ABSTRACT

The platform carbonates of the Natih Formation (Albian-Turonian) are hydrocarbon reservoirs throughout the Middle East. This study uses a high-resolution sequence stratigraphic model built from the Natih outcrops of the Adam Foothills Transect, in northern Oman, as the basis for the construction of synthetic seismic sections along the Natih outcrops. The model covers the seaward progression from a proximal carbonate platform to a distal intrashelf basin. Three third-order depositional sequences had been identified and correlated across the outcrops, and facies were mini-core plugged for petrophysical data measurement, which were then used to construct an impedance model from the stratigraphic model. This impedance model was used to construct synthetic seismic sections with a zero phase Ricker wavelet at varying peak frequencies (80 Hz, 60 Hz and 40 Hz). The high-frequency synthetic seismic identified seismic characteristics of specific depositional environments, notably the organic-rich intrashelf basin, the platform margin and the platform interior. These seismic characteristics were very subtle on the lower frequency data, but could nonetheless be identified. A seismic line passing through a nearly analogous setting in the subsurface Natih was used to try and identify the characteristics observed on the synthetic seismic data. Many of the synthetically-produced characteristics were found on the industry seismic, which shows that this method is a good way of using the detail of the outcrop to help with predictions on the lower frequency/resolution seismic.

INTRODUCTION

Study area

The “middle Cretaceous” carbonates are prolific hydrocarbon producers in the Middle East. In Oman, the Natih Formation was studied as it is both a reservoir and source rock interval with excellent outcrops, and it also produces from the subsurface nearby. The rudist-bearing, shallow-water carbonates of the Natih Formation (Albian-Turonian) (Figure 1) are hydrocarbon-bearing reservoirs. The Natih B and E source rocks are time equivalent organic-rich marl-limestones of the Shilaif and Khatiyah formations (Figure 1), which were deposited in intrashelf basins. The overlying regional shales of the Muti and Laffan formations (Figure 1) provide the seal for the play. The Natih Formation forms part of the Wasia Group, and lateral age-equivalent formations in the Gulf region are the Mauddud, Mishrif, and the basinal Shilaif-Khatiyah formations (Figure 1).

A high-resolution sequence stratigraphic study of the Natih Formation in northern Oman (van Buchem et al., 1996, 2002) demonstrated that there are distinctive and predictive patterns in the distribution and geometries of reservoir and source rock facies. The high-resolution sequence stratigraphic study was based on the excellent outcrops of the Natih Formation in both the Adam Foothills and the Jabal Akhdar area of northeastern Oman (Figure 2). This paper uses the Adam Foothills Transect of the study as a basis for the depth model that is used in a seismic forward model to construct synthetic seismic in the Natih Formation.

The Adam Foothills Transect is oriented perpendicular to the paleocoastline of the intrashelf basin and is ideally suited to illustrate the geometries and facies changes when passing from a proximal carbonate platform or ramp, into a more distal intrashelf basin (van Buchem et al., 2002). The Adam Foothills Transect composite outcrop section is nearly 100 km long and 350 m thick. The sections were measured within five jabals (hills, Figure 2), and correlation between jabals is based on the recognition of stratigraphic sequences comprising numerous high-resolution stratigraphic cycles and surfaces in the outcrops. The excellent quality and lateral extent of these outcrops allow the unique reconstruction of a high-resolution sequence stratigraphic section for the Adam Foothills Transect. The outcrop exposures also allow for drill-plug sampling to obtain velocity and density control of specific facies. The high-resolution sequence stratigraphic model and the synthetic seismic sections of the Natih outcrops allow us to establish predictive models for the time-equivalent Mishrif Formation throughout the Middle East.

Previous seismic work

Previously published work on the Natih Formation has described the interpretation and processing of seismic (2-D and 3-D). Articles have included seismic case studies of imaging fractures using shear data (Potters et al., 1999; Hitchings and Potters, 2000; van der Kolk et al., 2001), the use of various processing and volume visualization techniques to enhance the interpretation of subtle features (Whyte, 1995; Keating, 2001; Masaferro et al., 2003) and the interpretation of regional clinoform trends within the Natih E (Droste and van Steenwinkel, 2004). Droste and van Steenwinkel (2004) document two different types of clinoform geometries observed on seismic within the platform interior of the Natih E: low-angle (<1° dip) inclined surfaces within the prograding marly units of the lower Natih E, and higher angle (1°-2° dip) clinoforms within the aggrading platform derived grainstones of the upper Natih E. Their work mapped these clinoform belts in the Natih E, and included data from the fieldwork upon which this paper is based (figure 11 of Droste and van Steenwinkel, 2004). Their paper validates the regional extent of the clinoform geometry found in the Natih E, which is contained within the synthetic seismic line of the outcrop model for the Adam Foothills Transect (discussed in detail later in this paper).

Terminology for depositional geometries

The interpretation of depositional geometries is clear from seismic data, but more uncertain when reconstructed from lithological facies and facies sequences alone. The depositional geometry terminology used in this paper is illustrated in Figure 3. The depth of the intrashelf basin probably did not exceed 100 m during the development and infill of the Natih system. A slight ambiguity of depositional environment terminology within the Natih system has occurred because van Buchem et al. (2002) have followed the Burchette and Wright (1992) terminology of facies defined ramp depositional environments (i.e. inner-ramp, mid-ramp, outer-ramp and basin), while Droste and van Steenwinkel (2004) have followed the platform depositional environments identified with seismic geometries (i.e. platform, slope and basin). This paper uses the regional depositional geometries, at seismic scale, to define depositional profiles (Figure 3). For our purpose, similar facies exist along both depositional profiles, and in this paper their equivalence is defined as follows: platform ≈ inner-ramp, platform margin ≈ mid-ramp, slope ≈ outer-ramp, and intrashelf basin = intrashelf basin. The platform (inner-ramp) deposits are the most proximal, and the intrashelf basin deposits are the most distal.

