in thickness to the channels (Figures 7-12, 7-14), or the levees may even appear to have an inverted topography, whereas the channels appear to be elevated relative to the levees (Figure 7-8).

Figure 7-14.

(a) Seismic profile across a modern channel-levee system, northern deep Gulf of Mexico, illustrating elevated channel-fill sediments in contrast to the surrounding levee deposits. Inversion of depositional topography has occurred in a short amount of time. Green arrows indicate areas of the greatest amount of compaction in the levee, and yellow arrows indicate areas of less compaction (channel-fill sediments). (b) Schematic profile showing the decompacted sediments in profile in (a). Reprinted with permission of Henry Posamentier.

Figure 7-14.

(a) Seismic profile across a modern channel-levee system, northern deep Gulf of Mexico, illustrating elevated channel-fill sediments in contrast to the surrounding levee deposits. Inversion of depositional topography has occurred in a short amount of time. Green arrows indicate areas of the greatest amount of compaction in the levee, and yellow arrows indicate areas of less compaction (channel-fill sediments). (b) Schematic profile showing the decompacted sediments in profile in (a). Reprinted with permission of Henry Posamentier.

Shape in cross section

As we state above, in cross section, levees are described informally as having a “gull-wing” shape, that is, a double-wedge shape that tapers away from the central channel (Figures 7-1, 7-7 through 7-12, 7-14 through 7-17). In addition, levees tend to increase in their bathymetric elevation with respect to the adjacent channel, throughout the channel’s development (Figures 7-11, 7-14, 7-15). This upward increase is associated with the amount of finegrained material being transported through the channel as a result of larger and/or more frequent turbidity flows and the increasing length of time the channel is active. As levees grow, flow is more confined to the channel, and coarser sediment gets carried farther downfan.

Figure 7-15.

Seismic profile across an upper Pleistocene sequence (ca. 0.5 Ma) in the Mississippi Fan, northern deep Gulf of Mexico. Key elements are a mass-transport complex (MTC) at the base (see Chapter 9), overlain by channel-fill and levee-overbank sediments. The top of the MTC is an irregular surface, and it has been eroded into several channels (high-amplitude reflections). Note the vertical change in the amplitude of the levee reflections to the left (west), changing from high amplitude at the base (HAMP) to low amplitude at the top (LAMP). This vertical change is interpreted to be caused by a decrease in the grain size of the sediments. After Weimer (1990). Reprinted with permission of AAPG.

Figure 7-15.

Seismic profile across an upper Pleistocene sequence (ca. 0.5 Ma) in the Mississippi Fan, northern deep Gulf of Mexico. Key elements are a mass-transport complex (MTC) at the base (see Chapter 9), overlain by channel-fill and levee-overbank sediments. The top of the MTC is an irregular surface, and it has been eroded into several channels (high-amplitude reflections). Note the vertical change in the amplitude of the levee reflections to the left (west), changing from high amplitude at the base (HAMP) to low amplitude at the top (LAMP). This vertical change is interpreted to be caused by a decrease in the grain size of the sediments. After Weimer (1990). Reprinted with permission of AAPG.

Figure 7-16.

Schematic cross section across a channel-levee system, and the corresponding gamma-ray or SP logs. Note the vertical decrease in the grain sizes of the levee sediments. This figure illustrates two scales of levees: those outside of the main or master channel (shown in green and orange) and those within the master channel and associated with the smaller channels (shown in yellow). Modified from Beaubouef (2004).

Figure 7-16.

Schematic cross section across a channel-levee system, and the corresponding gamma-ray or SP logs. Note the vertical decrease in the grain sizes of the levee sediments. This figure illustrates two scales of levees: those outside of the main or master channel (shown in green and orange) and those within the master channel and associated with the smaller channels (shown in yellow). Modified from Beaubouef (2004).

Figure 7-17.

(a) An rms amplitude extraction map (20-ms gated window) of a channel-levee system, with a prominent crevasse splay, Miocene deposits, offshore Angola. The crevasse is fan shaped and resulted from an avulsion in the channel. (b) Inset seismic profile that shows the interval from which the attribute was extracted. See (a) for location of the profile. After Mayall and Stewart (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-17.

(a) An rms amplitude extraction map (20-ms gated window) of a channel-levee system, with a prominent crevasse splay, Miocene deposits, offshore Angola. The crevasse is fan shaped and resulted from an avulsion in the channel. (b) Inset seismic profile that shows the interval from which the attribute was extracted. See (a) for location of the profile. After Mayall and Stewart (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Edge relations

At the resolution level of seismic data, levee-overbank settings can have both transitional and lapout edge relations, depending on the depositional setting of the basin.

The updip contact for levee-overbank settings is transitional and depends largely on the exact location at which the aggradational channel develops as it transports sediments downdip within a basin. In unconfined, base-of-slope settings, levees initially develop where the aggra-dational channel enters the basin, such as from canyon-fed systems (Figures 7-2, 7-3). Channels and their levees also can develop on slopes, both within intraslope basins and within unconfined slopes. In basins with structural confinement (e.g., intraslope basins), levees can appear first where the channel passes into the basin and transitions from an erosional phase to an aggradational phase (Figure 7-9; see also Chapter 3, Chapter 6). In unconfined slopes, levees will tend to be more abundant on more gentle gradient portions, where sinuous channels are more common.

The distal lateral contacts in levee-overbank deposits can be transitional or abrupt. In general, levee and overbank settings become thinner away from the channel, to the point where, ultimately, they no longer exist. With small channels, levees tend to thin and pinch out across a fairly short distance (Figures 7-11, 7-12). In confined basins, such as intraslope basins, levee-overbank sediments can thin and/or lap out against the flank of the basin (Figure 7-11).

The levee’s proximal contact with the channel can occur within a fairly narrow zone where the levees end and the channel facies begin (see Chapter 6). The detailed nature of channel-levee transition is problematic and often not resolvable on seismic. With buried systems, where the levee and channel-fill sediments are within the resolution of seismic data, the transition from levee to channel may be indeterminable. In some cases, the seismic reflections associated with the channel fill terminate against the levee sediments; in other examples, the contact between channel and levee appears to be erosional. The nature of the leveed-channel margin is discussed in Chapter 6.

The levee’s downdip contacts are transitional in unconfined basins. Where the constructional, aggradational channel passes transitionally into a sheet, the levee also begins to thin because of a lack of overbanking processes (Figure 7-9, see also Figure 6-15, Figure 8-2, and Figure 8-8). In confined settings (such as intraslope basins), levees also mimic the distribution of channels. Where the channels bypass into a downslope basin, levees may onlap against the downdip portion of the basin.

Internal reflections

Levee-overbank sediments generally consist of laterally continuous, parallel to subparal-lel reflections, with variable amplitude. On 3D seismic, they have areally widespread reflections with variable amplitude. Where the levees are sufficiently thick—that is, in modern fans—internal discontinuities, erosion, and thinning can be imaged (Figure 7-15). In addition, crevasse splays (Figure 7-1) and slides can be imaged. With burial, sediment waves become compacted and cannot be imaged with seismic profiles.

With buried systems, our ability to recognize levee-overbank deposits depends on the thickness of the unit, the frequency of the seismic data, the depth of the unit, and the amount of compaction to which the unit has been subjected. In some Neogene petroleum-producing deepwater systems, the depositional relief of the levee can still be imaged (Figure 7-11). However, with many buried systems, the levee and associated channels commonly are at the resolution of one seismic wavelet (Figure 7-12). Interpretation is challenging in vertical profiles; 3D can often help us define the areas adjacent to channels.

In many producing fields, the rock physics of the levee-overbank deposits are such that there is a prominent amplitude response caused by the fluids within the levees (Pacht et al., 1992). As a consequence, the horizontal perspective is an area with high amplitude, which is indicative of the fluid distribution and not the sediment distribution (Figure 7-18).

Figure 7-18.

Amplitude extraction map of the Ram-Powell L sand interval superposed on a top L sand structure map, northern deep Gulf of Mexico. Producing levee sands correspond to areas of high amplitude, as a result of the effects of gas in the reservoir. Note that the adjacent channel (outlined by turquoise lines) has low amplitude and no pay. Location of Figure 7-13a is shown. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-18.

Amplitude extraction map of the Ram-Powell L sand interval superposed on a top L sand structure map, northern deep Gulf of Mexico. Producing levee sands correspond to areas of high amplitude, as a result of the effects of gas in the reservoir. Note that the adjacent channel (outlined by turquoise lines) has low amplitude and no pay. Location of Figure 7-13a is shown. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Wireline-log to seismic response

Levee-overbank sediments are primarily mud-prone, and silt-prone although good sands can develop locally. Consequently, the gamma-ray log response is highly variable in these sediments but is generally shaley or “ratty” in appearance. Beaubouef (2004) presented a schematic cross section across a channel-levee system in one depositional sequence; their cross section indicates the transition in sediments from the proximal to the distal levee (Figure 7-16). A variety of gamma-ray log patterns are present in levee sediments: both coarsening- and fining-upward trends, and a mixture crescentic (Figure 7-13a). As we discussed in Chapter 6 regarding channel fill, the basal zone of channels may be filled with shale-clast conglomerates, thus giving a shaley log response to the basal interval (and sometimes also reducing net-pay counts). Farther from the channels, the overall levee-overbank interval thins. We review examples here, to illustrate the widely varying sand content in levee facies: (1) modern examples from different-size systems (northeastern Gulf of Mexico), (2) coring results from modern deep-sea fans (Mississippi and Amazon Fans), and (3) one producing subsurface example (northern deep Gulf of Mexico). We then review two examples that illustrate important vertical changes in grain size in levees.

(1) Hackbarth and Shew (1994) presented an example of the Einstein channel, an upper Pleistocene channel in the northeastern deep Gulf of Mexico (Figure 7-19). This channel-levee system was cored at three sites to help calibrate the seismic facies to the grain sizes in the cores. At the proximal levee (borehole D, Figure 7-19), the core recovered at the bottom of the sequence slightly coarser-grained sediments that corresponded to higher-amplitude, subparal- lel reflections. The net:gross value is 45% in this interval. In the overlying sediments, the core recovered mostly clays, with a few silt layers; this grain size corresponds to the low-amplitude parallel facies. At the distal levee, the core penetrated almost entirely clay, with some silts (borehole A; Figure 7-19; note that no seismic profile is available at this position).

Figure 7-19.

(a) Seismic profile across the Einstein channel, northern deep Gulf of Mexico. Four seismic facies are present in the levee facies. Note the vertical change in seismic facies from high amplitude at the base of the levee to high continuity, low amplitude at the top. (b) Wireline-log cross section across the Einstein channel. Logs that penetrated the levee facies are dominantly clays and some interbedded silts. Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-19.

(a) Seismic profile across the Einstein channel, northern deep Gulf of Mexico. Four seismic facies are present in the levee facies. Note the vertical change in seismic facies from high amplitude at the base of the levee to high continuity, low amplitude at the top. (b) Wireline-log cross section across the Einstein channel. Logs that penetrated the levee facies are dominantly clays and some interbedded silts. Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-19.

(c) Seismic profile with superposed time-based wireline logs with lithologies plotted as determined from the cores. Note the vertical decrease in grain size in the levees in borehole D. After Hackbarth and Shew (1994). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-19.

(c) Seismic profile with superposed time-based wireline logs with lithologies plotted as determined from the cores. Note the vertical decrease in grain size in the levees in borehole D. After Hackbarth and Shew (1994). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Hackbarth and Shew’s (1994) study was an analog for the deeper Miocene prospects in the area. The authors noted that the deeper Miocene prospects had more sand in their levees than did the shallower Pleistocene Einstein channel (e.g., Figures 7-12, 7-13, 7-18), and they attributed these differences to three things. First, they suggested that this upper Pleistocene system may be farther updip and in a less sandy part of the overall depositional system than are the deep Miocene prospects. Second, the Einstein channel developed during a minor lowstand in sea level, with little storage of sand in the updip, deltaic part of the system. Third, the Einstein channel was the younger of two channel-levee systems deposited within the same depositional cycle; perhaps the Einstein was deposited in a later-stage, more sand-poor part of the sea-level cycle.

(2) Levee systems in major submarine fans were cored in two cruises of deep-sea sci entific drilling programs: The Mississippi Fan was cored during DSDP Leg 96 (Bouma et al., 1985, 1986), and the Amazon Fan was cored during ODP Leg 155 (Piper et al., 1997). DSDP Leg 96 cored the levee-overbank sediments in the middle portion of the Mississippi Fan at two sites (617 and 620). Both sites recovered primarily clays and silts; little sand was recovered. ODP Leg 155 cored the Amazon Fan levees at 6 sites (sites 939, 940, 934, 936, 944, and 946: Figures 7-2, 7-20, 7-21). Seven lithofacies were recovered in the cores, including interbedded silts, clays, discontinuous silt laminae, regular silt laminae, graded silt-to-clay beds, structure less silt beds, and graded to cross-bedded sand beds (Hiscott et al., 1997; Normark, Damuth et al., 1997; Piper and Deptuck, 1997). The most abundant coarser facies were silt laminae and thin sand beds with parallel to cross lamination, rarely with normal grading, and with climbing ripples. Sand constituted less than 10% of the sediment cored in the levees. Both of these stud ies were important for establishing the overall grain size of the levee sediments; however, no distinct vertical trends in grain size were reported in the Mississippi Fan cores. For the Amazon Fan, Pirmez and Imran (2003) reexamined the cores and reported subtle upward decreases in the grain size in the levees. They also considered the relative paucity of sand in the cores to have been the result of the cores being sampled too far from the channel.

Figure 7-20.

(a) Shaded bathymetric map of the highly sinuous channel, middle fan of the Amazon. Locations of ODP site 936 and seismic profile in (b) are shown. (b) Seismic profile across the channel-levee system. Note the extreme vertical exaggeration of the profile. Time-based gamma-ray log from ODP site 936 is shown. Wireline log indicates that the levee sediments are primarily clay, with some silt. See Figure 7-21 for detailed lithology from cores of site 936, and two additional levee sites (935, 940). After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-20.

(a) Shaded bathymetric map of the highly sinuous channel, middle fan of the Amazon. Locations of ODP site 936 and seismic profile in (b) are shown. (b) Seismic profile across the channel-levee system. Note the extreme vertical exaggeration of the profile. Time-based gamma-ray log from ODP site 936 is shown. Wireline log indicates that the levee sediments are primarily clay, with some silt. See Figure 7-21 for detailed lithology from cores of site 936, and two additional levee sites (935, 940). After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-21.

Grain-size logs based on cores from ODP sites 940, 936, and 935 through the Amazon Fan. Each site cored levee sediments and recovered primarily clays with some silt; the sand fraction is extremely small. The thick sand fraction in site 936, at 70–90 m, is through a HARP (high-amplitude reflection package) crevasse-splay package. See Figure 7-20 for seismic expression and time-based gamma-ray log of site 936. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-21.

Grain-size logs based on cores from ODP sites 940, 936, and 935 through the Amazon Fan. Each site cored levee sediments and recovered primarily clays with some silt; the sand fraction is extremely small. The thick sand fraction in site 936, at 70–90 m, is through a HARP (high-amplitude reflection package) crevasse-splay package. See Figure 7-20 for seismic expression and time-based gamma-ray log of site 936. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

(3) One example of a field producing from thin beds is the L sand of the Ram-Powell field in the northern deep Gulf of Mexico (Figures 7-12, 7-13). Clemenceau et al. (2000) presented a wireline-log cross section and map of the reservoir. In the proximal levee, the net:gross value was relatively high (60%), but it decreased to 28% along a 3-mi (7-km) length (Figures 7-13, 7-22). The corresponding amplitude was quite high, because of the gas content of the reservoir (Figures 7-14, 7-18). The distribution of sand in Figure 7-22 is of roughly similar dimensions to that interpreted in Figure 7-4, where higher amplitudes are about 2 km (3 miles) wide adjacent to the channel. We suggest that Figure 7-22 is a drilled, subsurface example quite similar of those images generated on the sea floor.

Figure 7-22.

Total sand map of the Ram Powell “L” sand interval, northern deep Gulf of Mexico. Total thickness of sands decreases from the proximal to the distal levee, from more than 100 ft (30 m) to 20 ft (6 m) across a distance of 3 mi (5 km). The net:gross value also decreases from 60 to 27%. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-22.

Total sand map of the Ram Powell “L” sand interval, northern deep Gulf of Mexico. Total thickness of sands decreases from the proximal to the distal levee, from more than 100 ft (30 m) to 20 ft (6 m) across a distance of 3 mi (5 km). The net:gross value also decreases from 60 to 27%. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

(4) An important feature is the distinct vertical change in reflection amplitudes in the levees in one depositional sequence within some modern submarine fans (Figure 7-15) (Weimer, 1990). This change is interpreted to have lithologic implications. Levee reflections have a higher amplitude at the base of a depositional sequence, and their amplitude becomes increasingly lower toward the top of the sequence. This vertical change is interpreted to reflect an overall decrease in the grain size and/or bed thickness of the levee sediments, from inter-bedded, slightly coarser-grained sediment at the levee’s base to more homogeneous, finer-grained sediment at its top. This interpreted vertical decrease in grain size and/or bed thickness is thought to indicate the overall evolution of the depositional system within one depositional sequence. As we discussed in Chapter 6 (Figure 6-6, Figure 6-7, Figure 6-20, Figure 6-26), distributary channels at the base of a sequence evolve upward into a single aggradational, laterally migrating channel (Figure 7-15). The distributary channels are associated with the higher-amplitude levee reflections. By contrast, the lower-amplitude levee reflections correspond to laterally migrating, aggradational channels with bathymetrically elevated levees. Thus, the vertical upward decrease in amplitude indicates the increasingly homogenized grain size of levee sediments, moving from levee base to top.

This upward decrease in grain size also has been noted in some levee deposits in outcrop (Miocene-New Zealand; Browne and Slatt, 2002; see below) and in producing fields: the Ram-Powell L sand (Figure 7-13) (Clemenceau et al., 2000) and the Tahoe M4.1 Sand (Figure 7-23) (Kendrick, 2000).

Figure 7-23.

Wireline logs and a core photograph of the M4.1 sand reservoir, Tahoe field, northern deep Gulf of Mexico. Gamma-ray log indicates thin interbeds of mud and sand, with an overall vertical decrease in grain size. Core photograph shows representative producing facies, consisting of very fine-grained sands, planar to ripple laminated (0–2 in. or 0.8 cm thick), with interbedded shales. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-23.

Wireline logs and a core photograph of the M4.1 sand reservoir, Tahoe field, northern deep Gulf of Mexico. Gamma-ray log indicates thin interbeds of mud and sand, with an overall vertical decrease in grain size. Core photograph shows representative producing facies, consisting of very fine-grained sands, planar to ripple laminated (0–2 in. or 0.8 cm thick), with interbedded shales. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

(5) Lateral migration of levee deposits within one sequence is another important process that can be responsible for vertical trends in the grain size of lithologies. Levees migrate both laterally and downfan, in association with the lateral and downfan migration of a channel (Cremer et al., 1985; Weimer, 1989, 1991; Cainelli, 1994; see also Chapter 6 of this book). On seismic profiles, levee deposits can be seen to overlie channel-fill deposits within the same depositional sequence (Figure 7-24). On wireline logs, fine-grained, possible thin beds overlie coarser-grained, channel-fill deposits. By wireline-log signature alone, moving upward this decrease in grain size may be interpreted as channel abandonment; however, in fact, this vertical upward decrease in grain size implies ongoing channel migration.

Figure 7-24.

Seismic profile across a late Pleistocene Mississippi Fan sequence (ca. 0.4 Ma). (a) uninterpreted profile, (b) interpreted profile. Note that the high-amplitude reflections (HAR), interpreted as channel-fill sands, migrate to the north (left) throughout the evolution of this sequence. Note also that the adjacent low-amplitude levee reflections migrate to the north, such that younger levee deposits overlie the older channel-fill deposits. Salt tongues deformed the sequence after its deposition. After Weimer (1989).

Figure 7-24.

Seismic profile across a late Pleistocene Mississippi Fan sequence (ca. 0.4 Ma). (a) uninterpreted profile, (b) interpreted profile. Note that the high-amplitude reflections (HAR), interpreted as channel-fill sands, migrate to the north (left) throughout the evolution of this sequence. Note also that the adjacent low-amplitude levee reflections migrate to the north, such that younger levee deposits overlie the older channel-fill deposits. Salt tongues deformed the sequence after its deposition. After Weimer (1989).

Development-Scale Characteristics

Important features of levee-overbank deposits at the development scale can be inferred from outcrops, cores, imaging logs, and subsurface reservoirs. We present examples of each, to illustrate the effect these features have on reservoir production performance. The most important features at this scale are net-to-gross value (at any given location in a levee, both laterally away from the related channel and/or down the depositional dip); lateral bed continuity; vertical bed connectivity; and bed-scale features, such as sedimentary structures, textures, and composition. These features are almost always beneath the resolution of conventional 2D and 3D seismic and are termed “subseismic-scale” features of deepwater reservoirs (Slatt and Weimer, 1999).

Examples of levee-overbank outcrops

In contrast with other deepwater architectural elements, there are relatively few levee-overbank deposits that are well exposed for any distance in outcrop. This is, in part, a result of their fine-grained nature, which makes them susceptible to surficial weathering. Those that are better preserved in outcrop are well cemented, and, thus, more resistant to weathering. Some of the better outcrop examples are shown in Figure 7-25, and listed in T a b l e 7 - 1 .

Figure 7-25.

Map showing location of outcrops of levee-overbank thin beds (orange dots) and producing fields (yellow dots) discussed in the text. The numbered outcrops correspond to those numbered in Table 7-1 .

Figure 7-25.

Map showing location of outcrops of levee-overbank thin beds (orange dots) and producing fields (yellow dots) discussed in the text. The numbered outcrops correspond to those numbered in Table 7-1 .

Table 7-1.

Outcrops with significant thin-bed levees used for reservoir modeling.

Below, we illustrate the four best outcrops in which lateral, vertical, and internal attributes of beds have been measured and documented. These outcrops represent mixed sand-mud-prone and mud-prone deepwater systems (Richards et al., 1998).

Miocene Mount Messenger Formation, New Zealand (a mixed sand-mud system)

The general geology of the Mount Messenger Formation was discussed in Chapter 6. Miocene leveed-channel strata along the west coast of the north island of New Zealand are well-exposed in a vertical beach cliff face and range to as much as 250 m thick and 4 km long. The beds dip only a few degrees, thus allowing observation and measurement of both lateral and vertical attributes of thin-bedded levee strata (Slatt et al., 1998; Browne et al., 2000; Browne and Slatt, 2002). This thick succession (>600 m thick; 1970 feet) comprises a series of vertically stacked, interleaving packages of thin-bedded levee-overbank strata; individual packages are as much as 10 m (33 ft) thick, with beds that generally become thinner upward (Figure 7-26). Relatively large erosional channels form a lesser part of the succession (Figure 7-26) (see also Chapter 4). Fine- to very-fine-grained (with a mean of 0.07 to 0.08 mm), planar-laminated Tb and climbing-ripple laminated Tc beds are the dominant sandstone types and are interbedded with Td-Te siltstones (Table 6-8 , Chapter 6). Scolicia burrows are particularly common in climbing-ripple beds, where they occur as escape burrows, sometimes with the parent echinoid preserved at the top of the burrow. An outcrop gamma-ray log of the succession exhibits a thin-bedded response, similar to that of an interval in which a gamma-ray log was collected from a truck (Jordan et al., 1994) and that of an interval from a nearby exploration well (Figure 7-27).

Figure 7-26.

Part of the main outcrop of the Mount Messenger Formation strata discussed in the text: (a) outcrop photograph and (b) line drawing interpretation of the outcrop. Note the channel downcutting through proximal-levee facies (Figure 7-28) and the fining-upward nature of the proximal-levee intervals. After Browne and Slatt (2002). Reprinted with permission of AAPG.

Figure 7-26.

Part of the main outcrop of the Mount Messenger Formation strata discussed in the text: (a) outcrop photograph and (b) line drawing interpretation of the outcrop. Note the channel downcutting through proximal-levee facies (Figure 7-28) and the fining-upward nature of the proximal-levee intervals. After Browne and Slatt (2002). Reprinted with permission of AAPG.

Figure 7-27.

(a) Outcrop gamma-ray log (obtained with a handheld scintillometer) of 600+ m of Mount Messenger strata exposed on the Pukearuhe Beach, New Zealand. The lower 150 m comprises basin-floor fan strata, and the upper 450 m comprises mainly levee deposits with associated channel fill (modified from Diridoni, 1996). (b) Pukearuhe #1 exploration well, located approximately 1 km from the Pukearuhe Beach section and downdip of the beach, so that the same strata in outcrop and on the outcrop gamma-ray log occur on the subsurface well log (modified from Browne and Slatt, 2002). Note that a and b are shown at the same scale. (c) Outcrop gamma-ray log obtained with a logging truck across thin-bedded facies on Pukearuhe Beach (photograph compliments of D. Jordan). Reprinted with permission of AAPG.

Figure 7-27.

(a) Outcrop gamma-ray log (obtained with a handheld scintillometer) of 600+ m of Mount Messenger strata exposed on the Pukearuhe Beach, New Zealand. The lower 150 m comprises basin-floor fan strata, and the upper 450 m comprises mainly levee deposits with associated channel fill (modified from Diridoni, 1996). (b) Pukearuhe #1 exploration well, located approximately 1 km from the Pukearuhe Beach section and downdip of the beach, so that the same strata in outcrop and on the outcrop gamma-ray log occur on the subsurface well log (modified from Browne and Slatt, 2002). Note that a and b are shown at the same scale. (c) Outcrop gamma-ray log obtained with a logging truck across thin-bedded facies on Pukearuhe Beach (photograph compliments of D. Jordan). Reprinted with permission of AAPG.

Relatively small-scale variations in stratification styles and sedimentary features, like those that were observed in outcrop and in core and borehole-image logs from two wells drilled behind the outcrop, allow the differentiation of proximal- from distal-levee beds. Proximal-levee beds are characterized by relatively high net:gross values (60-80%); high dip angles (10-300) arranged in packages with an upward-decreasing dip angle (Figure 7-28); and abundant small-scale, mud-lined scour surfaces at the tops of individual beds (Figure 7-29). These features, plus the abundance of Tb and Tc beds, reflect rapid deposition when the uppermost parts of sediment gravity flows overtopped the channel margin and flow velocity decreased quickly. The abundance of scour surfaces (Figure 7-29) is interpreted to represent turbulence when the flow overtopped the bank, in much the same manner that a turbulent flow generates scours in the channel-lobe transition zone of Mutti and Normark (1987, 1991). By contrast, distal-levee beds are characterized by a higher proportion of Td-Te intervals, a lower net:gross value (20-50%), lower-angle and more uniform dip patterns (Figure 7-28), and fewer mud-lined scours. This implies that deposition occurred farther from the sediment-source channel, within the lower-gradient, lower-energy portion of the levee wedge.

Figure 7-28.

(a) Dipmeter logs from shallow boreholes obtained from behind the Pukearuhe Beach outcrop, showing the three characteristic dip patterns for fine-grained channel-fill, proximal-, and distal-levee facies. (b) Outcrop expression of the left-hand edge of the channel fill (Figure 7-26a) onlapping the complex channel margin. To the right is the proximal-levee facies. Note the variable dip angles and orientations of the thin beds in the proximal-levee deposit. (c) Outcrop expression of distal-levee beds. Shown are six levee packages, each with slightly different dip magnitudes and orientations. Note the shallower and more uniform dips compared with the proximal-levee facies in (b). (Modified from Browne and Slatt, 2002).

Figure 7-28.

(a) Dipmeter logs from shallow boreholes obtained from behind the Pukearuhe Beach outcrop, showing the three characteristic dip patterns for fine-grained channel-fill, proximal-, and distal-levee facies. (b) Outcrop expression of the left-hand edge of the channel fill (Figure 7-26a) onlapping the complex channel margin. To the right is the proximal-levee facies. Note the variable dip angles and orientations of the thin beds in the proximal-levee deposit. (c) Outcrop expression of distal-levee beds. Shown are six levee packages, each with slightly different dip magnitudes and orientations. Note the shallower and more uniform dips compared with the proximal-levee facies in (b). (Modified from Browne and Slatt, 2002).

Figure 7-29.

(a) Portion of a borehole-image log from the Central borehole, Pukearuhe Beach. Particularly significant is the change in dip at about 42.3 m, which corresponds to the base of a channel fill. (Photograph provided by J. Coleman, 2002). (b) Core of the base of channel fill illustrated on the image log, showing a basal shale clast conglomerate underlain by more shallowly dipping thin levee beds. (c) Outcrop photograph showing erosional scour surface cutting into horizontal beds; the erosional surface is overlain by a mud drape (darker color). (d) Subtle erosional scour on top of a sand bed (light color) at 0.77-m depth on the image log.

Figure 7-29.

(a) Portion of a borehole-image log from the Central borehole, Pukearuhe Beach. Particularly significant is the change in dip at about 42.3 m, which corresponds to the base of a channel fill. (Photograph provided by J. Coleman, 2002). (b) Core of the base of channel fill illustrated on the image log, showing a basal shale clast conglomerate underlain by more shallowly dipping thin levee beds. (c) Outcrop photograph showing erosional scour surface cutting into horizontal beds; the erosional surface is overlain by a mud drape (darker color). (d) Subtle erosional scour on top of a sand bed (light color) at 0.77-m depth on the image log.

Continuity and connectivity also differ between proximal- and distal-levee beds. In proximal-levee beds, the presence of mud-lined scours and somewhat larger erosional surfaces reduces lateral continuity, but the high net:gross value results in relatively good vertical connectivity (Figure 7-28). By contrast, distal-levee beds are more laterally continuous, because they generally lack mud-lined scours. However, individual sands lack vertical connectivity because of their lower net:gross values and greater abundance of interbedded muds (Td-Te) (Figure 7-28).

A 150Hz 2D seismic profile (Figure 7-30) was acquired along the beach front in the area shown in Figure 7-26 (Browne and Slatt, 2002). Resolution was later improved by special processing. This profile, which is oriented oblique to depositional strike, shows a complex stratigraphy of interleaving levee sets and channels containing visible lateral accretion bedding. These complexities, plus the lack of precise positioning of the wells and seismic line in true 3D space preclude an accurate overlay of the dip logs (Figure 7-30) onto the seismic line. Probably, the breaks in dip pattern correspond to the boundaries interpreted on the line (Figure 7-30b).

Figure 7-30.

High frequency (150 Hz) seismic reflection profile acquired along the beach in front of the cliff face as shown in Figure 7-26. The profile was processed using Thin MANTM, which removes the seismic wavelet and broadens the spectrum robustly without introducing noise. The resulting profile allows for the resolution of stratigraphic detail that is not apparent with the lower frequency seismic wavelet. Many stratal surfaces are recognized. The two intervals bounded by blue lines are interpreted as sheet sandstones. They occur along strike and down the structural dip from sheet sandstones that occur in outcrop farther to the north (Chapter 8; Figure 8-26). Black lines are interpreted as channel bases. Several of the channels appear to be filled with beds that exhibit downlapping, lateral accretion features (Chapter 6). Blue reflections are positive and red reflections are negative amplitude.

Figure 7-30.

High frequency (150 Hz) seismic reflection profile acquired along the beach in front of the cliff face as shown in Figure 7-26. The profile was processed using Thin MANTM, which removes the seismic wavelet and broadens the spectrum robustly without introducing noise. The resulting profile allows for the resolution of stratigraphic detail that is not apparent with the lower frequency seismic wavelet. Many stratal surfaces are recognized. The two intervals bounded by blue lines are interpreted as sheet sandstones. They occur along strike and down the structural dip from sheet sandstones that occur in outcrop farther to the north (Chapter 8; Figure 8-26). Black lines are interpreted as channel bases. Several of the channels appear to be filled with beds that exhibit downlapping, lateral accretion features (Chapter 6). Blue reflections are positive and red reflections are negative amplitude.

Minipermeameter permeability measurements made on cores from behind the outcrop revealed that the mean permeability values are 456 md for Tb beds (141 measurements), 275 md for Tc beds (77 measurements), and 50 md for Td-Te beds (781 measurements). Measurements made on a single bed comprising a lower 10-cm-thick (4 in) Tb interval and an upper 27-cm-thick (10.5 in) Tc interval show a positive correlation between grain size and permeability (Browne and Slatt, 2002).