OUTCROP DESCRIPTION

Sequence stratigraphy

The Adam Foothills Transect passes from a proximal platform or carbonate ramp to a more distal intrashelf basin, in a southeast to northwest direction (van Buchem et al., 2002). The synthetic seismic sections in this paper are based on the high-resolution sequence stratigraphic model (Figure 4) of the Adam Foothills Transect of van Buchem et al. (1996, 2002). This sequence stratigraphic model was constructed from the geometrical and genetic relationships between the intrashelf basin (maximum water depth of 100 m) and the adjacent shallow-water carbonate platform of the Natih Formation. The van Buchem et al. work (1996, 2002) subdivides the Natih into three larger scale accommodation cycles (third-order cycles of 0.5-3 my) that comprise the Natih E, the Natih C and D, and the Natih A and B members (Figure 4). The three third-order cycles (Sequences I – III) typically consist of a basal organic rich marl-limestone which is overlain by a prograding platform of rudstones layers and incised by tidal channels (van Buchem et al., 1996). A further refinement of this model (van Buchem et al., 2002) has distinguished two types of depositional systems: (1) a carbonate platform/margin/organic matter-rich intrashelf basin setting (Natih E and Natih A and B members – white arrow bars on Figure 4); and (2) a flat to low-angle mixed carbonate/clay-dominated ramp (Natih F and Natih C and D members – black arrow bars on Figure 4).

The Natih E and F members are a third-order sequence, which was deposited within an initial flat to low-angle mixed carbonate/clay-dominated ramp setting (Natih F – black arrow bar on Figure 4) that evolved into a carbonate platform/margin/organic matter-rich intrashelf basin setting (Sequence I – white arrow bar on Figure 4). This sequence is characterised by distinctive facies within the intrashelf basin, the slope or outer-ramp, the platform margin or mid-ramp and the platform or inner-ramp environments. The intrashelf basin is composed of either organic-rich/organic-poor limestone couplets (grey on Figure 4), or mudstones and wackestones with abundant chert (light blue with black dashes on Figure 4). The slope or outer-ramp area is composed of muddy wackestones (light blue on Figure 4). The platform margin or mid-ramp region is composed of lower energy (i.e. muddier and bioturbated wackestone to packstone) facies, or higher energy cleaner coarse-grained bioclastic cross-bedded facies (dark blue on Figure 4). These two facies are found within low-angle clinoforms with a basinward transition from higher energy to lower-energy facies along the clinoforms. The platform or inner-ramp consists of wackestones to packstones (yellow and orange on Figure 4) with a rich faunal association dominated by rudists (Philip et al., 1995). Incised, multi-story channels occur within the upper part of the platform or inner-ramp environment. The incisions are found at the top of the Natih E (Figure 4) and were formed during subaerial exposure of the platform.

Sequence I consists of four medium scale fourth-order sequences (cycles I-1 to I-4 on Figure 4). In the study area, Sequence I has a very uniform thickness, but is composed of the succession of the two different types of depositional systems. The initial deposition in Sequence I occurs on a flat to low-angle mixed carbonate-clay ramp (Natih F, cycle I-1, black arrow bar on Figure 4). This is followed by the development of a carbonate platform to intrashelf basin topography (Natih E, cycles I-2, I-3, and I-4, white arrow bar on Figure 4). During the transgressive part of Sequence I (increasing accommodation cycle – blue triangle on Figure 4), the flat-bedded geometries (cycle I-1 on Figure 4) changed into a very low-angle ramp with very low-angle clinoforms on the outer-ramp or slope and organic-rich basinal deposits in a distal position (cycle I-2 on Figure 4). During the regressive part of Sequence I (decreasing accommodation cycle – red triangle on Figure 4), the carbonate platform margin/organic-rich intrashelf basin depositional environment developed and the whole system progressively stepped basinward (cycles I-3 and I-4). A clear platform margin was built up at this time (top cycle I-3 on Figure 4). Droste and van Steenwinkel (2004) have mapped the regional maximum extent of the Natih E platform margin on seismic (refer to their figure 11). The top of Sequence I exhibits subaerial erosion (Homewood et al., 2002) in the proximal outcrops (cycle I-4 on Figure 4).

The Natih C and D members form an entire third-order sequence within a mixed carbonate-clay ramp environment (Sequence II, black arrow bar on Figure 4). Flat-bedded units with few lateral facies changes characterize the facies within this setting. There are two main environments and associated facies, a clay-rich environment with thin carbonate beds (i.e. clayey marls – green, and carbonate beds – dark blue on Figure 4), and a carbonate-dominated environment with thin clayey interbeds (light blue on Figure 4). Iron encrusted hardgrounds (hachured lines and Fe symbol on Figure 4) are common within the carbonate-dominated setting.

Sequence II consists of three medium-scale, fourth-order sequences (cycles II-1 to II-3 on Figure 4). The cycles within this mixed carbonate-clay-rich ramp environment show various changes from clay-rich to carbonate-rich facies depending on the regional clay supply (south of the study area) (van Buchem et al., 2002). In general, the beds are flat-lying except for the subtle wedges of carbonate-rich layers thinning to the northwest during the progradation of bioclastic intervals (dark blue in Figure 4). There is no intrashelf basin developed in the study area during Sequence II (van Buchem et al., 2002). The initial deposition in Sequence II (cycle II-1) is a shallow-water, rudist-rich ramp, which is overlain by ramp interior claystones. These are separated by an erosional unconformity. The ramp then changes into a clay-rich system with numerous iron-crusted hardgrounds, and then returns to a carbonate dominated system with two regional carbonate marker beds at the top of cycle II-2. A carbonate-rich ramp is then developed during the transgression at the top of Sequence II in cycle II-3.

The Natih A and B members form an entire third-order sequence within a carbonate platform/margin/organic matter-rich intrashelf basin setting (Sequence III – white arrow bar on Figure 4), similar to the Natih E member. In the Adam Foothills Transect, only the intrashelf basin and slope facies are present (Figure 4).

Sequence III consists of four medium-scale, fourth-order sequences (cycles III-1 to III-4 on Figure 4). In general, the organization in Sequence III is very similar to that of Sequence I, but thicker (van Buchem et al., 2002). Initially there was a flat-bedded mixed carbonate-clay ramp (cycle black arrow bar on Figure 4) which developed into an organic-rich intrashelf basin (cycles III-2 to III-4, white arrow bar on Figure 4), with an adjacent carbonate platform. In the Adam Foothills Transect only the intrashelf basin and slope facies are seen in Sequence III, and various unconformities truncate the upper part of Sequence III (Figure 4).

Along the Adam Foothills Transect, Sequence I within the Natih E and F exhibits all the stratal geometries and facies encountered within the Natih Formation. Certain facies might be thicker in other sequences/members, but their lateral transitional changes are best imaged on outcrop in Sequence I. These characteristics were modelled on the synthetic seismic sections (discussed in detail in a following section), and can be applied to similar depositional style carbonate ramps throughout the Middle East (i.e. the Natih, Mishrif and Shu’aiba Formations).

Outcrop geometries and stratal surfaces

The high-resolution sequence stratigraphic model illustrates that the Natih Formation is made of two alternating types of depositional systems: (1) a carbonate-dominated platform bordering an intrashelf basin; and (2) a flat to low-angle mixed carbonate-clay ramp (van Buchem et al., 2002). These systems have specific geometries, stratal surfaces and facies associations, which were traced between the outcrop jabals. These characteristics were incorporated into the impedance model (Figure 5) that was used for the synthetic seismic modelling and are described below.

Carbonate-dominated platform bordering an organic-rich intrashelf basin

Sequence I best illustrates this depositional system (white arrow bar on Figure 4). The most distinct geometry is the platform margin that is made of thickening low-angle clinoforms, due to rudist shoal buildups in this area (dark blue in cycles I-3 and I-4 of Figure 4). A 0.15° dip angle is estimated for these clinoforms by connecting the stratal surfaces in cycle I-3 between the Madmar 2-1 and Salakh 1 outcrops (about 10 km apart), since the clinoform dip angles are too low to be measured at one single location. Progradation within the platform interior, at a lower dip angle than the rudist shoals, is characterised by lateral facies, and velocity and density changes along the clinoforms in the model (orange-yellow-dark blue-light blue in Sequence I of Figure 4).

Droste and van Steenwinkel (2004) discuss the difficulty of identifying seismically observed Natih clinoforms (0.1° - 2° dip) in wells that are 10 km apart in dip direction. This problem also occurs in the outcrop model. At the time of the outcrop study, we recognized that the 0.15° dip of the rudist margin clinoform geometry (discussed above) was probably too low, but until the Droste and van Steenwinkel (2004) paper we could not quantify this error. We suggest that the two types of clinoform geometries documented by Droste and van Steenwinkel (2004) on seismic (i.e. <1° dip in marly units and 1° - 2° dip in grainstone units) correspond to the two types of clinoform geometries in our high-resolution stratigraphic outcrop model (i.e. very low-angle of the platform interior, and the rudist shoal platform margin). Thus our outcrop model incorporates the two different clinoform geometries, but at lower dip angles than those actually observed from subsurface seismic data.

The erosional channels at the top of the platform facies are documented from measured sections in the Madar and Madmar outcrops. Variations in the shape of the channels, and the overlying nature of the dolomitised layers, are incorporated into the impedance model (Figure 5) to study their various effects on the seismic. A dolomitised, laterally extensive stratal surface above a regional exposure surface (top of cycle I-4) is built into the model because of the high velocity and density associated with this surface. The thin organic-rich intrashelf basinal limestone layers can be traced laterally across the outcrops, so are included within the impedance model (Figure 5) and are represented by their appropriate velocities and densities.

Flat to low-angle mixed carbonate-clay ramp

Sequence II best illustrates this depositional system (black arrow bar on Figure 4) and is composed of flat-bedded to low-angle, clay-dominated ramps, with numerous iron-crusted hardgrounds (cycles I-1, II-2 and III-1 of Figure 4), that alternate with carbonate-dominated ramps (cycles I-1 and II-3 of Figure 4). The hardgrounds are distinct, laterally extensive time lines with a high layer interval velocity and density, so some representative layers were incorporated within the impedance model. Thin wedges of carbonate-rich layers with extremely low-angle progradation can occur as the clay-dominated ramps change to carbonate-dominated ramps. These were incorporated into the impedance model (cycles II-2 and II-3 of Figure 5) as either a wedge shaped geometry, if the layer was thick (cycle II-2), or by laterally varying velocities and densities in the thinner layers to mimic the changing facies within the prograding unit (cycle II-3).

Impedance model

The Adam Foothills Transect is 100 km long and 320 m thick (Figure 4), and the stratigraphic model has been constructed from more than 2,000 m of measured section. The average height of each measured section was 140 m. A variety of facies in the measured sections were mini-core plugged for petrophysical data used in the impedance model. The impedance model (Figure 5), used as the depth input into the construction of the synthetic seismic sections, is based on the original high-resolution sequence model of van Buchem et al. (1996), which was slightly modified in the most recent work of van Buchem et al. (2002) (Figure 4). The impedance model (Figure 5) simplified the high-resolution sequence model by incorporating the main geometries, stratal surfaces, and lateral facies variations from the outcrops. The model layer thickness ranges from 2.5 – 35 m, and some of the thinnest layers will clearly not be resolved on the seismic. However the model was constructed to include as much of the geologic detail as possible to simulate a more realistic composite response of the seismic signal passing through a similar geologic setting. The regional scale Adam Foothills Transect did not allow us to study in detail the progradation within the platform or the incision on the top of the platform, which is the topic of future work.