Upper Cretaceous Dad Sandstone Member, Lewis Shale, southern Wyoming

The general geology of the Dad sandstone is discussed in Chapter 6. Because of their fine-grained nature, levee-overbank strata associated with the nine channels within the Spine 1 Lewis Shale outcrop are not readily exposed without trenching. When trenched, thin-bedded intervals with a relatively high net:gross value and Tb and Tc (climbing ripple) intervals occur near channel-fill sandstones, and muddier siltstones and thin sandstones occur farther away (Witton-Barnes et al., 2000). A 570-m (1700-ft) well was drilled, and 195 m (580 ft) of the interval was cored, behind the Spine 1 outcrop (Figure 7-31) (Goolsby et al., 2001). That core reveals a variety of thin-bedded, fine-grained strata. One core penetrated a sandstone layer that is interpreted to be channel-fill strata, on the basis of an erosional surface at its base and shale rip-up clasts within the basal sandstone (Figure 7-32a). This sandstone immediately overlies a high gamma-ray mudstone, which is interpreted to be a condensed section that has been partially eroded by the overlying channel sandstone (Figure 7-32b). Another example is a succession of thin-bedded sandstones and mudstones with a relatively high overall net:gross value (Figure 7-32c). In all of these cored intervals, occasional small erosional scours and rippled beds are evident. Slumped beds are uncommon in these cores, but large rotated blocks of levee beds do occur in the outcrop.

Figure 7-31.

Conventional well logs of borehole (CSM Strat Test #61) drilled behind the Spine 1 Lewis Shale outcrop. The gray-pink core gamma scan shows the intervals that were continuously cored. Locations of cores shown in Figure 7-31 are provided. (Logs provided by S. Goolsby, 2002).

Figure 7-31.

Conventional well logs of borehole (CSM Strat Test #61) drilled behind the Spine 1 Lewis Shale outcrop. The gray-pink core gamma scan shows the intervals that were continuously cored. Locations of cores shown in Figure 7-31 are provided. (Logs provided by S. Goolsby, 2002).

Figures 7-32.

Core photographs from the CSM Strat Test #61 well. Stratigraphic positions of the core are shown in Figure 7-30. (a) The upper 2 m (6 ft) of core is composed of sandstone (light colored) with abundant shale rip-up clasts, convoluted beds, burrowed beds, and rippled beds. The sandstones are interpreted to be a basal channel-fill interval. The lower part of the core and its continuation in core (b) are shaley and do not contain many thin sandstone beds. The gamma-ray response of this shaley interval is quite high (Figure 7-30), suggesting that it might be an organic-rich condensed section (similar condensed sections occur in outcrops). (c) High-net-sand interval of thin beds that appear shaley on the gamma-ray log (Figure 7-30).

Figures 7-32.

Core photographs from the CSM Strat Test #61 well. Stratigraphic positions of the core are shown in Figure 7-30. (a) The upper 2 m (6 ft) of core is composed of sandstone (light colored) with abundant shale rip-up clasts, convoluted beds, burrowed beds, and rippled beds. The sandstones are interpreted to be a basal channel-fill interval. The lower part of the core and its continuation in core (b) are shaley and do not contain many thin sandstone beds. The gamma-ray response of this shaley interval is quite high (Figure 7-30), suggesting that it might be an organic-rich condensed section (similar condensed sections occur in outcrops). (c) High-net-sand interval of thin beds that appear shaley on the gamma-ray log (Figure 7-30).

Levee beds can be traced away from one of the Spine 1 channel sandstones (Figure 7-33). Two thin channel sandstones are juxtaposed, and two sets of initially proximal-, then distal-levee beds occur progressively away from the channel sandstones. The proximal-levee beds exhibit significant cut-and-fill features and a high net sand; the distal-levee beds are flatter-bedded and thinner (Figure 7-33), in much the same manner as are the Mount Messenger beds described above (Figures 7-26, 7-27, and 7-28). Paleocurrent indicators within the levee beds also are oriented in a direction away from the channel sandstone.

Figure 7-33.

(a) Photograph of channel-fill sandstone and adjacent proximal and distal-levee beds on the Spine 1 outcrop of the Dad sandstone. Two sets of channel fill and levees are present. The lower figures -are close-ups showing (b) the low angle and uniform dips of the distal-levee beds and (c) the higher dip angle and cut-and-fill features of the proximal-levee beds.

Figure 7-33.

(a) Photograph of channel-fill sandstone and adjacent proximal and distal-levee beds on the Spine 1 outcrop of the Dad sandstone. Two sets of channel fill and levees are present. The lower figures -are close-ups showing (b) the low angle and uniform dips of the distal-levee beds and (c) the higher dip angle and cut-and-fill features of the proximal-levee beds.

Middle Miocene Whakataki Formation, Wairarapa, New Zealand

The Whakataki Formation is a succession of middle Miocene, thin-bedded sandstones and mudstones that are superbly exposed as 60-70o dipping strata along a 1+km long (1.6 miles), wave-cut bench on the north island of New Zealand (Figure 7-34) (Edbrooke and Browne, 1996; Field, 2005). The strata are interpreted to have been deposited in a deep water medial to distal levee-overbank setting. Rapid sedimentation of individual beds is implied by the sedimentary features, indicating the beds are not distal, basin-plain facies. Calculations by Field (2005) suggest an overall sedimentation rate of up to 1500m (4920 ft) of the formation to be 3 m.y.

Figure 7-34.

(a) Photograph of a wave-cut bench at low tide, exposing thin beds of the Miocene Whakataki Formation, Whakataki Beach, North Island, New Zealand. (b) Aerial photograph of the beach with pseudo gamma ray log derived from hand-held scintillometer overlain. Three lithofacies are delineated: A-C. Select bed numbers (S1 and S 20) are noted from Edbrooke and Browne (1996). The main picture (a) shows a lateral view of the thin beds along the bench, for a distance of several hundred meters. Sandstone beds are light colored and composed of repetitive Bouma Tb-Tc intervals separated by thin, dark shales. After Field (2005). Reproduced with permission of Brad Field. (c) Photograph of one bed, showing Bouma Tb (parallel-lamination) and Tc (climbing-ripple) sandstone beds.

Figure 7-34.

(a) Photograph of a wave-cut bench at low tide, exposing thin beds of the Miocene Whakataki Formation, Whakataki Beach, North Island, New Zealand. (b) Aerial photograph of the beach with pseudo gamma ray log derived from hand-held scintillometer overlain. Three lithofacies are delineated: A-C. Select bed numbers (S1 and S 20) are noted from Edbrooke and Browne (1996). The main picture (a) shows a lateral view of the thin beds along the bench, for a distance of several hundred meters. Sandstone beds are light colored and composed of repetitive Bouma Tb-Tc intervals separated by thin, dark shales. After Field (2005). Reproduced with permission of Brad Field. (c) Photograph of one bed, showing Bouma Tb (parallel-lamination) and Tc (climbing-ripple) sandstone beds.

Emphasis of this study was a 32m-thick (105 ft) stratigraphic interval consisting of 360 sandstone beds and intervening mudstones (Figure 7-34). An outcrop gamma ray log across the outcrop, and a graph of bed thickness (Figure 7-35) depict the thin-bedded stratigraphy. Based on the estimated age and thickness of the entire formation, this 32m (105 ft) thick interval would have been deposited over a 64,000 year interval, or an average of one sandstone bed every 177 years. Within the 32m (105 ft) interval, three lithofacies associations have been recognized. Lithofacies association A consists of packets of beds several meters thick in which the thicknesses of individual sandstones generally decrease up-section. The range of sandstone thickness is 0.1-40cm (0.04-16 in) (64% of beds are <10cm thick (4 in)). Net sand is 74%. Intervening, highly bioturbated mudstone beds are generally 1-3cm (0.4-1.3 in) thick. Sandstone beds are parallel laminated Bouma Tb, ripple- and climbing-ripple-laminated Bouma Tc. Bouma Ta units are lacking. Lithofacies association B is an 82cm (2.7 ft) thick unit composed of 80% sandstone occurring in thin beds, each <8cm (3.1 in) thick. Most of these sandstones are massive, though with graded tops. Lithofacies association C consists of a series of sedimentary packages in which sandstone bed thickness increases up-section. Bed thickness and sedimentary structures are similar to those of association A, except that the thicker sandstones have thinner Tb and more dominant Tc units. Net sand is 51% and 55% of the sandstone beds are <10 cm (4 in) thick.

Figure 7-35.

Graph of bed thickness versus bed numbers (all sandstone and mudstone beds are shown). Lithofacies are shown: A (oldest), B, and C (youngest). After Field (2005). Reproduced with permission of Brad Field.

Figure 7-35.

Graph of bed thickness versus bed numbers (all sandstone and mudstone beds are shown). Lithofacies are shown: A (oldest), B, and C (youngest). After Field (2005). Reproduced with permission of Brad Field.

Net sand values can be quite variable over short intervals (e.g. if only 50 bed thicknesses are measured, then the net sand can vary between 65 and 95%) (Figure 7-36). Variations in bed thickness appear to be cyclic, with wavelengths of 1m (3.3 ft)(10 bed sandstone packages) and possibly 14m (50 ft)(200 bed sandstone package).

Figure 7-36.

Graph showing net:gross values for the interval studied as a 50-bed moving average. After Field (2005). Lithofacies associations A (oldest), B, and C (youngest) are delineated. Overall net:gross is 74% (dashed line). Reproduced with permission of Brad Field.

Figure 7-36.

Graph showing net:gross values for the interval studied as a 50-bed moving average. After Field (2005). Lithofacies associations A (oldest), B, and C (youngest) are delineated. Overall net:gross is 74% (dashed line). Reproduced with permission of Brad Field.

Lateral continuity measurements over a 3.7m portion of the measured section reveal that across a 200-m distance, 90% of the beds as thin as 5cm (2 in) are correlative, and for a 400- to 500-m distance (1300-1650 ft), about 80% of the beds are correlative (Figure 7-37). Termination of beds is almost always the result of depositional pinch-out rather than erosional truncation.

Figure 7-37.

Measurements of thin beds shown in Figure 7-34 for a distance of 500 m. (a) Percentage of beds that are continuous as a function of distance for the 500-m measured interval of thin beds. Approximately 80% of the thin beds are continuous for the 500-m distance. (b) Line drawing of the length of the bench that was measured, the stratification, and the location and spacing of the measured sections that form the basis for the graph in (a). Modified from Edbrooke and Browne (1996).

Figure 7-37.

Measurements of thin beds shown in Figure 7-34 for a distance of 500 m. (a) Percentage of beds that are continuous as a function of distance for the 500-m measured interval of thin beds. Approximately 80% of the thin beds are continuous for the 500-m distance. (b) Line drawing of the length of the bench that was measured, the stratification, and the location and spacing of the measured sections that form the basis for the graph in (a). Modified from Edbrooke and Browne (1996).

A nearby offshore exploration well, the Titihaoa-1 was drilled by Amoco New Zealand Exploration Ltd.. A 14 m (46 ft) interval of a borehole image log reveals 140 sandstone beds which occur in packages 0.5-1.5m (1.6- 5 ft) in thickness (Figure 7-38). A seismic line that crosses the well location shows a typical gull-wing levee and channel morphology (Figure 7-39).

Figure 7-38.

Borehole image log from Titihaoa-1 well showing thin beds equivalent to those exposed at the Whakataki Beach. After Field (2005). Reproduced with permission of Brad Field.

Figure 7-38.

Borehole image log from Titihaoa-1 well showing thin beds equivalent to those exposed at the Whakataki Beach. After Field (2005). Reproduced with permission of Brad Field.

Figure 7-39.

Flattened seismic profile across the Titihaoa-1 well, offshore eastern New Zealand. Interval penetrated by the borehole image log is shown, and corresponds to the slightly hum-mocky reflections of the levees, and adjacent channel-fill deposits. After Field (2005). Reproduced with permission of Brad Field.

Figure 7-39.

Flattened seismic profile across the Titihaoa-1 well, offshore eastern New Zealand. Interval penetrated by the borehole image log is shown, and corresponds to the slightly hum-mocky reflections of the levees, and adjacent channel-fill deposits. After Field (2005). Reproduced with permission of Brad Field.

This study provides important information for thin-bed reservoir evaluation: (1) good lateral continuity of medial- to distal-levee thin beds; (2) pitfalls in using average values of net sand for volumetric analysis of thin-bedded intervals; and (3) potential predictive cyclicity in stratigraphic thickness trends of thin-bedded intervals.

Cerro Toro Formation, Chile

The general geology of the Cerro Torro Formation is discussed Chapter 6. As we indicate in that discussion, there is disagreement about whether the thin-bedded strata there are levee-overbank or basin-floor distal sheet deposits. DeVries and Lindholm (1994) favor the former interpretation. They traced and measured the thicknesses of 91 “levee” beds that could be traced laterally. Only two of the beds are laterally discontinuous across the 150-m (500-ft) outcrop measured, 30% of the beds do not thin laterally, and the remainder thin at a decreasing rate. Thicker sandstone beds (defined as >5 cm [2 in] thick) thin for a shorter distance than do thinner beds (<5 cm [2 in] thick) (Figure 7-40). Over the distance of 150 m (500 ft), only 35% of the beds that are greater than 5 cm (2 in) thick retain their original thickness, as opposed to more than 60% of the beds that are less than 5 cm (2 in) thick. The most proximal-levee package has a net sand of 42%, whereas 2134 m (7000 ft) in a more distal direction, net sand is 20%.

Figure 7-40.

Percentage of original thickness of beds for a distance of 167 m (500 ft) in thin beds of the Cerro Toro Formation, Chile. Bed measurements are subdivided into to beds that are greater than and those that are less than 5 cm (2 in) (modified from Devries and Lindholm 1994).

Figure 7-40.

Percentage of original thickness of beds for a distance of 167 m (500 ft) in thin beds of the Cerro Toro Formation, Chile. Bed measurements are subdivided into to beds that are greater than and those that are less than 5 cm (2 in) (modified from Devries and Lindholm 1994).

Beaubouef (2004) summarized additional characteristics of levee-overbank facies (Table 7-2) and confirmed that the following features change from the proximal- to the distal-levee areas: net sand decreases, the range of bed thickness decreases, and lateral continuity increases (Figure 7-16). Thinner beds have higher lateral continuity than do thicker levee beds. This is the same condition that was observed in the Mount Messenger Formation and is a result of there having been lower depositional energies and erosional capabilities in the more distal areas.

Table 7-2.

Levee-overbank facies, Cerro Toro Formation, Chile.

Slide scars and mass-movement deposits are associated with the levee strata. The largest such deposit occurs about 914 m (3000 ft) away from the channel margin, in the proximal-levee setting, and is 46 m thick by 91 m wide (150 ft thick by 300 ft wide). Crevasse-splay deposits also occur in the proximal-levee setting and are as much as 71 m (560 ft) wide and 10 m (33 ft) thick. These deposits are thought to be sediment gravity flows that overtopped the channel margin and filled slump-scar depressions on the levee.

Barton et al. (2006) have extended the study to include a 250m (800ft.) long photomo-saic and measured sections from a 25m (80ft.) thick interval interpreted as levee strata that sit approximately 1km from an associated large channel. The orientation of this outcrop is oblique to the overall paleocurrent direction. This interval thins from 25m (80 ft.) to 16m (52 ft) and decreases in net sand from 35 to 20% over the 250 m (800 ft) lateral distance. Four facies recognized are burrowed mudstones (laterally extensive 0.5-1.0m (1.6-3.3 ft) thick), parallel-laminated mudstones and siltstones fine-grained graded sandstones with occasional ripple tops, and fine- to medium-grained, parallel laminated sandstones. The graded sandstone beds are 1-30cm (0.4-12 in) thick; only 20% of the beds >5cm thick are laterally continuous across the outcrop, and the other 80% pinch out or reach thicknesses of <1cm. The parallel laminated sandstones are 10-60cm (4-24 in) thick; only 15% of the beds extend the length of the outcrop and average bed length is 100m (330ft.). This facies stacks to form low relief (0.3-1.2m; 1ft-4ft from crest to trough) sediment wave-like features with a wavelength from crest to crest of about 30m (100ft.). The direction of wave migration is opposite the paleocurrent direction, which led Barton et al. (2006) to the conclusion that the waves were formed by bottom current reworking and winnowing after deposition.

Reservoir implications of outcrop characteristics

Lateral continuity

Because levee-overbank beds are so thin, it is difficult to trace and correlate them over great distances. The measurements reported above indicate that there is thin-bed continuity for lengths of a few hundred meters, but data on longer bed lengths are not available. In the Whakataki Formation example, the thin beds appear to be continuous for as much as thousands of meters, based on both examination of the outcrop (Figure 7-34) and extrapolation of continuity measurements (Figures 7-35, 7-37). Judging by the high flow rates in such reservoirs as Ram-Powell and Tahoe (discussed below), lateral continuity of thin bedded reservoirs can be extensive.

Lateral continuity is also thought to vary between proximal- and distal-levee beds. In the former, the presence of erosional scours, which are often mud lined, may reduce the continuity of any single bed or a group of beds. In the latter, the net:gross value is lower, and the greater proportion of mudstone beds interbedded with sandstones deposited in a relatively lower-energy environment provides greater long-distance continuity.

Vertical connectivity

Vertical connectivity is easier to measure at a single outcrop or wellbore, but lateral variations in vertical connectivity are probably quite common. Proximal-levee deposits with a relatively high net:gross value and with a relatively high proportion of erosionally scoured beds will tend to have better vertical connectivity than do distal-levee deposits that are more evenly interbedded. Thus, there is a trade-off in reservoir parameters between proximal- and distal-levee beds. Proximal-levee beds have a higher net:gross value and greater vertical connectivity, but they have less lateral continuity than do lower net:gross value distal-levee beds.

The effect of shales on continuity and connectivity

Within levee deposits, the two main types of shales are apt to be shale interbeds and shale-lined scour surfaces, both of which may have a pronounced effect on continuity and connectivity. Laterally continuous shales can isolate stacked, thin-bedded sandstones of the distal levee. If the shales are eroded or otherwise nonexistent in the proximal-levee setting, their absence promotes vertical connectivity. Mud-lined scours, which are prevalent on at least some proximal-levee deposits (Figure 7-29), can provide baffles but not barriers to fluid flow. It is not known whether mud linings are the fine-grained tails of the sediment gravity flow that scoured the underlying bed or are hemipelagic mud that was deposited long after the scour formed.

Textural, structural, and compositional characteristics of levee-overbank deposits

Most of the sediments deposited on levees come from the tops of sediment gravity flows that overtop channel banks. For this reason, levee beds tend to be fine-grained (i.e., are fine sand and/or silt). These beds may be interbedded with thin hemipelagic beds deposited during periods of quiescence on the levee surface. Such hemipelagic beds are finer-grained than their sediment-gravity-flow counterparts.

Many workers have found that the sandy parts of levee beds tend to be populated with a relatively high proportion of Bouma Tb and Tc (climbing-ripple) intervals. Sedimentary structures associated with rapid deposition of high sediment volumes are to be expected on the parts of the levees where sediment suspensions rapidly overtop the channel margins and spread across the levee surface. In terms of proximity indicators, the proportion of Tc intervals should increase at the expense of Tb intervals, as one moves from the proximal to the distal levee. This fact, coupled with the presence or absence of mud-lined scours, should provide some means of predicting relative proximity to feeder channels.

Identifying levee-overbank beds in subsurface cores and downhole logs

Identification of levee-overbank beds using conventional wireline logs is difficult, if not impossible, because of the limited vertical bed resolution of conventional logging tools. Therefore, borehole-image logs and cores can help considerably in detailed interpretations. The general appearances of levee-overbank thin beds in borehole-image logs and cores are shown in Figures 7-29, 7-31, and 7-32. Mud-lined scours, Tb and Tc beds, and variations in dip patterns can be recognized on image logs (Figure 7-29). All of these features can be used to differentiate proximal- from distal-levee beds. Dip patterns can also be recognized using a dip-meter log (Figure 7-28). These same sedimentary features can be recognized in cores, although it is actually sometimes much easier to see these features on the image logs, particularly if the cores contain only a narrow range in grain size. Such features on dip logs and in core were used to differentiate proximal from distal-levee facies in the L Sand of the Ram-Powell field, as we describe below. This differentiation led to placement of an excellent horizontal well in the proximal-levee facies (Figure 7-13).

Quantifying thin-bedded net pay

A problem that is specific to thin beds is the accurate estimation of sand content. Since conventional well logs appear shaley, significant pay can be overlooked in volumetric calculations (see Figure 13-18). A variety of tools and techniques have been applied to wireline logs to reduce the uncertainty. Three such techniques are multicomponent induction logging dipole sonic imaging, and nuclear magnetic resonance logging.

Multicomponent induction logging provides a measurement of both Rh and Rv in thin bedded strata (Munkholm et al., 2002). A Tensor Resistivity Petrophysical analysis allows for removal of the laminar shale effects to determine sand porosity and saturation. The model greatly enhances Sw in thin sands and quantifies low Sw and high porosities in thin laminae where open hole logs indicated only shale. This method of analysis allows volumetric upside to be realized in thin bedded reservoirs which can represent a significant volume in deep marine systems.

The dipole sonic imaging uses a Dipole Shear Sonic imager tool to measure shear velocities (as well as standard P-wave velocities) in unconsolidated, gas-bearing sands (Bastia and Klimentos, 2004). One case study has shown that thinly-bedded sand/silt/shale packages exhibit different Vp-Vs trends than blocky, thicker sands within the same formation. This information can be applied to mapping thin-bedded intervals from seismic data.

Nuclear magnetic resonance (NMR) logging is another tool that has proven to be particularly useful in differentiating fluids within thin-bedded strata. The basic theory behind nuclear magnetic resonance logging is that protons in hydrogen atoms within pore spaces of a rock react to an induced magnetic field by precessing at characteristic frequencies and by aligning themselves with the induced field. When the induced magnetic field is switched off, the protons respond by realigning themselves with the earth’s natural magnetic field. The process of realignment or relaxation is controlled by pulse sequences and amplitudes generated in the logging tool. The strength and rate of alignment are proportional to the volume of hydrogen atoms, the amount of water, and the effective size of the pore in which the protons are located. The rate of relaxation is modeled as a sum of exponentials whose characteristic time is inversely proportional to the pore size. The amplitude at any relaxation time, therefore, corresponds to the contribution made by the volume of protons existing in that pore size. The sum of all contributions is a measure of porosity. Thus, this distribution of relaxation times, referred to as a T2 distribution, can be interpreted as a pore-size frequency distribution, which yields insights into reservoir quality and producibility. Interpretations of these distributions in terms of free and bound water have helped geoscientists develop empirical methods for predicting permeability from NMR logs (Coates et al., 1999). Additionally, changing the fluids to oil or gas alters the relaxation times significantly.

Nuclear magnetic resonance can be measured both by a downhole logging tool and in the laboratory. In fact, to improve the accuracy of determinations of rock and fluid properties in a wellbore, it is best to calibrate the log response by first measuring the same properties on core samples.

Communication between channel and levee beds

Another issue of importance to levee deposits and their associated channel fill is whether or not the two different sand types are in communication. If they are, then it is possible to drain the levee beds of hydrocarbons by producing into the channel sands, or vice versa. If they are not, then additional drilling is required to capture hydrocarbons from levees and channels.

Some performance calculations indicate communication between channel and levee beds. For example, for thin bed reservoirs, the initial Ro values that are calculated are too low. As a field produces, adjacent channel fill reservoirs are over -producing compared to reservoir modeling and simulation. The thin beds are re-evaluated and (1) the Ro values are increased and (2) they clearly are in communication with the channel-fill reservoirs.

Besides predicting the connectivity of channel and levee sands by such well performance, the only other way in which the small-scale connectivity can be established is by outcrop examination. Figure 6-43 and Figure 6-52 show the margins of channel sandstones and adjacent thin-beds from two outcrops; in both cases, the channel margin contains slumped and contorted beds, which appear to separate the two sandstone bodies. An analogous situation has been observed on subsurface seismic records (Figure 6-44 and Figure 6-48a), and could lead to either partial or complete compartmentalization depending upon the extend of slumping. On the other hand, Kirschner and Bouma (2000; their Figure 4) show a channel margin through which they claim adjacent levee sands are connected. Similar relations are noted in two Permian fields in west Texas and in nearby coeval outcrop examples (Dutton and Barton, 1999; Dutton et al. (2003).

Crevasse-splays and sediment waves

Crevasse-splays are often seen on seismic amplitude extractions to be associated with leveed channels (Figures 7-1, 7-5, 7-6, 7-17). A coarsening- and thickening-upward well log response within an interpreted crevasse splay was illustrated by Roberts and Compani (1996). An outcrop of the Jackfork Sandstone in central Arkansas is also interpreted as containing two crevasse splays within a series of channelized sandstone strata (Figure 7-41). In this particular example, channel sandstones are amalgamated at erosional bases, levee beds are thin, and comprised of Bouma Tb and Tc beds, and the crevasse splays are generally massive or graded Bouma Ta beds with flat bases and tops. The only producing example that has been mentioned in the literature is the Falcon Field (discussed below). In this example, the crevasse splay was cored.

Figure 7-41.

Photograph of outcrop of the Jackfork Sandstone at McCain Mall in Little Rock (south central Arkansas) showing a lower interval of lenticular, amalgamated channel sandstones, overlain by a thinning-upward interval of levee beds, then by a thickening-upward interval interpreted as a crevasse splay. Mr. Charlie Stone for scale. See Slatt et al (2000) for discussion of section.

Figure 7-41.

Photograph of outcrop of the Jackfork Sandstone at McCain Mall in Little Rock (south central Arkansas) showing a lower interval of lenticular, amalgamated channel sandstones, overlain by a thinning-upward interval of levee beds, then by a thickening-upward interval interpreted as a crevasse splay. Mr. Charlie Stone for scale. See Slatt et al (2000) for discussion of section.

Recognition of sediment waves in the subsurface is challenging due to the scale of the features and diagnostic criteria. With burial, these features will compact and their wave form geometries will be obscured. The sediment distribution would be linear in bar forms, potentially with distinct linear permeability and production trends. However, these have yet to be recognized in the subsurface. Barton et al. (2006) described outcrop examples of sediment waves from the Upper Cretaceous Cerro Toro Formation in Chile

Examples of Thin-Bedded Levee Reservoirs

Because levee reservoirs have not been primary targets for development, there are fewer examples of producing fields in levee-overbank wedges than in the other types of deepwater reservoirs we discuss in Chapter 6 and Chapter 8. In major mud-dominated continental margins, thin bed reservoirs are common. Our experience is that thin-bed levee reservoirs have been discovered in many basins globally during the past five years, so that many companies are beginning to develop them: such examples include the offshore India discoveries (Bastia et al., 2003; 2004), offshore western Nile (Egypt (Munkholm et al., 2002; Dolson et al., 2005), Mio-Pliocene of Nigeria, Upper Cretaceous of Sacramento Basin of California (Nilsen et al., 1994), and Ceiba field, Equatorial Guinea (Dailly et al., 2002; Weaver et al., 2002).

The following is a brief summary of the performance of three such fields from the northern deep Gulf of Mexico. For these fields, we describe the basic setting of the field, trap, and rates of production; show examples of seismic profile and log data; and then discuss the important production history of the reservoir. In addition, there are many unpublished examples in the northern Gulf of Mexico, and a few representative examples are included in Weimer et al. (1998).

“L” Sand, Ram-Powell field, northern Gulf of Mexico

Key references

Location

The Ram-Powell field is located 130 mi (210 km) southeast of New Orleans, Louisiana, U.S.A., in the eastern Gulf of Mexico continental slope, in Viosca Knoll blocks 911, 912, 913, 956, and 957 (Figure 7-18). The field rests in 1067 m (3200 ft) of water.

Significance

Five wells were drilled into the L sand in the leveed channel, on the assumption that channel sands would be present and petroleum-bearing. All five wells penetrated the 500- to 677-m- (1500- to 2000-ft-) wide channel, and they encountered good sands, all of which were water-bearing (Figure 7-42). Wells drilled into a high-amplitude reflection zone adjacent to the channel were gas-charged. A strategy was developed to drill a 830-m (2500-ft) horizontal well parallel to the channel margin, but within adjacent strata that had been interpreted to be proximal-levee beds on the basis of core, dip logs, location relative to the channel, and outcrop analogs.

Figure 7-42.

Well paths of the VK A-1STB2 horizontal well and the appraisal well VK 912-2 through the Ram-Powell L sand, northern deep Gulf of Mexico. After Bramlett and Craig (2002). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-42.

Well paths of the VK A-1STB2 horizontal well and the appraisal well VK 912-2 through the Ram-Powell L sand, northern deep Gulf of Mexico. After Bramlett and Craig (2002). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Trap

The L sand is considered to be one of the few purely stratigraphic traps in the northern deep Gulf of Mexico. The sands pinch out updip.

Size

The L sand reservoir is approximately 6 km (4 mi) long by 3 km (2 mi) across (Figures 7-18 and 7-22). Gross thickness varies from about 33 m (100 ft) proximal to the channel margin to about 13 m (42 ft) at the edge of the field. The reservoir comprises 18.2 km2 (4500 acres), based on material balance.

Age and depositional environment

Unlike fields in the western and central northern Gulf of Mexico, which are located in ponded salt minibasins, the middle Miocene Ram-Powell reservoir sands were deposited on the slope and toe-of-slope environment within a lowstand systems tract. The L sand was deposited as a leveed-channel system.

Hydrocarbons in place and production characteristics

The L sand contains 250–300 billion ft3 gas. Its 690m (2297ft) long horizontal well was drilled and is an open hole that is gravel packed. Well performance exceeded expectations, with a peak flow rate of 109 MCFPD and 9500 BCPD. By the end of 2001, cumulative production was 102.1 BCF of gas and 6.5 (MMBC).

The well produced 4.8 MMBO and 61.5 BCF (15.4 MBOE) from September 1997 to May 2000. A four-day well test in a 17-m- (50-ft-) thick distal levee flowed 23 MCFPD and 2700 BOCPD.

New information became available after this well was drilled. The proximal levees comprise four distinct, upward-thinning, thin-bedded packages, all of which are separated by shale beds. At least two of these units have different fluid levels, and pressure data suggest that they act as separate flow units. The post-production MDT data indicate that connectivity occurs across a large area, from the proximal- to distal-levee deposits. Through time, connectivity seems to be increasing because of the breakdown of intrareservoir barriers, which is manifested by an increase in pressure (from 4 to 7 psi), followed by flattening in the pressure-decline curve. Current estimates are that this one well will drain the entire reservoir. Pressure tests indicate that the levee and adjacent channel sandstones are not in communication.

Seismic and well-log expression

A single seismic-reflection event characterizes the Ram-Powell L sand (Figure 7-12). Both the left-side levee and the channel are transparent, but the gas-charged right-side levee wedge exhibits a high-amplitude reflection. Well logs of proximal-levee strata exhibit a fine-scale interbedded nature, whereas distal-levee beds exhibit a shalier log response (Figure 7-13).

Thickness, lateral continuity, and aspect ratio

Gross thickness of the L sand varies, from about 30 m (100 ft) at the proximal levee to about 12 m (40 ft) at the distal levee (Figure 7-13). The gas-charged eastern levee extends 500 m (16,500 ft) to the east of the channel margin. Well tests indicate that there is good lateral continuity and communication across the entire 4000-acre (16 km2) proximal-levee deposit.

Vertical connectivity and net:gross

Net sand varies from 50 to 60% in the proximal levee, to approximately 20% in the distal levee (Figure 7-13).

Aspect ratio

No accurate calculations are publicly available.

Sedimentary texture, composition, and structures determined from core and borehole-image logs

Development planning was significantly influenced by core, because the interbedded thin sandstones and siltstones appeared as a low-resistivity interval on well logs and would not otherwise have been considered to be pay. In fact, while they were drilling the well, the drillers frequently called the office asking when the pay zone would be reached. In reality, they were already well into that zone! In core, proximal-levee beds are somewhat thicker than distal-levee beds and exhibit higher dip angles (Figure 7-13). Dip logs exhibit the same characteristics as those described above for the Mount Messenger Formation; that is, proximal-levee beds are characterized by relatively high and variable dip magnitudes, and distal-levee beds are characterized by lower and more uniform dip magnitudes (Figure 7-13). Proximal-levee sandstones are 0.4–32 cm (0.25–12 in.) thick, and mudstones generally are less than 2.5 cm (1in.) thick and never are greater than 5 cm (2 in.) thick. Bouma Tc, Td, and Te beds are common, with fewer Tb beds present. Distal-levee sandstones are about the same thickness as are the proximal-levee beds, but they have a slightly finer grain size.