The construction of the impedance model (Figure 5) began with a detailed depth model. Due to the regional nature of the transect, the depth model was drawn at a VE = 100x (vertical scale of 1 cm = 5 m and horizontal of 1 cm = 500 m). This was done to capture as many details as possible of the high-resolution sequence stratigraphic model (Figure 4). The basal stratal surface of the model was drawn at a regional tilt of 0.01° towards the basin, i.e. to the northwest. This represented a slight initial topography, which existed before the development of Sequence I, and has been shown from subsurface work on the Natih (van Buchem et al., 2002).

Numerous measured sections from each jabal were spaced at their appropriate horizontal distance to construct the model. This gave a more precise representation of the actual facies than the schematic high-resolution sequence stratigraphic model of the van Buchem et al. articles (1996 and 2002; Figure 4). The geometries of the stratal surfaces were constructed by connecting the surfaces between the measured sections (Figure 5). This resulted in steeper clinoforms (0.15° in cycle I-3 and 0.04° in cycle I-2) and larger erosional channels (up to 2.5 km wide and 20 m deep) than those shown on the van Buchem et al. outcrop model (Figure 4). The minimum bed thickness used was 2.5 m to represent the regional dolomitised erosional surfaces and hardgrounds of cycles II-1 and III-1 (6,340 m/s, 2.64 gm/cc layers in Figure 5). Similar facies within the prograding intervals of Sequence I were grouped into 10 m thick units instead of using individual prograding beds, which could not be correlated from outcrop to outcrop, especially over the 40 km spacing between the Jabal Madar and Jabal Madmar outcrops (see Figures 3 and 4). Units of laterally continuous facies with thin vertical alternations of clay-rich and carbonate-rich layers were grouped into one layer in cycles II-1 and II-2 (Figure 5). The thin prograding carbonate layers at the top of cycle II-2 were grouped into one layer.

Sequence I comprises the lower half of the model (Figure 5) and can be divided into 4 medium scale cycles to I-4). Cycle I-1 is a flat to low-angle mixed carbonate-clay ramp, and comprises the lowermost layer of the model. This uniform layer was given a constant velocity, density, and thickness of 50 m, so the base of the model would not interfere with the first impedance layer of the model. At the base of cycle I-2, a carbonate platform starts to develop, as can be seen by the three pulses of very low-angle progradation of bioclastic packstone and grainstone wedges (R arrows on Figure 5). Laterally equivalent to the platform is an organic-rich intrashelf basin, with layers of oyster-rich organic limestone beds (gray on Figure 5). The overall thickness of cycle I-2 ranges from 55 m thick in the platform to 35 m in the intrashelf basin. The sequence boundary at the top of cycle I-2 is marked in red on Figure 5. Cycle I-3 is made of two distinctive progradational pulses (R arrows on Figure 5). These wedges contain shallower facies, mainly the bioclastic grainstones of the platform to margin facies. Laterally equivalent to the platform margin facies are the distal wackestones and chert facies of the slope. The overall thickness of cycle I-3 ranges from about 45 m in the platform to 32 m in the more distal section. The sequence boundary at the top of cycle I-3 (Figure 5) is found at the top of the highest angle clinoform in the model (0.15°). Cycle I-4 contains the shallowest facies of the platform and the last prograding pulse in Sequence I (R arrow on Figure 5). The stratal surface at the top of cycle I-4 is an erosional sequence boundary at the top of Sequence I. This surface is represented by a 2.5 m thick layer, which corresponds to the dolomitised erosional surface seen at outcrop. This layer has been drawn on top of some channels and at the base of others to simulate the various configurations seen in the outcrops. The overall thickness of cycle I-4 varies from 20 m on the platform to 35 m in the slope. This cycle is not present in the outcrops to the northwest (Figure 4), so a constant thickness from Salakh 2 has been carried across the impedance model (Figure 5).

Sequence II is made of 3 medium scale cycles in a flat to low-angle mixed carbonate-clay ramp setting (II-1, II-2, II-3 on Figure 5). Cycle II-1 is a carbonate-dominated mixed ramp and is represented by one layer that varies in thickness from 7.5 m to 20 m. This layer is made of proximal rudist floatstones and tidal deposits, and has been given one average velocity and density, with channel features at its base and top. The channels are given different shapes and sizes (1-2.75 km wide and 5-15 m deep) to respect the variations seen on the outcrops. Another dolomitised layer (2.5 m thick) represents the sequence boundary at the top of cycle II-1 (Figure 5). Cycle II-2 is a clay-dominated ramp made of alternating units of carbonate rich layers of bioclastic packstones and green shales. The thickness of cycle II-2 ranges from 30 m to 20 m. The thin lower beds of this cycle are grouped into a basal very thin green shale layer, which is covered by a very thin carbonate rich layer (5 m) that pinches out to the northwest (purple in Figure 5), and is then covered by a thicker layer of green shales. The sequence boundary of cycle II-2 lays on the top of the upper carbonate unit with bioclastic material that progrades to the northwest (R arrow on Figure 5). This layer was continued and thinned across the impedance model (Figure 5 to the northwest) to demonstrate the prograding geometry through an area where continuous outcrop was missing (note gap in Figure 4 in this area). The changing lithology of this upper unit is represented by a lateral impedance change. Cycle II-3 is a carbonate-dominated mixed ramp. The thin beds of this cycle have been divided into two layers, a lower layer of alternating marls and thin carbonates, represented by a laterally changing impedance layer, and an overlying mudstone layer. Cycle II-3 ranges from 15 m to 25 m. The top of Sequence II is a sequence boundary represented by a facies change where shallower rudist facies overlie the mudstones at the top of cycle II-3 (red line on Figure 5).