The thin-bedded nature of the reservoir interval that was observed in core initially led to the hypothesis that reservoir connectivity was poor. On this assumption, the initial development plan in 1993 was to drill more than 30 wells at 120-acre (49-ha) spacing. However, a production test in the distal-levee facies suggested good reservoir connectivity throughout 300–700 acres (120–280 ha). These data, plus information gained by comparing features from core and dip logs from this field with similar features in the Mount Messenger outcrops (Slatt et al., 1998), led the operator to reduce the number of development wells first to six, then to three, and eventually to the one producing well. This well is currently one of the highest cumulative producers in the Gulf of Mexico (Figure 7-42).

Reservoir quality

Sand porosities range from 15 to 32%, with a mean of 28%. Permeabilities range from less than 10 md to 1000 md, with an average of 300 md. The one producing well, VK 912A-1ST1BP2, indicates a permeability of 90 md, based on pressure transient analysis. Well-test permeabilities are consistently lower than core-plug permeabilities.

Drive mechanism

The field has gas-depletion drive.

M4.1 Sand, Tahoe field, Northern Gulf of Mexico

Key references

Location

The Tahoe field is in Viosca Knoll block 783, in the northern deep Gulf of Mexico, 130 mi (210 km) east-southeast of New Orleans, Louisiana, U.S.A. The field is in 366 to 457 m (1200 to 1500 ft) of water.

Significance

The Tahoe field, which was discovered in 1984, was one of the first thin-bedded reservoirs discovered and developed in the northern deepwater Gulf of Mexico. Two reservoirs are present; the M4.1 is the deeper and main reservoir. The operator (Shell Oil Company) initially was unsure whether these thin beds could produce at high enough rates to warrant development. Several outcrop analog studies were done to address this issue and resulted in the coring and testing of the 783-4ST2 well, which contained significant, thin sand beds (Figure 7-23). Results indicated that these beds could produce at the high rates of 29 MCFPD and 950 BOCPD.

Initially, in January 1994, the entire reservoir was developed with one well; a second phase of development occurred in 1996 with four additional producing wells.

Trap

The field lies on a faulted anticline (Figures 7-43, 7-44).

Figure 7-43.

Depth-based seismic profile across the Tahoe field, northern deep Gulf of Mexico. The producing interval (M4.1) corresponds to the high-amplitude reflection labeled levee-splay. Time-based log shows grain sizes of the interval. Note also the channel-fill reflection. See Figure 7-38b for location of the profile. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-43.

Depth-based seismic profile across the Tahoe field, northern deep Gulf of Mexico. The producing interval (M4.1) corresponds to the high-amplitude reflection labeled levee-splay. Time-based log shows grain sizes of the interval. Note also the channel-fill reflection. See Figure 7-38b for location of the profile. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-44.

(a) Structure contour map of the top M4.1 reservoir level, Tahoe field, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown. Note that the channel (lavender) segments the east from the west levee reservoirs.

Figure 7-44.

(a) Structure contour map of the top M4.1 reservoir level, Tahoe field, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown. Note that the channel (lavender) segments the east from the west levee reservoirs.

Figure 7-44.

(b) Amplitude extraction map of the M4.1 reservoir level, superposed with structure contour and faults. Distribution of the gas reservoir is indicated by the hotter colors (reds, purples, orange). Location of Figure 7-37 is shown. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-44.

(b) Amplitude extraction map of the M4.1 reservoir level, superposed with structure contour and faults. Distribution of the gas reservoir is indicated by the hotter colors (reds, purples, orange). Location of Figure 7-37 is shown. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Size

The field is about 5.4 by 7.2 km (3 by 4 mi), based on the seismic-amplitude anomaly that is associated with the field (Figure 7-44). A prominent channel dissects the field.

Age and depositional environment

The field consists of upper Miocene reservoirs deposited in a levee-overbank setting. The slope was primarily unconfined structurally. The zone of channelization is about 4.8 km (3 mi) wide and tens of miles long.

Hydrocarbons in place and production characteristics

Nothing has been published. However, More than 17 million bbl equivalents of gas and condensate had been produced from four wells through mid-2000. The oil-water contact is shallower in the west levee than it is in the east levee, indicating that communication does not extend across the entire field (Figure 7-45). Pressures in the west levee depleted over time throughout the entire stratigraphic interval, but only depleted in the upper part of the east levee. However, the lower portion of the eastern levee remained at original pressure. This suggests that stratigraphic boundaries separated both sides of the channels.

Figure 7-45.

(a) Pressure profiles from the M4.1 sand, Tahoe field, northern deep Gulf of Mexico: before (left) and after (right) production. Before production, pressure indicated that the east and west levees had similar formation pressures. Two years after production, differential depletion occurred in an offsetting well in the west levee reservoir. In contrast, the east levee had partial depletion in the upper levee but none in the lower levee. (b) Schematic cross section across the M4.1 reservoir level, showing the pattern of depletion in the two levees. Blue zones indicate where the pressure in the thin beds had depleted by early production in the 783-4ST2 well. Blue hachured zones represent zones of partial completion. White areas indicate zones of no depletion or no data. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-45.

(a) Pressure profiles from the M4.1 sand, Tahoe field, northern deep Gulf of Mexico: before (left) and after (right) production. Before production, pressure indicated that the east and west levees had similar formation pressures. Two years after production, differential depletion occurred in an offsetting well in the west levee reservoir. In contrast, the east levee had partial depletion in the upper levee but none in the lower levee. (b) Schematic cross section across the M4.1 reservoir level, showing the pattern of depletion in the two levees. Blue zones indicate where the pressure in the thin beds had depleted by early production in the 783-4ST2 well. Blue hachured zones represent zones of partial completion. White areas indicate zones of no depletion or no data. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Seismic and well-log expression

The M4.1 sand has a typical gull-wing appearance on seismic records (Figure 7-43). On an amplitude extraction map, the reservoir corresponds to the area of prominent high amplitude (Figure 7-44). The channel appears as a linear low-amplitude area, with occasional higher-amplitude values. On the gamma-ray log, the reservoir consists of interbedded sand and shales that, overall, fine and thin upward (Figure 7-23). This occurs in both the eastern and western levees.

One important lesson learned from developing the Tahoe field was that log-based estimates of sand-bed thickness and quality should not be used to predict sand-bed continuity. Initial development plans considered the lower reservoir to have better-developed sands than did the upper part. Consequently, to avoid a biased result, the upper sands were tested, and the lower interval was not. As the field was produced, the lower interval was recognized to have good sands but less continuity than the upper interval, as the models of Hackbarth and Shew (1994) predicted (Figure 7-19).

Thickness and lateral continuity

The reservoir is about 65 m (190 ft) thick (Figure 7-23). Production indicates that some sand beds are greater than 3 km (9840 ft) in length, whereas other are less than 670 m (2200 ft) long.

Aspect ratio

This has not been published.

Vertical connectivity and net:gross values

The average net:gross value in the No.4 ST2 well was 50% in a 30-m (100-ft) core. Vertical connectivity at the individual bed scale is minimal because of the thinly interbedded nature of the strata (Figure 7-23).

Sedimentary texture, composition, and structures determined from core and borehole-image logs

Cores recovered very thinly bedded and laminated sands and silts that are interbedded with shales. There is an equal number of thin beds (0.4 to 4 in. [1 to 10 cm] thick) and laminae (each less than 1 cm thick) (Figure 7-23). In the No.4 ST2 core, more than 1200 discrete layers (lamina and thin beds) were noted. Some laminae are so thin that the formation microscanner (FMS) log could not resolve individual layers.

Reservoir quality

Porosity averages 27% and average permeability is about 70 md for individual beds and laminae. Permeability values range from a few millidarcys to 500 md, depending on grain size and bed thickness. Some of the beds are cemented with ferroan calcite, siderite, and authigenic clay.

Drive mechanism

The drive mechanism has not published.

M57 Sand, Falcon field, northern deep Gulf of Mexico

Key references

Abdulah et al. (2004); Mark Bengtson, Justin Bellamy, and David Keller (personal communication, 2005).

Location

The Falcon field is in East Breaks blocks 579 and 623, in the northern deep Gulf of Mexico, 100 mi (162 km) east of Corpus Christi, Texas, U.S.A. The field is in 3400 ft (1037 m) of water, and is produced by a two-well subsea tieback to a host platform.

Significance

The Falcon field was discovered in April 2001, and has been producing since mid-2003. Two distinct reservoirs are present; the primary reservoir is east of the channel and a smaller accumulation lies west of the channel (Figure 7-46). A prominent channel is the lateral seal downdip of the field. This field illustrates the inherent problems in estimating sand content based on logs only, without integrating cores and image logs. Also, the excellent lateral continuity of thin beds is indicated by two wells draining the entire eastern area.

Figure 7-46.

Location map of Falcon field, East Breaks 579 and 623, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown by red area, and blue dashed line shows gas/water contact. Note that the channel (green) segments the east from the west levee reservoirs. Slight offset in the gas/water contact is shown by the channel. Regional dip to the southwest is shown by red arrow. Corrected stratigraphic dips in the eastern levee are shown, as determined from image logs. Modified from Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-46.

Location map of Falcon field, East Breaks 579 and 623, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown by red area, and blue dashed line shows gas/water contact. Note that the channel (green) segments the east from the west levee reservoirs. Slight offset in the gas/water contact is shown by the channel. Regional dip to the southwest is shown by red arrow. Corrected stratigraphic dips in the eastern levee are shown, as determined from image logs. Modified from Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Trap

The field consists of a combined stratigraphic-structural trap. The field is located on the western flank of a salt feature that dips to the southwest (Figure 7-47). The updip stratigraphic trap is the depositional limit of levee beds that were deposited on the flank of the underlying salt feature. The downdip limit in both the Eastern and Western levee is demarcated by a seis-mically-defined flat spot. The East and West levee gas accumulations are separated by a shale-filled channel. This same channel forms the updip trap in the West levee. Although the Western levee is highly faulted, the Eastern levee appears to be unfaulted.

Figure 7-47.

(a) Two thin-section photographs illustrate the excellent porosity and permeability in the proximal and distal levee facies. (b) Core photograph of proximal (lower core) and distal (upper core) levee facies of the Falcon field. Sands consist of unconsolidated, well-sorted, fine- to very fine-grained sands with climbing ripples. (c) Image and wireline logs (gamma-ray, resistivity and neutron) illustrate the levee facies for the entire M57 reservoir interval. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-47.

(a) Two thin-section photographs illustrate the excellent porosity and permeability in the proximal and distal levee facies. (b) Core photograph of proximal (lower core) and distal (upper core) levee facies of the Falcon field. Sands consist of unconsolidated, well-sorted, fine- to very fine-grained sands with climbing ripples. (c) Image and wireline logs (gamma-ray, resistivity and neutron) illustrate the levee facies for the entire M57 reservoir interval. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Size

The field is about 2000 acres, based on the seismic-amplitude anomaly that is associated with the field (Figure 7-46).

Age and depositional environment

The field consists of middle Miocene reservoirs deposited in a levee-overbank setting. The slope had an irregular bathymetry, with which the channel and levee deposits responded during deposition. The zone of channelization is about 0.5 mi (0.8 km) wide and tens of miles long. Channel morphology varies from straight to sinuous along its course. Levees change in thickness and morphology along the channel. Levees were deformed locally by post-deposi-tional processes such as slides and faults.

Hydrocarbons in place and production characteristics

To date, no reserves have been published. Falcon Field produces from two wells— EB579#2 and EB623#2. Through mid-2003, the two-well development was producing about 185 MCFPD and 65 BOCPD from the eastern levee (Figure 7-46). Deviated wells were used to minimize the costs and complexity of seafloor facilities. The Tomahawk (EB623#3) well also produced from the western levee (Figure 7-46).

Seismic and well-log expression

No seismic profiles have been published. On an amplitude extraction map, the reservoir corresponds to the area of prominent high-amplitude, low-impedance anomalies. The intervening channel appears as a linear low-amplitude area, with occasional higher-amplitude values thought to be gas-charged slide deposits. Gas-water contacts are clearly imaged as flatspots on both sides of the channel (Figure 7-46).

Reservoir occurs at different structural elevations. To the east, the seismically-defined flat spot has been penetrated by drilling. Production and pressure data also confirm the two separate reservoirs. Production history best fits reservoir models with a sealing channel and edge-drive aquifer in communication with both sides of the channel.

On the gamma-ray log, the reservoir consists of interbedded sand and shales that, overall, fine and become thinner upward (Figure 7-47). One important lesson learned from developing the Falcon field was that log-based estimates of sand content versus core-based measurements are quite different (Figures 7-47, 7-48). In the proximal to medial levee facies, sand content is overestimated by 35% by conventional and high-resolution logs. In the distal facies, both sets of logs considerably underestimate sand content.

Figure 7-48.

(a) Annotated core description from the East Breaks 579#2 well, with number of laminations determined from core, estimated from logs and from V Clay content (sampling 2 samples /foot). (b) Calculated net:gross from laminations count. (c) Percent error in measurement of net sand calculated by comparing cores with log measurements. In the proximal facies, the percent net:gross was overestimated by up to 35 %. In the distal levee facies, the percent net:gross was underestimated by up to 70%. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-48.

(a) Annotated core description from the East Breaks 579#2 well, with number of laminations determined from core, estimated from logs and from V Clay content (sampling 2 samples /foot). (b) Calculated net:gross from laminations count. (c) Percent error in measurement of net sand calculated by comparing cores with log measurements. In the proximal facies, the percent net:gross was overestimated by up to 35 %. In the distal levee facies, the percent net:gross was underestimated by up to 70%. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Thickness and lateral continuity

The reservoir is on average 30 m (100 ft) thick (Figure 7-47). Based on production, the individual beds appear to extend across the entire amplitude as two wells effectively draining the reservoir.

Aspect ratio

This has not been published.

Vertical connectivity and net:gross values

Based on whole core data (EB 579#2 well), average net:gross is 58 %. In the proximal-levee facies, net:gross values are as high as 84%. Vertical connectivity at the individual bed scale is good in the proximal-medial levee facies, and minimal in the distal levee facies because of the thinly interbedded nature of the strata (Figures 7-47, 7-48). The Miocene Mt. Messenger Formation, described above, provides an outcrop analog to explain the lateral variation in vertical connectivity between proximal and distal levee beds (Figure 7-27).

Sedimentary texture, composition, and structures determined from core and borehole-image logs

Core in the EB-579 #2 well recovered poorly consolidated, well sorted fine- to very finegrained sands. Sands are highly laminated with climbing ripples. Four general facies were recognized: (1) Proximal–medial levee has amalgamated sand laminations grading upward into lower net:gross facies, consisting of alternating sand/shale laminations. Climbing ripples are common. Locally, flame structures, load structures and sparse burrowing are also present. (2) Distal levee has lower net:gross (20%) with interlaminated sands and shales. Sands are planar laminated and contain both climbing and current ripples, with sparse burrowing on tops of the beds. (3) Interchannel splay is characterized by net:gross of 50-60%. Localized zones of excellent quality amalgamated sand laminae reach a thickness of tens of meters. This facies is markedly different than the other two facies in that it is composed of a succession of upward thickening, upward coarsening sand-rich beds with sharp basal contacts, similar to the sequence shown in Figure 7-48. This facies overlies the fine-grained, basin-floor facies. (4) Fine-grained basin-floor sediments consist of shales and some silts that underlie and overlie the entire reservoir interval. Laminations, deformed beds, and large-scale bedforms are present in this facies.

Borehole image logs were collected in four wells, and help illustrate the internal architecture of the reservoir (Figures 7-40, 7-41). Once adjusted for regional structural dip, the image logs show that the levee sediments consistently dip away from the channel axis, and are perpendicular to it. Two stacked levee packages are present. The lower package averages 5.5 degrees of dip, varying from 5 to 8 degrees. The upper levee package has an average dip of 4.5 degrees, with values varying from 4 to 6 degrees. Dipmeter measurements of current ripples indicate that paleo-flow was sub-parallel to the channel, and moved in a down-channel direction.

Reservoir quality

Average porosity in the cored interval is 31.4% and average permeability is about 915 md, ranging from 0.06 to 6020 md. Based on capillary pressure analysis from whole core, Sw is calculated as 7-10%.

Drive mechanism

The drive mechanism is a weak water drive.

Summary: Lessons learned

  1. 1.

    Levee-overbank deposits constitute large volumes of the sediments in mud-rich and mixed-mud-sand deepwater systems. The sediments are derived from the overbanking of finer-grained portions of turbidity currents as they travel down an aggradational channel. Subenvironments include proximal and distal levees, crevasse splays, slides, and sediment waves.

  2. 2.

    In plan view, levee-overbank deposits parallel the trends of the channels and can have extensive areal distribution laterally away from the channel. Amplitude extractions of seafloor images suggest that sand-rich portions occur in the proximal-levee and in crevasse-splay deposits. Crevasse-splay deposits tend to be concentrated at the downcurrent end of a bend or meander loop in the channel. They have the potential to be good reservoirs.

  3. 3.

    On vertical seismic profiles, levees have a gull-wing appearance; that is, they have a double-wedge shape that thins away from the channel-fill sediments. Upon burial, levees lose their bathymetric expression when the mud-rich sediments differentially compact more than do the surrounding channel-fill deposits. It may be possible to use these criteria to estimate the sand content of channels; that is, the more sand there is in the channel, the greater will be its relief above the levees upon burial. Likewise, if the levees have bathymetric relief above the channel fill, then the levees might be more sand-prone than the fill.

  4. 4.

    Recognizing the effects of compaction on levee sediments is important when one is reconstructing the thickness of buried levee-overbank systems. Because levee-overbank sediments have a higher percentage of mud than do channel-fill sediments, levees are more susceptible to compaction. With increasing burial, levees will compact more than channels, to the point at which the levees and channels will appear equal in thickness, or they may even appear to have an inverted topography in which the channels appear to be elevated relative to the levees.

  5. 5.

    Levee sediments are primarily mud and silt with thin-bedded sands occurring throughout. Their appearance on gamma-ray logs is ratty and commonly fines and/or thins upward in association with one channel. They are frequently referred to as low-resistivity, low-contrast pay. Many pay zones probably have been overlooked because of their shaley appearance on conventional well logs. Also, levee beds that may not have been considered to be pay in volumetric calculations from conventional well logs may be reclassified when borehole-image logs or cores are obtained.

  6. 6.

    Distinct facies changes are present when we compare proximal-levee facies with distal-levee facies. Characteristic features in outcrops, cores, and borehole logs include the following. (a) Proximal facies consist of (i) a relatively high net-sand content and vertical connectivity, even though the sand beds are thin and beneath conventional well-log resolution; (ii) relatively high dip angles and variable dip directions, (iii) a somewhat coarser grain size than their distal counterparts; (iv) common Bouma Tb and Tc (climbing ripples) beds; and (v) small-scale, mud-lined scour surfaces on the tops of thin beds. (b) Distal facies consist of (i) a lower net-sand content and vertical connectivity; (ii) lower dip angles and more uniformity of dip direction; (iii) somewhat finer grain size than their proximal counterparts; (iv) Bouma Tb and Tc beds, which are perhaps slightly thinner than their proximal counterparts; and (v) fewer small-scale, mud-lined scour surfaces. These criteria can be used to reduce the uncertainty in interpreting proximal- versus distal-levee beds.

  7. 7.

    Thin-bedded reservoirs are found in many basins globally but have been documented for only a few fields. Production rates can be quite high during initial development, then decrease rapidly, and ultimately produce at a lower plateau for many years. Complex pressure and fluid distributions can occur in these reservoirs; however, in some, the continuity and connectivity appear to be relatively high.

  8. 8.

    Production from one horizontal well in one field indicates that large areas can be drained where net:gross values are fairly high. To date, this has proven to be most effective in high net:gross levees with gas reservoirs. Movement of oil in thin-bedded reservoirs is still unknown.

  9. 9.

    A potentially viable drilling scenario would be horizontal well that is within the higher-net sand proximal levee beds near the channel margin, and drilled parallel to that margin.

  10. 10.

    Limited production information suggests that bed continuity is greater in the shallower beds within a levee succession. As a channel begins to fill, a spill phase occurs with overbanking turbidity flows. In lower parts of a levee succession, good sands may be present; however, bed continuity is less the result of erosion of successive flows.

  11. 11.

    In some reservoirs, levee beds are not in pressure or fluid communication with adjacent channel-fill strata, even though the latter might be sandier. Outcrop observations suggest the channel margin is a complex, slumped zone that could comprise a barrier to fluid communication. Some thin-bed reservoirs appear to be connected to adjacent channels, while others appear not to be. The degree of fluid communication across this zone will depend upon the degree of complexity of the channel margin strata. When developing a drilling scenario, one should not assume that drilling a single vertical well into either the channel fill or into the levee will drain hydrocarbons from both facies. Instead, slanted or horizontal wells that penetrate both facies might prove more valuable.

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Figures & Tables

Figure 7-1.

Block diagram of a channel-levee system, illustrating the key subenvironments in the levee-overbank systems: proximal and distal levees, slides, crevasse splays, and sediment waves. Modified from Roberts and Compani (1996).

Figure 7-1.

Block diagram of a channel-levee system, illustrating the key subenvironments in the levee-overbank systems: proximal and distal levees, slides, crevasse splays, and sediment waves. Modified from Roberts and Compani (1996).

Figure 7-2.

(a) Map showing the distribution of late Pleistocene channels of the Amazon Fan, offshore Brazil. Also included are ODP Leg 155 sites (numbers in ellipses), and two prominent mass-transport deposits (shaded gray). Approximately 90% of the fan surface consists of levee-overbank sediments, with two slides originating from the slope. (b) Schematic cross section of the channel-levee systems of the Amazon Fan. Note the predominance of muddy sediment, as determined from cores, and the offset stacking patterns in the lev-eed-channels through time. Locations of the ODP sites are shown. After Piper et al. (1997).

Figure 7-2.

(a) Map showing the distribution of late Pleistocene channels of the Amazon Fan, offshore Brazil. Also included are ODP Leg 155 sites (numbers in ellipses), and two prominent mass-transport deposits (shaded gray). Approximately 90% of the fan surface consists of levee-overbank sediments, with two slides originating from the slope. (b) Schematic cross section of the channel-levee systems of the Amazon Fan. Note the predominance of muddy sediment, as determined from cores, and the offset stacking patterns in the lev-eed-channels through time. Locations of the ODP sites are shown. After Piper et al. (1997).

Figure 7-3.

False-color image derived from the GLORIA II side-scan sonar image of the Mississippi Fan surface. Youngest channel is shown by white line (labeled). Brighter colors correspond to layers that are interpreted to be sand-rich. Each depositional lobe (red and yellow colors) at the termini of the channels is interpreted to be a sand-rich area, in contrast to other portions of the fan surface. Blue areas represent finer-grained, overbank sediments. Note the linear distribution of sediment (yellow and green colors) in the overbank areas close to the youngest channel (outlined by dashed line). These areas may contain slightly coarser-grained material than does the surrounding overbank. After Wen et al. (1995). Reprinted with permission of Chapman-Hall and Neil Kenyon.

Figure 7-3.

False-color image derived from the GLORIA II side-scan sonar image of the Mississippi Fan surface. Youngest channel is shown by white line (labeled). Brighter colors correspond to layers that are interpreted to be sand-rich. Each depositional lobe (red and yellow colors) at the termini of the channels is interpreted to be a sand-rich area, in contrast to other portions of the fan surface. Blue areas represent finer-grained, overbank sediments. Note the linear distribution of sediment (yellow and green colors) in the overbank areas close to the youngest channel (outlined by dashed line). These areas may contain slightly coarser-grained material than does the surrounding overbank. After Wen et al. (1995). Reprinted with permission of Chapman-Hall and Neil Kenyon.

Figure 7-4.

Seismic-amplitude displays of channel and overbank settings, modern seafloor, Block 221, the Nigerian slope. (a) Detailed image of sinuous channel and overbank areas (Ov). Proximal levees consist of areas of higher amplitude (yellow). Possible sediment waves are also present (Sw). Channel fill consists of high amplitudes (Ha). Scours associated with slides (Sc), fault traces (Ft) and mud volcanoes (MV) are present. Sediment waves trend subperpendicular to the channel and subparallel to the trend of the levees. The sediment waves are bottom current indicators. Slides form randomly in the levee-overbank areas. (b) Detail of the same channel as in (a), but this time 40 km downdip. Channel has a slightly braided appearance. Overbank areas consist of high amplitudes, suggesting that coarser-grained sediments spilled into the area. Areas are about 3 km in width on either side of the channel. Arcuate slide scars (Sa) are present. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-4.

Seismic-amplitude displays of channel and overbank settings, modern seafloor, Block 221, the Nigerian slope. (a) Detailed image of sinuous channel and overbank areas (Ov). Proximal levees consist of areas of higher amplitude (yellow). Possible sediment waves are also present (Sw). Channel fill consists of high amplitudes (Ha). Scours associated with slides (Sc), fault traces (Ft) and mud volcanoes (MV) are present. Sediment waves trend subperpendicular to the channel and subparallel to the trend of the levees. The sediment waves are bottom current indicators. Slides form randomly in the levee-overbank areas. (b) Detail of the same channel as in (a), but this time 40 km downdip. Channel has a slightly braided appearance. Overbank areas consist of high amplitudes, suggesting that coarser-grained sediments spilled into the area. Areas are about 3 km in width on either side of the channel. Arcuate slide scars (Sa) are present. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-5.

An rms amplitude extraction map 170 ms below the seafloor in one intraslope miniba-sin in the Brunei continental slope. The upslope channel passes through shale ridges to an elongate, sheet deposit (depositional lobe). Finer-grained slope and overbank areas are indicated by the blue color. Note the possible coarser-grained levee sediments (orange color) next to the channel and the prominent crevasse-splay deposit (orange) next to the sinuous channel. After Demyttenaere et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-5.

An rms amplitude extraction map 170 ms below the seafloor in one intraslope miniba-sin in the Brunei continental slope. The upslope channel passes through shale ridges to an elongate, sheet deposit (depositional lobe). Finer-grained slope and overbank areas are indicated by the blue color. Note the possible coarser-grained levee sediments (orange color) next to the channel and the prominent crevasse-splay deposit (orange) next to the sinuous channel. After Demyttenaere et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-6.

Amplitude extraction maps and interpreted line drawings of upper Pleistocene (a) and (b) crevasse splays and (c) overbank splays, northern deep Gulf of Mexico. Overbank splays are similar to crevasse splays but differ in size and the absence of levee deposits associated with them. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Figure 7-6.

Amplitude extraction maps and interpreted line drawings of upper Pleistocene (a) and (b) crevasse splays and (c) overbank splays, northern deep Gulf of Mexico. Overbank splays are similar to crevasse splays but differ in size and the absence of levee deposits associated with them. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Figure 7-7.

(a) Dip attribute map 48 ms below the seafloor of a late Pleistocene channel-overbank system, Makassar Straits, eastern Borneo. Overbank splays and sediment waves are present. (b) and (c) Seismic profiles across the channel-levee system. Note the prominent sediment waves at the top of the levee. Locations of these seismic profiles are shown in (a). After Posamentier et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-7.

(a) Dip attribute map 48 ms below the seafloor of a late Pleistocene channel-overbank system, Makassar Straits, eastern Borneo. Overbank splays and sediment waves are present. (b) and (c) Seismic profiles across the channel-levee system. Note the prominent sediment waves at the top of the levee. Locations of these seismic profiles are shown in (a). After Posamentier et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-8.

(a) 3D perspective of a late Pleistocene channel, northeastern deep Gulf of Mexico. Image is derived from 3D seismic data and illuminated from the west. View is to the north. Florida Escarpment forms the vertical image to the east. Note the overall elevation in the channel-levee system, sediment waves in the eastern levee, oblique to the overall trend of the channels, and downfan bifurcation of the channel. Location of the enlargement in (b) is shown. (b) Enlargement of the channel in (a), illustrating the elevated levees and channel. Location of the profile in (c) is shown. (c) Seismic profile across the channel-levee system. High-amplitude reflections (channel fill) migrate to the right (east) and aggrade. At the surface, the channel is 625 m wide, and the levees have 6–7 m of depositional relief. See (b) for location of the profile. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Figure 7-8.

(a) 3D perspective of a late Pleistocene channel, northeastern deep Gulf of Mexico. Image is derived from 3D seismic data and illuminated from the west. View is to the north. Florida Escarpment forms the vertical image to the east. Note the overall elevation in the channel-levee system, sediment waves in the eastern levee, oblique to the overall trend of the channels, and downfan bifurcation of the channel. Location of the enlargement in (b) is shown. (b) Enlargement of the channel in (a), illustrating the elevated levees and channel. Location of the profile in (c) is shown. (c) Seismic profile across the channel-levee system. High-amplitude reflections (channel fill) migrate to the right (east) and aggrade. At the surface, the channel is 625 m wide, and the levees have 6–7 m of depositional relief. See (b) for location of the profile. After Posamentier and Kolla (2003). Reprinted with permission of SEPM.

Figure 7-9.

Four seismic profiles across one intraslope minibasin, northern deep Gulf of Mexico, illustrating the downslope changes in geometries in one channel and the associated levees from: (a) an erosional channel with slight bathymetric relief on the levees, to (b) and (c) strongly aggra-dational levees with prominent bathymetric relief, to (d) erosional channels with levees that have little relief. (e) Inset map shows locations of the seismic profiles. After Badalini et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-9.

Four seismic profiles across one intraslope minibasin, northern deep Gulf of Mexico, illustrating the downslope changes in geometries in one channel and the associated levees from: (a) an erosional channel with slight bathymetric relief on the levees, to (b) and (c) strongly aggra-dational levees with prominent bathymetric relief, to (d) erosional channels with levees that have little relief. (e) Inset map shows locations of the seismic profiles. After Badalini et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-10.

(a) Seismic profile along the right (southern) side of one levee crest adjacent to a late Pleistocene sinuous channel, Makassar Straits, eastern Borneo. The profile is flattened on the top of the levee crest. Levee reflections are low amplitude and discontinuous. Overall, the levee thickness decreases downfan. Note that the thickness of the levee sediments is greater along the outer channel bend and thinner along the inner bends. (b) Azimuth dip map of the sinuous channel, showing the location of the seismic profile. After Posamentier and Kolla (2003). Reprinted with permission of the SEPM.

Figure 7-10.

(a) Seismic profile along the right (southern) side of one levee crest adjacent to a late Pleistocene sinuous channel, Makassar Straits, eastern Borneo. The profile is flattened on the top of the levee crest. Levee reflections are low amplitude and discontinuous. Overall, the levee thickness decreases downfan. Note that the thickness of the levee sediments is greater along the outer channel bend and thinner along the inner bends. (b) Azimuth dip map of the sinuous channel, showing the location of the seismic profile. After Posamentier and Kolla (2003). Reprinted with permission of the SEPM.

Figure 7-11.

(a) Seismic profile across an intraslope basin, offshore Angola. The sequence is composed of a mass-transport deposit (labeled “sand debrite”) at the base, consisting of low-amplitude, chaotic, mounded, and hummocky reflections, overlain by amalgamated channelized and channel-levee systems. Levee reflections are low amplitude, with variable continuity, and lap out against the side of the basin. (b) Wireline logs through a channel-levee system, Angola. Thin bedded sands are deposited near the top of the sequence associated with the leveed channel. After Sikkema and Wojcik (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-11.

(a) Seismic profile across an intraslope basin, offshore Angola. The sequence is composed of a mass-transport deposit (labeled “sand debrite”) at the base, consisting of low-amplitude, chaotic, mounded, and hummocky reflections, overlain by amalgamated channelized and channel-levee systems. Levee reflections are low amplitude, with variable continuity, and lap out against the side of the basin. (b) Wireline logs through a channel-levee system, Angola. Thin bedded sands are deposited near the top of the sequence associated with the leveed channel. After Sikkema and Wojcik (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-12.