Sequence III starts with the flat to low-angle mixed carbonate-clay ramp of cycle III-1 (Figure 5). The initial rudist packstone facies are represented by a single average impedance layer that thickens slightly to the northwest (pink in Figure 5). Alternating layers of shales, mudstones and hardgrounds cover this layer. The top of cycle III-1 is a flooding surface represented by the uppermost hardground (blue on Figure 5). Cycle III-1 ranges in thickness from 28 m to 45 m. Cycle III-2 comprises the top part of the model and represents the development of another organic-rich intrashelf basin, which is part of a carbonate platform/organic-rich intrashelf basin depositional system. In the model (Figure 5), this cycle is a uniform basinal limestone-marl except for distinct layers of organic-rich oyster beds on the northwest side (ranging in thickness from 2.5 to 7.5 m). The last stratal surface of the impedance model is the maximum flooding surface near the top of cycle III-2. A layer of uniform mudstones overlies this cycle, and the impedance model (Figure 5) does not contain the large erosional surface seen on the outcrop section (Figure 4).

SYNTHETIC SEISMIC MODEL OF THE OUTCROP

The technique of using seismic forward modelling to construct synthetic seismic sections of carbonate outcrops to aid in the interpretation of low frequency industry data has been used in many locations (Biddle et al., 1992; Stafleu et al., 1994; Stafleu and Sonnenfeld, 1994; Anselmetti et al., 1997; Schwab and Eberli, 2000; Bracco Gartner et al., 2002; Janson, 2002). Most of these studies have been in areas where the carbonate platforms are thick and have clearly exposed platform margins. This study looks at a slightly different type of carbonate system, a flat to low-angle mixed carbonate-clay ramp that evolves into a carbonate platform/margin/organic-rich intrashelf basin. This type of carbonate system is common among the prolific hydrocarbon producers throughout the Cretaceous of the Middle East, and the conclusions from this study may be broadly applied throughout the region.

Methodology

The initial step in the seismic forward modelling of an outcrop is to construct a depth model from outcrop measurements. The detailed high-resolution sequence stratigraphic depth model of the Adam Foothills Transect (Figure 4) was simplified, as described in the previous section, and this was used to create the impedance model (Figure 5) that was used as the input to the seismic forward modelling.

After the initial sequence stratigraphic framework had been established from the measured sections, the outcrop samples were taken for measurement of velocities and densities. Eighty-five, oriented, one inch-diameter core plug samples were taken by drilling from representative facies at different outcrops (asterisks on Figure 4). Numerous core plugs were taken from similar facies to get a good average velocity and density for representative lithologic facies. Anomalously high or low values were not used in the model. Ultrasonic compressional-wave (Vp), 2 shear-wave (Vs) velocities and densities (Table 1) were measured under varying confining and pore-fluid pressures in the Petrophysics Laboratory at the University of Miami. Details on this technique can be found in Christensen (1985), and Schwab and Eberli (2000). In this study, all the petrophysical values were determined at 20 Mpa, which is equivalent to about 1,000 m depth of burial, similar to production depths. Only the Vp and density values were used in this study, the Vs values will be used in future work.

The measured core plug Vp values range from a high of 6,340 m/s for the dolomitised hardgrounds to a low of 5,240 m/s for the platform orbitalene grainstones. The measured density values range from a high of 2.66 gm/cc in the intrashelf basinal mudstones to a low of 2.40 gm/cc in the platform orbitalene grainstones. The measured density values are comparable to the subsurface equivalent log values, but the core plug velocities are generally 25% higher than the subsurface equivalent log velocities. However, the trend of the measured velocities is similar to that of the log velocities, i.e. the grainstones are slowest and the mudstones/muddy wackestones are fastest. The large difference between the outcrop and log-derived velocities is mainly due to the different burial history of the outcrop compared to the subsurface equivalent producing fields (see Figure 2 of van Buchem et al., 2002). The Adam Foothills outcrops were part of the obduction front and fold and thrust belt, while the producing fields were not. This burial history led to greater cementation and compaction of the outcrops, hence much higher velocities for the measured outcrop core plug samples versus the equivalent subsurface log velocities. In addition, meteoric diagenesis of the outcrop after uplift could also have increased the outcrop velocities. Nonetheless these effects should have been similar across the different facies, which is demonstrated by the measured changes in velocities and densities of the various facies samples (Table 1). Consequently it is valid to use the measured outcrop velocities and densities for the impedance model, though they are higher than the subsurface equivalent log values.

The impedance model for the Adam Foothills Transect (Figure 5) was constructed from the simplified depth model using the Vp and density data from the outcrop sample core plugs (Table 1). Outcrop facies work and thin section work on the core plugs (Mettraux, 1997) were used to assign core plugs to facies specific impedance layers in the impedance model. The layer properties for the various facies within the impedance model are found in Table 2, and were determined by averaging all samples within a specific facies. Figure 6a is a plot of Vp versus density for the samples used to determine the layer properties. In general it shows that the faster velocities and higher densities occur in the mudstones of the intrashelf basin and slope or outer-ramp setting (diamonds and triangles on Figure 6a), while the slower velocities and lower densities occur in the grainstones of the platform margin or mid-ramp setting (squares on Figure 6a). The exception are the wackestones/packstones of the platform margin or mid-ramp (green squares on Figure 6a). Where these facies are grainier, or less heavily cemented, they tend to be slower and lighter than the muddier wackestones/packstones (green squares on Figure 6a). The rudist packstones of the platform interior or inner-ramp (dashed lines on Figure 6a) have higher velocities and densities, due to their high mud content. Figure 6a demonstrates that the mud content appears to be the main control on the velocities and densities of the outcrop samples, i.e. the grainstones are slower and lighter than the mudstones, wackestones and packstones.