(a) Seismic profile across the Ram-Powell field, northern deep Gulf of Mexico. A high-amplitude reflection associated with the L sand reservoir is shown and is interpreted to be caused by gas content. See Figure 7-19 for location of the profile. After Clemenceau et al. (2000). (b) Detailed seismic profile illustrating the reflection associated with the L sand reservoir. A possible low-amplitude reflection associated with channel-fill sediments is to the west (left) of the levee reservoir. After Kendrick (2000). Both profiles are reprinted with permission of Gulf Coast Section SEPM Foundation.

Figure 7-12.

(a) Seismic profile across the Ram-Powell field, northern deep Gulf of Mexico. A high-amplitude reflection associated with the L sand reservoir is shown and is interpreted to be caused by gas content. See Figure 7-19 for location of the profile. After Clemenceau et al. (2000). (b) Detailed seismic profile illustrating the reflection associated with the L sand reservoir. A possible low-amplitude reflection associated with channel-fill sediments is to the west (left) of the levee reservoir. After Kendrick (2000). Both profiles are reprinted with permission of Gulf Coast Section SEPM Foundation.

Figure 7-13.

(a) Wireline-log cross section across the Ram-Powell L sand reservoir. Proximal-levee reservoirs consist of 100 ft (30 m) of 60% net:gross sands, whereas the distal levee, 3 mi (5 km) to the east (right), has a net:gross value of 27%. Note the overall upward fining and thinning in both the proximal- and distal-levee sediments. Channel-fill sands are water wet. (b) Dipmeter logs for proximal- and distal-levee facies, Ram-Powell field, showing the two styles of dip patterns. (c) Photograph (normal and UV light) of cores through proximal- and distal-levee facies, showing the higher net sand and greater dip angles in the proximal-levee facies than in the distal-levee beds. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-13.

(a) Wireline-log cross section across the Ram-Powell L sand reservoir. Proximal-levee reservoirs consist of 100 ft (30 m) of 60% net:gross sands, whereas the distal levee, 3 mi (5 km) to the east (right), has a net:gross value of 27%. Note the overall upward fining and thinning in both the proximal- and distal-levee sediments. Channel-fill sands are water wet. (b) Dipmeter logs for proximal- and distal-levee facies, Ram-Powell field, showing the two styles of dip patterns. (c) Photograph (normal and UV light) of cores through proximal- and distal-levee facies, showing the higher net sand and greater dip angles in the proximal-levee facies than in the distal-levee beds. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-14.

(a) Seismic profile across a modern channel-levee system, northern deep Gulf of Mexico, illustrating elevated channel-fill sediments in contrast to the surrounding levee deposits. Inversion of depositional topography has occurred in a short amount of time. Green arrows indicate areas of the greatest amount of compaction in the levee, and yellow arrows indicate areas of less compaction (channel-fill sediments). (b) Schematic profile showing the decompacted sediments in profile in (a). Reprinted with permission of Henry Posamentier.

Figure 7-14.

(a) Seismic profile across a modern channel-levee system, northern deep Gulf of Mexico, illustrating elevated channel-fill sediments in contrast to the surrounding levee deposits. Inversion of depositional topography has occurred in a short amount of time. Green arrows indicate areas of the greatest amount of compaction in the levee, and yellow arrows indicate areas of less compaction (channel-fill sediments). (b) Schematic profile showing the decompacted sediments in profile in (a). Reprinted with permission of Henry Posamentier.

Figure 7-15.

Seismic profile across an upper Pleistocene sequence (ca. 0.5 Ma) in the Mississippi Fan, northern deep Gulf of Mexico. Key elements are a mass-transport complex (MTC) at the base (see Chapter 9), overlain by channel-fill and levee-overbank sediments. The top of the MTC is an irregular surface, and it has been eroded into several channels (high-amplitude reflections). Note the vertical change in the amplitude of the levee reflections to the left (west), changing from high amplitude at the base (HAMP) to low amplitude at the top (LAMP). This vertical change is interpreted to be caused by a decrease in the grain size of the sediments. After Weimer (1990). Reprinted with permission of AAPG.

Figure 7-15.

Seismic profile across an upper Pleistocene sequence (ca. 0.5 Ma) in the Mississippi Fan, northern deep Gulf of Mexico. Key elements are a mass-transport complex (MTC) at the base (see Chapter 9), overlain by channel-fill and levee-overbank sediments. The top of the MTC is an irregular surface, and it has been eroded into several channels (high-amplitude reflections). Note the vertical change in the amplitude of the levee reflections to the left (west), changing from high amplitude at the base (HAMP) to low amplitude at the top (LAMP). This vertical change is interpreted to be caused by a decrease in the grain size of the sediments. After Weimer (1990). Reprinted with permission of AAPG.

Figure 7-16.

Schematic cross section across a channel-levee system, and the corresponding gamma-ray or SP logs. Note the vertical decrease in the grain sizes of the levee sediments. This figure illustrates two scales of levees: those outside of the main or master channel (shown in green and orange) and those within the master channel and associated with the smaller channels (shown in yellow). Modified from Beaubouef (2004).

Figure 7-16.

Schematic cross section across a channel-levee system, and the corresponding gamma-ray or SP logs. Note the vertical decrease in the grain sizes of the levee sediments. This figure illustrates two scales of levees: those outside of the main or master channel (shown in green and orange) and those within the master channel and associated with the smaller channels (shown in yellow). Modified from Beaubouef (2004).

Figure 7-17.

(a) An rms amplitude extraction map (20-ms gated window) of a channel-levee system, with a prominent crevasse splay, Miocene deposits, offshore Angola. The crevasse is fan shaped and resulted from an avulsion in the channel. (b) Inset seismic profile that shows the interval from which the attribute was extracted. See (a) for location of the profile. After Mayall and Stewart (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-17.

(a) An rms amplitude extraction map (20-ms gated window) of a channel-levee system, with a prominent crevasse splay, Miocene deposits, offshore Angola. The crevasse is fan shaped and resulted from an avulsion in the channel. (b) Inset seismic profile that shows the interval from which the attribute was extracted. See (a) for location of the profile. After Mayall and Stewart (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-18.

Amplitude extraction map of the Ram-Powell L sand interval superposed on a top L sand structure map, northern deep Gulf of Mexico. Producing levee sands correspond to areas of high amplitude, as a result of the effects of gas in the reservoir. Note that the adjacent channel (outlined by turquoise lines) has low amplitude and no pay. Location of Figure 7-13a is shown. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-18.

Amplitude extraction map of the Ram-Powell L sand interval superposed on a top L sand structure map, northern deep Gulf of Mexico. Producing levee sands correspond to areas of high amplitude, as a result of the effects of gas in the reservoir. Note that the adjacent channel (outlined by turquoise lines) has low amplitude and no pay. Location of Figure 7-13a is shown. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-19.

(a) Seismic profile across the Einstein channel, northern deep Gulf of Mexico. Four seismic facies are present in the levee facies. Note the vertical change in seismic facies from high amplitude at the base of the levee to high continuity, low amplitude at the top. (b) Wireline-log cross section across the Einstein channel. Logs that penetrated the levee facies are dominantly clays and some interbedded silts. Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-19.

(a) Seismic profile across the Einstein channel, northern deep Gulf of Mexico. Four seismic facies are present in the levee facies. Note the vertical change in seismic facies from high amplitude at the base of the levee to high continuity, low amplitude at the top. (b) Wireline-log cross section across the Einstein channel. Logs that penetrated the levee facies are dominantly clays and some interbedded silts. Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-19.

(c) Seismic profile with superposed time-based wireline logs with lithologies plotted as determined from the cores. Note the vertical decrease in grain size in the levees in borehole D. After Hackbarth and Shew (1994). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-19.

(c) Seismic profile with superposed time-based wireline logs with lithologies plotted as determined from the cores. Note the vertical decrease in grain size in the levees in borehole D. After Hackbarth and Shew (1994). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-20.

(a) Shaded bathymetric map of the highly sinuous channel, middle fan of the Amazon. Locations of ODP site 936 and seismic profile in (b) are shown. (b) Seismic profile across the channel-levee system. Note the extreme vertical exaggeration of the profile. Time-based gamma-ray log from ODP site 936 is shown. Wireline log indicates that the levee sediments are primarily clay, with some silt. See Figure 7-21 for detailed lithology from cores of site 936, and two additional levee sites (935, 940). After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-20.

(a) Shaded bathymetric map of the highly sinuous channel, middle fan of the Amazon. Locations of ODP site 936 and seismic profile in (b) are shown. (b) Seismic profile across the channel-levee system. Note the extreme vertical exaggeration of the profile. Time-based gamma-ray log from ODP site 936 is shown. Wireline log indicates that the levee sediments are primarily clay, with some silt. See Figure 7-21 for detailed lithology from cores of site 936, and two additional levee sites (935, 940). After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-21.

Grain-size logs based on cores from ODP sites 940, 936, and 935 through the Amazon Fan. Each site cored levee sediments and recovered primarily clays with some silt; the sand fraction is extremely small. The thick sand fraction in site 936, at 70–90 m, is through a HARP (high-amplitude reflection package) crevasse-splay package. See Figure 7-20 for seismic expression and time-based gamma-ray log of site 936. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-21.

Grain-size logs based on cores from ODP sites 940, 936, and 935 through the Amazon Fan. Each site cored levee sediments and recovered primarily clays with some silt; the sand fraction is extremely small. The thick sand fraction in site 936, at 70–90 m, is through a HARP (high-amplitude reflection package) crevasse-splay package. See Figure 7-20 for seismic expression and time-based gamma-ray log of site 936. After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-22.

Total sand map of the Ram Powell “L” sand interval, northern deep Gulf of Mexico. Total thickness of sands decreases from the proximal to the distal levee, from more than 100 ft (30 m) to 20 ft (6 m) across a distance of 3 mi (5 km). The net:gross value also decreases from 60 to 27%. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-22.

Total sand map of the Ram Powell “L” sand interval, northern deep Gulf of Mexico. Total thickness of sands decreases from the proximal to the distal levee, from more than 100 ft (30 m) to 20 ft (6 m) across a distance of 3 mi (5 km). The net:gross value also decreases from 60 to 27%. After Clemenceau et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-23.

Wireline logs and a core photograph of the M4.1 sand reservoir, Tahoe field, northern deep Gulf of Mexico. Gamma-ray log indicates thin interbeds of mud and sand, with an overall vertical decrease in grain size. Core photograph shows representative producing facies, consisting of very fine-grained sands, planar to ripple laminated (0–2 in. or 0.8 cm thick), with interbedded shales. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-23.

Wireline logs and a core photograph of the M4.1 sand reservoir, Tahoe field, northern deep Gulf of Mexico. Gamma-ray log indicates thin interbeds of mud and sand, with an overall vertical decrease in grain size. Core photograph shows representative producing facies, consisting of very fine-grained sands, planar to ripple laminated (0–2 in. or 0.8 cm thick), with interbedded shales. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-24.

Seismic profile across a late Pleistocene Mississippi Fan sequence (ca. 0.4 Ma). (a) uninterpreted profile, (b) interpreted profile. Note that the high-amplitude reflections (HAR), interpreted as channel-fill sands, migrate to the north (left) throughout the evolution of this sequence. Note also that the adjacent low-amplitude levee reflections migrate to the north, such that younger levee deposits overlie the older channel-fill deposits. Salt tongues deformed the sequence after its deposition. After Weimer (1989).

Figure 7-24.

Seismic profile across a late Pleistocene Mississippi Fan sequence (ca. 0.4 Ma). (a) uninterpreted profile, (b) interpreted profile. Note that the high-amplitude reflections (HAR), interpreted as channel-fill sands, migrate to the north (left) throughout the evolution of this sequence. Note also that the adjacent low-amplitude levee reflections migrate to the north, such that younger levee deposits overlie the older channel-fill deposits. Salt tongues deformed the sequence after its deposition. After Weimer (1989).

Figure 7-25.

Map showing location of outcrops of levee-overbank thin beds (orange dots) and producing fields (yellow dots) discussed in the text. The numbered outcrops correspond to those numbered in Table 7-1 .

Figure 7-25.

Map showing location of outcrops of levee-overbank thin beds (orange dots) and producing fields (yellow dots) discussed in the text. The numbered outcrops correspond to those numbered in Table 7-1 .

Figure 7-26.

Part of the main outcrop of the Mount Messenger Formation strata discussed in the text: (a) outcrop photograph and (b) line drawing interpretation of the outcrop. Note the channel downcutting through proximal-levee facies (Figure 7-28) and the fining-upward nature of the proximal-levee intervals. After Browne and Slatt (2002). Reprinted with permission of AAPG.

Figure 7-26.

Part of the main outcrop of the Mount Messenger Formation strata discussed in the text: (a) outcrop photograph and (b) line drawing interpretation of the outcrop. Note the channel downcutting through proximal-levee facies (Figure 7-28) and the fining-upward nature of the proximal-levee intervals. After Browne and Slatt (2002). Reprinted with permission of AAPG.

Figure 7-27.

(a) Outcrop gamma-ray log (obtained with a handheld scintillometer) of 600+ m of Mount Messenger strata exposed on the Pukearuhe Beach, New Zealand. The lower 150 m comprises basin-floor fan strata, and the upper 450 m comprises mainly levee deposits with associated channel fill (modified from Diridoni, 1996). (b) Pukearuhe #1 exploration well, located approximately 1 km from the Pukearuhe Beach section and downdip of the beach, so that the same strata in outcrop and on the outcrop gamma-ray log occur on the subsurface well log (modified from Browne and Slatt, 2002). Note that a and b are shown at the same scale. (c) Outcrop gamma-ray log obtained with a logging truck across thin-bedded facies on Pukearuhe Beach (photograph compliments of D. Jordan). Reprinted with permission of AAPG.

Figure 7-27.

(a) Outcrop gamma-ray log (obtained with a handheld scintillometer) of 600+ m of Mount Messenger strata exposed on the Pukearuhe Beach, New Zealand. The lower 150 m comprises basin-floor fan strata, and the upper 450 m comprises mainly levee deposits with associated channel fill (modified from Diridoni, 1996). (b) Pukearuhe #1 exploration well, located approximately 1 km from the Pukearuhe Beach section and downdip of the beach, so that the same strata in outcrop and on the outcrop gamma-ray log occur on the subsurface well log (modified from Browne and Slatt, 2002). Note that a and b are shown at the same scale. (c) Outcrop gamma-ray log obtained with a logging truck across thin-bedded facies on Pukearuhe Beach (photograph compliments of D. Jordan). Reprinted with permission of AAPG.

Figure 7-28.

(a) Dipmeter logs from shallow boreholes obtained from behind the Pukearuhe Beach outcrop, showing the three characteristic dip patterns for fine-grained channel-fill, proximal-, and distal-levee facies. (b) Outcrop expression of the left-hand edge of the channel fill (Figure 7-26a) onlapping the complex channel margin. To the right is the proximal-levee facies. Note the variable dip angles and orientations of the thin beds in the proximal-levee deposit. (c) Outcrop expression of distal-levee beds. Shown are six levee packages, each with slightly different dip magnitudes and orientations. Note the shallower and more uniform dips compared with the proximal-levee facies in (b). (Modified from Browne and Slatt, 2002).

Figure 7-28.

(a) Dipmeter logs from shallow boreholes obtained from behind the Pukearuhe Beach outcrop, showing the three characteristic dip patterns for fine-grained channel-fill, proximal-, and distal-levee facies. (b) Outcrop expression of the left-hand edge of the channel fill (Figure 7-26a) onlapping the complex channel margin. To the right is the proximal-levee facies. Note the variable dip angles and orientations of the thin beds in the proximal-levee deposit. (c) Outcrop expression of distal-levee beds. Shown are six levee packages, each with slightly different dip magnitudes and orientations. Note the shallower and more uniform dips compared with the proximal-levee facies in (b). (Modified from Browne and Slatt, 2002).

Figure 7-29.

(a) Portion of a borehole-image log from the Central borehole, Pukearuhe Beach. Particularly significant is the change in dip at about 42.3 m, which corresponds to the base of a channel fill. (Photograph provided by J. Coleman, 2002). (b) Core of the base of channel fill illustrated on the image log, showing a basal shale clast conglomerate underlain by more shallowly dipping thin levee beds. (c) Outcrop photograph showing erosional scour surface cutting into horizontal beds; the erosional surface is overlain by a mud drape (darker color). (d) Subtle erosional scour on top of a sand bed (light color) at 0.77-m depth on the image log.