The general velocity/facies model used in this work is illustrated in Figure 6b. In the intrashelf basin, velocities ranged from 5,900 m/sec for the organic-rich limestones to 6,200 m/sec in the laterally equivalent calcareous mudstones, to 6,000 m/sec in the mudstones/wackestones with chert. In the muddier part of the slope or outer-ramp environment, an average velocity of 6,160 m/sec was used for the muddy wackestones. Within the platform margin or mid-ramp region, the velocities ranged from 6,140 m/sec for the packstones/wackestones to 5,810 m/sec for the bioclastic grainstones. The platform or inner-ramp velocities ranged from 5,240 m/sec in the orbitolinid grainstones to 6,010 m/sec and then 6,100 m/sec in the rudist packstones/wackestones, as the facies became muddier on the platform or inner-ramp. A core plug derived velocity of 6,340 m/sec was used for the dolomitised and hardground layers. The green shales could not be properly cored at the outcrop, so a velocity of 5,300 m/sec was estimated from sonic logs of subsurface equivalent rocks. A standard tidal channel infill velocity of 5,800 m/sec was used for all channels in the model, since the channels tend to be filled with grainstones and very little mud (similar to the bioclastic grainstone facies).

The SattleggerTM modelling program was used to construct a synthetic seismic section of the impedance model at varying peak frequencies (80, 60 and 40 Hz; Figure 7). This software package uses normal incidence ray tracing, of the primary energy (no transmission loss or multiple reflections), to create a spike synthetic seismogram of the impedance model (reflection coefficients), which is convolved with a seismic wavelet to create a synthetic seismic section. The synthetic seismic sections were made by convolution with zero phase Ricker, SEG reverse polarity, wavelets of varying peak frequencies. With SEG reverse polarity, a decrease in acoustic impedance (change from fast to slow velocity) corresponds to a black peak on the synthetics (i.e. positive number, wiggle excursion to the right on Figures 7, 8 and 9). The synthetic seismic sections were constructed using a 2 ms sampling rate and a 50 m trace spacing. The varying peak frequency synthetic seismic sections were analyzed to identify the various seismic characteristics associated with a flat to low-angle mixed carbonate-clay ramp that evolves into a carbonate platform/margin/organic-rich intrashelf basin. Finally, the 40 Hz peak frequency synthetic seismic was compared with an exploration seismic line (average peak frequency of 45 Hz in the Natih) in an analogous geological setting to see if the actual seismic contained seismic facies and geometries similar to those predicted from the synthetic seismic modelling of the outcrop.

As previously discussed, it was known from the start that the thin beds within the depth model were below the tuning thickness of the seismic, and therefore could not be individually resolved. If a bed thickness is greater than the tuning thickness, then the top and bottom of the bed will be uniquely resolved. As the bed thickness falls below the tuning thickness (1/4λ), the reflections from the top and bottom of the bed interfere constructively then destructively so they cannot be uniquely resolved (Sheriff, 1985; Widess, 1973). With this in mind, the aim of the project was to investigate the level of detail the outcrop geology could provide in order to better interpret the response of the seismic signal in a flat to low-angle mixed carbonate-clay ramp that evolves into a carbonate platform/margin/organic-rich intrashelf basin.

The horizontal scale of the synthetic seismic sections has been drastically reduced in Figure 7 to allow illustration of the full 100 km long model. In Figure 7, the impedance model (Figure 5) in time (red lines) is overlain on the synthetic seismic sections. Enlargements of the interesting features in the platform margin and platform interior areas are found in Figures 8 and 9. For ease in description of the synthetic seismic sections (Figures 7, 8 and 9), the eight major reflections have been numbered on the figures, and these numbers will be referenced in the following sections.

80 Hertz

On the 80 Hz peak frequency synthetic seismic section (Figure 7a) the tuning thickness (i.e. 1/4λ - Sheriff, 1985) ranges from 16 m to 20 m depending on the interval velocity of the layer. Thus many of the layers on the original impedance model (Figure 5) combine to form composite reflections (Figure 7).

Reflection 1 is caused by the basal impedance layer of cycle I-2 (R arrow on Figure 5). This reflection straddles the 100 ms time line of the model, and can be confused with this due to the reduced scale of Figure 7. Reflection 1 laterally changes amplitude and time due to interference with the overlying thin organic-rich basinal beds as the first prograding unit of cycle I-2 thins basinward around SP1600 (R arrow on Figure 5). A faint extra reflection, above reflection 1, can be seen between SP1200-820 (Figures 7a and 8a), which is caused by the impedance contrast in the first transgressive unit of cycle I-2 (T arrow on Figure 5). This extra reflection loses amplitude to the right of SP1250 due to the thinning of this unit. In this area it is also clear that the extra reflection is located below reflection 2. The extra reflection merges with reflection 1 near SP800 and causes a phase change in reflection 1, due to the decreasing impedance contrasts in the overlying transgressive unit. Basinward of SP800, reflection 1 is very low amplitude and continuous due to the constant, slight impedance contrasts with the overlying basinal layers. There is a slight time shift ‘pull-up’ effect between SP800-300 due to a shallower high impedance layer in cycle II-2, which is discussed later in the description of reflection 6.