Figure 7-29.

(a) Portion of a borehole-image log from the Central borehole, Pukearuhe Beach. Particularly significant is the change in dip at about 42.3 m, which corresponds to the base of a channel fill. (Photograph provided by J. Coleman, 2002). (b) Core of the base of channel fill illustrated on the image log, showing a basal shale clast conglomerate underlain by more shallowly dipping thin levee beds. (c) Outcrop photograph showing erosional scour surface cutting into horizontal beds; the erosional surface is overlain by a mud drape (darker color). (d) Subtle erosional scour on top of a sand bed (light color) at 0.77-m depth on the image log.

Figure 7-30.

High frequency (150 Hz) seismic reflection profile acquired along the beach in front of the cliff face as shown in Figure 7-26. The profile was processed using Thin MANTM, which removes the seismic wavelet and broadens the spectrum robustly without introducing noise. The resulting profile allows for the resolution of stratigraphic detail that is not apparent with the lower frequency seismic wavelet. Many stratal surfaces are recognized. The two intervals bounded by blue lines are interpreted as sheet sandstones. They occur along strike and down the structural dip from sheet sandstones that occur in outcrop farther to the north (Chapter 8; Figure 8-26). Black lines are interpreted as channel bases. Several of the channels appear to be filled with beds that exhibit downlapping, lateral accretion features (Chapter 6). Blue reflections are positive and red reflections are negative amplitude.

Figure 7-30.

High frequency (150 Hz) seismic reflection profile acquired along the beach in front of the cliff face as shown in Figure 7-26. The profile was processed using Thin MANTM, which removes the seismic wavelet and broadens the spectrum robustly without introducing noise. The resulting profile allows for the resolution of stratigraphic detail that is not apparent with the lower frequency seismic wavelet. Many stratal surfaces are recognized. The two intervals bounded by blue lines are interpreted as sheet sandstones. They occur along strike and down the structural dip from sheet sandstones that occur in outcrop farther to the north (Chapter 8; Figure 8-26). Black lines are interpreted as channel bases. Several of the channels appear to be filled with beds that exhibit downlapping, lateral accretion features (Chapter 6). Blue reflections are positive and red reflections are negative amplitude.

Figure 7-31.

Conventional well logs of borehole (CSM Strat Test #61) drilled behind the Spine 1 Lewis Shale outcrop. The gray-pink core gamma scan shows the intervals that were continuously cored. Locations of cores shown in Figure 7-31 are provided. (Logs provided by S. Goolsby, 2002).

Figure 7-31.

Conventional well logs of borehole (CSM Strat Test #61) drilled behind the Spine 1 Lewis Shale outcrop. The gray-pink core gamma scan shows the intervals that were continuously cored. Locations of cores shown in Figure 7-31 are provided. (Logs provided by S. Goolsby, 2002).

Figures 7-32.

Core photographs from the CSM Strat Test #61 well. Stratigraphic positions of the core are shown in Figure 7-30. (a) The upper 2 m (6 ft) of core is composed of sandstone (light colored) with abundant shale rip-up clasts, convoluted beds, burrowed beds, and rippled beds. The sandstones are interpreted to be a basal channel-fill interval. The lower part of the core and its continuation in core (b) are shaley and do not contain many thin sandstone beds. The gamma-ray response of this shaley interval is quite high (Figure 7-30), suggesting that it might be an organic-rich condensed section (similar condensed sections occur in outcrops). (c) High-net-sand interval of thin beds that appear shaley on the gamma-ray log (Figure 7-30).

Figures 7-32.

Core photographs from the CSM Strat Test #61 well. Stratigraphic positions of the core are shown in Figure 7-30. (a) The upper 2 m (6 ft) of core is composed of sandstone (light colored) with abundant shale rip-up clasts, convoluted beds, burrowed beds, and rippled beds. The sandstones are interpreted to be a basal channel-fill interval. The lower part of the core and its continuation in core (b) are shaley and do not contain many thin sandstone beds. The gamma-ray response of this shaley interval is quite high (Figure 7-30), suggesting that it might be an organic-rich condensed section (similar condensed sections occur in outcrops). (c) High-net-sand interval of thin beds that appear shaley on the gamma-ray log (Figure 7-30).

Figure 7-33.

(a) Photograph of channel-fill sandstone and adjacent proximal and distal-levee beds on the Spine 1 outcrop of the Dad sandstone. Two sets of channel fill and levees are present. The lower figures -are close-ups showing (b) the low angle and uniform dips of the distal-levee beds and (c) the higher dip angle and cut-and-fill features of the proximal-levee beds.

Figure 7-33.

(a) Photograph of channel-fill sandstone and adjacent proximal and distal-levee beds on the Spine 1 outcrop of the Dad sandstone. Two sets of channel fill and levees are present. The lower figures -are close-ups showing (b) the low angle and uniform dips of the distal-levee beds and (c) the higher dip angle and cut-and-fill features of the proximal-levee beds.

Figure 7-34.

(a) Photograph of a wave-cut bench at low tide, exposing thin beds of the Miocene Whakataki Formation, Whakataki Beach, North Island, New Zealand. (b) Aerial photograph of the beach with pseudo gamma ray log derived from hand-held scintillometer overlain. Three lithofacies are delineated: A-C. Select bed numbers (S1 and S 20) are noted from Edbrooke and Browne (1996). The main picture (a) shows a lateral view of the thin beds along the bench, for a distance of several hundred meters. Sandstone beds are light colored and composed of repetitive Bouma Tb-Tc intervals separated by thin, dark shales. After Field (2005). Reproduced with permission of Brad Field. (c) Photograph of one bed, showing Bouma Tb (parallel-lamination) and Tc (climbing-ripple) sandstone beds.

Figure 7-34.

(a) Photograph of a wave-cut bench at low tide, exposing thin beds of the Miocene Whakataki Formation, Whakataki Beach, North Island, New Zealand. (b) Aerial photograph of the beach with pseudo gamma ray log derived from hand-held scintillometer overlain. Three lithofacies are delineated: A-C. Select bed numbers (S1 and S 20) are noted from Edbrooke and Browne (1996). The main picture (a) shows a lateral view of the thin beds along the bench, for a distance of several hundred meters. Sandstone beds are light colored and composed of repetitive Bouma Tb-Tc intervals separated by thin, dark shales. After Field (2005). Reproduced with permission of Brad Field. (c) Photograph of one bed, showing Bouma Tb (parallel-lamination) and Tc (climbing-ripple) sandstone beds.

Figure 7-35.

Graph of bed thickness versus bed numbers (all sandstone and mudstone beds are shown). Lithofacies are shown: A (oldest), B, and C (youngest). After Field (2005). Reproduced with permission of Brad Field.

Figure 7-35.

Graph of bed thickness versus bed numbers (all sandstone and mudstone beds are shown). Lithofacies are shown: A (oldest), B, and C (youngest). After Field (2005). Reproduced with permission of Brad Field.

Figure 7-36.

Graph showing net:gross values for the interval studied as a 50-bed moving average. After Field (2005). Lithofacies associations A (oldest), B, and C (youngest) are delineated. Overall net:gross is 74% (dashed line). Reproduced with permission of Brad Field.

Figure 7-36.

Graph showing net:gross values for the interval studied as a 50-bed moving average. After Field (2005). Lithofacies associations A (oldest), B, and C (youngest) are delineated. Overall net:gross is 74% (dashed line). Reproduced with permission of Brad Field.

Figure 7-37.

Measurements of thin beds shown in Figure 7-34 for a distance of 500 m. (a) Percentage of beds that are continuous as a function of distance for the 500-m measured interval of thin beds. Approximately 80% of the thin beds are continuous for the 500-m distance. (b) Line drawing of the length of the bench that was measured, the stratification, and the location and spacing of the measured sections that form the basis for the graph in (a). Modified from Edbrooke and Browne (1996).

Figure 7-37.

Measurements of thin beds shown in Figure 7-34 for a distance of 500 m. (a) Percentage of beds that are continuous as a function of distance for the 500-m measured interval of thin beds. Approximately 80% of the thin beds are continuous for the 500-m distance. (b) Line drawing of the length of the bench that was measured, the stratification, and the location and spacing of the measured sections that form the basis for the graph in (a). Modified from Edbrooke and Browne (1996).

Figure 7-38.

Borehole image log from Titihaoa-1 well showing thin beds equivalent to those exposed at the Whakataki Beach. After Field (2005). Reproduced with permission of Brad Field.

Figure 7-38.

Borehole image log from Titihaoa-1 well showing thin beds equivalent to those exposed at the Whakataki Beach. After Field (2005). Reproduced with permission of Brad Field.

Figure 7-39.

Flattened seismic profile across the Titihaoa-1 well, offshore eastern New Zealand. Interval penetrated by the borehole image log is shown, and corresponds to the slightly hum-mocky reflections of the levees, and adjacent channel-fill deposits. After Field (2005). Reproduced with permission of Brad Field.

Figure 7-39.

Flattened seismic profile across the Titihaoa-1 well, offshore eastern New Zealand. Interval penetrated by the borehole image log is shown, and corresponds to the slightly hum-mocky reflections of the levees, and adjacent channel-fill deposits. After Field (2005). Reproduced with permission of Brad Field.

Figure 7-40.

Percentage of original thickness of beds for a distance of 167 m (500 ft) in thin beds of the Cerro Toro Formation, Chile. Bed measurements are subdivided into to beds that are greater than and those that are less than 5 cm (2 in) (modified from Devries and Lindholm 1994).

Figure 7-40.

Percentage of original thickness of beds for a distance of 167 m (500 ft) in thin beds of the Cerro Toro Formation, Chile. Bed measurements are subdivided into to beds that are greater than and those that are less than 5 cm (2 in) (modified from Devries and Lindholm 1994).

Figure 7-41.

Photograph of outcrop of the Jackfork Sandstone at McCain Mall in Little Rock (south central Arkansas) showing a lower interval of lenticular, amalgamated channel sandstones, overlain by a thinning-upward interval of levee beds, then by a thickening-upward interval interpreted as a crevasse splay. Mr. Charlie Stone for scale. See Slatt et al (2000) for discussion of section.

Figure 7-41.

Photograph of outcrop of the Jackfork Sandstone at McCain Mall in Little Rock (south central Arkansas) showing a lower interval of lenticular, amalgamated channel sandstones, overlain by a thinning-upward interval of levee beds, then by a thickening-upward interval interpreted as a crevasse splay. Mr. Charlie Stone for scale. See Slatt et al (2000) for discussion of section.

Figure 7-42.

Well paths of the VK A-1STB2 horizontal well and the appraisal well VK 912-2 through the Ram-Powell L sand, northern deep Gulf of Mexico. After Bramlett and Craig (2002). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-42.

Well paths of the VK A-1STB2 horizontal well and the appraisal well VK 912-2 through the Ram-Powell L sand, northern deep Gulf of Mexico. After Bramlett and Craig (2002). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-43.

Depth-based seismic profile across the Tahoe field, northern deep Gulf of Mexico. The producing interval (M4.1) corresponds to the high-amplitude reflection labeled levee-splay. Time-based log shows grain sizes of the interval. Note also the channel-fill reflection. See Figure 7-38b for location of the profile. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-43.

Depth-based seismic profile across the Tahoe field, northern deep Gulf of Mexico. The producing interval (M4.1) corresponds to the high-amplitude reflection labeled levee-splay. Time-based log shows grain sizes of the interval. Note also the channel-fill reflection. See Figure 7-38b for location of the profile. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-44.

(a) Structure contour map of the top M4.1 reservoir level, Tahoe field, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown. Note that the channel (lavender) segments the east from the west levee reservoirs.

Figure 7-44.

(a) Structure contour map of the top M4.1 reservoir level, Tahoe field, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown. Note that the channel (lavender) segments the east from the west levee reservoirs.

Figure 7-44.

(b) Amplitude extraction map of the M4.1 reservoir level, superposed with structure contour and faults. Distribution of the gas reservoir is indicated by the hotter colors (reds, purples, orange). Location of Figure 7-37 is shown. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-44.

(b) Amplitude extraction map of the M4.1 reservoir level, superposed with structure contour and faults. Distribution of the gas reservoir is indicated by the hotter colors (reds, purples, orange). Location of Figure 7-37 is shown. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-45.

(a) Pressure profiles from the M4.1 sand, Tahoe field, northern deep Gulf of Mexico: before (left) and after (right) production. Before production, pressure indicated that the east and west levees had similar formation pressures. Two years after production, differential depletion occurred in an offsetting well in the west levee reservoir. In contrast, the east levee had partial depletion in the upper levee but none in the lower levee. (b) Schematic cross section across the M4.1 reservoir level, showing the pattern of depletion in the two levees. Blue zones indicate where the pressure in the thin beds had depleted by early production in the 783-4ST2 well. Blue hachured zones represent zones of partial completion. White areas indicate zones of no depletion or no data. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-45.

(a) Pressure profiles from the M4.1 sand, Tahoe field, northern deep Gulf of Mexico: before (left) and after (right) production. Before production, pressure indicated that the east and west levees had similar formation pressures. Two years after production, differential depletion occurred in an offsetting well in the west levee reservoir. In contrast, the east levee had partial depletion in the upper levee but none in the lower levee. (b) Schematic cross section across the M4.1 reservoir level, showing the pattern of depletion in the two levees. Blue zones indicate where the pressure in the thin beds had depleted by early production in the 783-4ST2 well. Blue hachured zones represent zones of partial completion. White areas indicate zones of no depletion or no data. After Kendrick (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Figure 7-46.

Location map of Falcon field, East Breaks 579 and 623, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown by red area, and blue dashed line shows gas/water contact. Note that the channel (green) segments the east from the west levee reservoirs. Slight offset in the gas/water contact is shown by the channel. Regional dip to the southwest is shown by red arrow. Corrected stratigraphic dips in the eastern levee are shown, as determined from image logs. Modified from Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-46.

Location map of Falcon field, East Breaks 579 and 623, northern deep Gulf of Mexico. Distribution of reservoir fluids is shown by red area, and blue dashed line shows gas/water contact. Note that the channel (green) segments the east from the west levee reservoirs. Slight offset in the gas/water contact is shown by the channel. Regional dip to the southwest is shown by red arrow. Corrected stratigraphic dips in the eastern levee are shown, as determined from image logs. Modified from Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-47.

(a) Two thin-section photographs illustrate the excellent porosity and permeability in the proximal and distal levee facies. (b) Core photograph of proximal (lower core) and distal (upper core) levee facies of the Falcon field. Sands consist of unconsolidated, well-sorted, fine- to very fine-grained sands with climbing ripples. (c) Image and wireline logs (gamma-ray, resistivity and neutron) illustrate the levee facies for the entire M57 reservoir interval. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-47.

(a) Two thin-section photographs illustrate the excellent porosity and permeability in the proximal and distal levee facies. (b) Core photograph of proximal (lower core) and distal (upper core) levee facies of the Falcon field. Sands consist of unconsolidated, well-sorted, fine- to very fine-grained sands with climbing ripples. (c) Image and wireline logs (gamma-ray, resistivity and neutron) illustrate the levee facies for the entire M57 reservoir interval. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-48.

(a) Annotated core description from the East Breaks 579#2 well, with number of laminations determined from core, estimated from logs and from V Clay content (sampling 2 samples /foot). (b) Calculated net:gross from laminations count. (c) Percent error in measurement of net sand calculated by comparing cores with log measurements. In the proximal facies, the percent net:gross was overestimated by up to 35 %. In the distal levee facies, the percent net:gross was underestimated by up to 70%. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Figure 7-48.

(a) Annotated core description from the East Breaks 579#2 well, with number of laminations determined from core, estimated from logs and from V Clay content (sampling 2 samples /foot). (b) Calculated net:gross from laminations count. (c) Percent error in measurement of net sand calculated by comparing cores with log measurements. In the proximal facies, the percent net:gross was overestimated by up to 35 %. In the distal levee facies, the percent net:gross was underestimated by up to 70%. After Abdulah et al. (2004). Reprinted with permission of AAPG and Ken Abdulah.

Table 7-1.

Outcrops with significant thin-bed levees used for reservoir modeling.

Table 7-2.

Levee-overbank facies, Cerro Toro Formation, Chile.

Contents