Reflection 2 appears to follow the grainstones of the upper prograding units of cycle I-2 (R arrows on Figure 5). This reflection loses amplitude towards the left around SP1600 (Figures 7a and 9a), where the units thin below resolution. Reflection 3 laterally changes amplitude and phase to the center as it follows the lower prograding unit in cycle I-3 (R arrow on Figure 5). It is a high amplitude, continuous reflection on the right and then loses amplitude towards the left around SP1550 (Figures 7a and 9a) where there is a difference in impedance contrast with the underlying transgressive wedge of cycle I-3 (T arrow on Figure 5). There is another change around SP1000 due to the laterally changing impedance within the prograding unit. Basinward of this point (SP950-750 – Figures 7a and 8a), the reflection increases in amplitude due to the interference of the overlying progradation in cycle I-3 (R arrow on Figure 5) and then disappears when the progradation wedges out (SP750 – Figures 7a and 8a). There is a continuous, high frequency extra reflection between reflections 3 and 4 from SP 750-1600 (Figures 7a and 8a). This reflection appears to follow the impedance contrasts associated with the transgressive wedge in cycle I-3 (T arrow in Figure 5).

Reflection 4 is a variable amplitude reflection that follows the progradation in cycle I-4 (R arrow on Figure 5) on the platform interior. The amplitude variations are related to the impedance contrasts with the underlying upper progradation unit and transgressive unit within cycle I-3. Reflection 4 takes a dip in time near SP1000 (Figures 7a and 8a) due to the impedance contrast with the underlying rudist shoal platform margin. Basinward of SP750 (Figures 7a and 8a) the amplitude is drastically reduced due to the disappearance of the platform margin and the low impedance contrast of the overlying mudstones.

Reflection 5 is made of two thin high frequency reflections (Figures 7a, 8a and 9a) that are due to the high velocity and density dolomitised layers at the top and bottom of cycle II-1 (blue layers on Figure 5). The channel, centered at SP1850 (Figures 7a and 9a), can be seen to “dim” the upper reflection and cause slight amplitude and time distortions on the underlying reflections. The channel at SP1480 increases the amplitude of the lower reflection. Around SP1450 the lower reflection merges with reflection 4 (Figures 7a and 9a), which is probably due to the impedance changes of the underlying layer in this area. Reflection 5 continues as a lower amplitude single upper reflection until around SP1000 where it disappears and then at SP800 once again becomes a double reflection (Figures 7a and 8a). Between SP1000 and SP800, the upper reflection is probably interfering destructively with the underlying cycle I-4. The smaller channels along the upper reflection (SP1380 and SP900) are not imaged. The channel centered at SP980 (Figure 5) is poorly imaged due to the interference with the progradation below reflection 4 (cycle I-3), although on a larger scale (Figure 8a) one can see a slight amplitude change and time ‘sag’ along reflection 4 due to the channel. Basinward of SP780, reflection 5 is once again a continuous, double reflection, probably due to the constant thickness of cycle II-1.

Reflection 6 is a continuous, high amplitude reflection due to the impedance contrast of the carbonate layer over the green shales within cycle II-2. There is a slight doming of reflection 6 between SP300 and 800, which follows the shape of the prograding carbonate layer in cycle II-2 (R arrow on Figure 5). This impedance contrast also causes a slight time shift ‘pull-up’ effect on the underlying reflections (5, 4 and 1) in this area. Reflection 7 is a continuous, variable amplitude reflection at the top of cycle II-3 (Figure 5). The amplitude loss from SP800-2000 (Figure 7a) is probably due to the destructive interference of the thinning underlying beds. Reflection 8 is a continuous, variable amplitude reflection that represents the maximum flooding surface at the top of cycle III-1 (Figure 5). The amplitude loss from SP800-2000 (Figure 7a) is due to interference from the thinning layers in the upper part of cycle III-1. In the top unit of the model above reflection 8 (Figure 7a), only the upper two organic-rich oyster beds cause reflections.

Over a short distance (i.e. 1-25 km), the Natih Formation resembles a series of ‘railroad track’ isopach reflections. However in detail, there are subtle seismic characteristic changes which may be attributed to facies changes. The 80 Hz peak frequency synthetic seismic section (Figure 7a) shows the clearest main seismic characteristics: an intrashelf basinal seismic facies, a platform margin geometry, and a platform interior seismic facies.

Intrashelf basinal seismic facies – high amplitude, continuous reflections (above reflection 8 on Figure 7a) occur where low impedance organic-rich oyster facies have developed in the intrashelf basin during Sequences 1 and 3 (Figure 5).

Platform margin geometry - location of the platform margin imaged by the high amplitude clinoform of the low impedance rudist shoal (reflection 4 – Figures 7a and 8a) within Sequence I (Figure 5).

Platform interior seismic facies - channels within the platform interior of Sequences I and II (Figure 5) are seen as amplitude changes (reflection 5 - Figures 7a and 9a) and as time ‘sags’ and amplitude changes on underlying reflections (reflection 4).

60 Hertz

On the 60 Hz peak frequency synthetic seismic section (Figure 7b) the tuning thickness (i.e. 1/4λ - Sheriff, 1985) ranges from 22 m to 26.5 m depending on the interval velocity of the layer, thus many of the layers on the original impedance model (Figure 5) combine to form composite reflections. This decrease in resolution is demonstrated when comparing the 80 Hz peak frequency section (Figure 7a) to the 60 Hz peak frequency section (Figure 7b), where some of the subtleties of the higher frequencies have been lost and the number of reflections and amplitude changes have been reduced. All observations are similar to the previously described 80 Hz peak frequency section except for reflections 2, and 3.

Reflection 2 is no longer a clear reflection (Figures 7b and 9b), since the grainstones of the prograding units of cycle I-2 (R arrow on Figure 5) thin below resolution. Reflection 3 is a lower frequency, higher amplitude reflection which is a composite of the lower part of cycle I-3 and the upper part of cycle I-2 (Figure 5). Reflection 3 has a slight dip in time between SP900-760 (Figures 7b and 8b), which is a ‘push down’ effect due to the overlying lower velocity and density prograding layer in cycle I-3 (the high amplitude shoal platform margin of reflection 4). Beyond SP760, reflection 3 is very low amplitude due to the low impedance contrasts of the basinal layers (Figures 7b and 8b).

The major changes in the 60 Hz peak frequency section compared to the 80 Hz peak frequency section (Figure 7b), are a loss in amplitude of some of the continuous reflections (reflections 1, 2, 7 and 8), the inability to image progradation or architecture within the platform interior of Sequence I (reflections 2 and 3), and the inability to image some of the time lines in the mixed carbonate-clay ramp of Sequence II (reflection 5). Nonetheless, subtle changes in reflection character, i.e. amplitude, phase, time distortion and geometry, which are associated with facies changes, allow the identification of the main seismic characteristics.

Intrashelf basinal seismic facies – image reflections due to the organic-rich oyster facies of the intrashelf basin (above reflection 8 – Figure 7b).

Platform margin geometry - clearly image the rudist shoal platform margin (reflection 4 – Figures 7b and 8b), and reflection 3 displays a ‘push down’ time effect due to the overlying prograding shoal platform margin.

Platform interior seismic facies – image one platform interior erosional channel as an amplitude change (reflection 5 – Figures 7b and 9b), and another causes a slight phase change along reflection 4.

40 Hertz

On the 40 Hz peak frequency synthetic seismic section (Figure 7c) the tuning thickness (i.e. 1/4λ -Sheriff, 1985) ranges from 33 m to 40 m depending on the interval velocity of the layer, thus most of the layers on the original impedance model (Figure 5) combine to form composite reflections. Thus, the 40 Hz peak frequency section (Figure 7c) shows the least detail of all the synthetic seismic sections (Figure 7). This synthetic seismic section is closest to actual seismic, though newer data can have a higher peak frequency. Despite the loss in resolution, similar general observations to those previously described for the 60 Hz peak frequency model can be made, except for reflections 2, 6, 7 and 8.

Reflection 2 does not occur. Reflections 6 and 7 have merged together into one low frequency, continuous variable amplitude reflection (Figure 7c). This composite reflection changes amplitude to the right of SP 800 (Figures 7c and 8c), due to the thinning and lateral impedance change within the carbonate layer of cycle II-2 (R arrow on Figure 5). There is no reflection 8, probably due to destructive interference of the layers in cycle III-1.

The major changes in the 40 Hz peak frequency section compared to the 60 Hz peak frequency section (Figure 7c) are that the clear definition of the intrashelf basinal and platform interior seismic facies are lost. The rudist shoal platform margin geometry can still be identified by the phase change along reflection 4 at the platform margin (SP760 – Figure 8c), and the time ‘push down’ below it along reflection 3 (SP760-900 – Figure 8c). Basinward of the platform margin (SP 750 to the left), reflection 4 rises slightly in time and has an amplitude decrease and frequency increase due to the interference between the top and base of cycle I-4.

Intrashelf basinal seismic facies – only one low amplitude reflection above reflection 8 represents the layers of organic-rich oyster facies (Figure 7c).

Platform margin geometry - the rudist shoal platform margin is imaged as a slight dip in reflection 4 (Figure 8c), and appears to be continuous with a younger section that infills the rudist shoal topography (reflection 4/5). Reflection 5 appears to onlap reflection 4 (SP850 - Figure 8c), which is an interpretation ‘pitfall’ that would suggest the shoal platform margin is slightly younger than its actually age.

Platform interior seismic facies - no clear evidence of the channels in the platform interior (Figure 9c).

DISCUSSION

This study has shown the importance of using the highest possible peak frequency seismic data to identify subtle changes in reflection character, i.e. amplitude, phase, time distortion and geometry, which are associated with facies changes. These subtle seismic characteristics are key observations, especially when calibrated with outcrop, in an integrated seismic sequence stratigraphic interpretation.

In this study, a high-resolution sequence stratigraphic outcrop model, and synthetic seismic sections of this model, were used to demonstrate the occurrence of seismic characteristics specific to certain depositional systems. The identification of these seismic characteristics can be used to predict the probable location of these depositional systems in subsurface areas of similar age and depositional setting. The reliability of the interpretation of these features on lower frequency industry seismic (i.e. 40 Hz) is improved by the observations of the features on high frequency synthetic seismic data (i.e. 80 Hz) and the change in their clarity is documented with a stepped decrease of resolution as the peak frequency of the synthetic seismic decreases (i.e. 80 Hz – 60 Hz – 40 Hz – Figure 7). This panel of decreasing frequencies illustrates how the seismic characteristics change, and gives confidence for the interpretation of the subtle features seen on lower frequency as being due to genuine geological features that can be imaged at higher frequencies.

The regional 100 km Adam Foothills Transect (Figures 4 and 5) is built from two different types of depositional systems: (1) carbonate-dominated platform bordering an organic-rich intrashelf basin; and (2) mixed carbonate-clay ramp. These systems can be divided into three general areas (intrashelf basin, platform margin/mid-ramp and platform/ramp interior), with associated seismic characteristics that distinguish each of these areas (Figure 7). The distinguishing seismic characteristics for the depositional systems are: an intrashelf basinal seismic facies, a platform margin geometry and a platform interior seismic facies.

The platform margin geometry (reflection 4 on Figures 7 and 8) is the most distinct seismic characteristic in a low-angle carbonate-dominated platform bordering an organic-rich intrashelf basin. Reflection 4 (Figures 7 and 8) is caused by the last progradational unit of cycle I-3 (Figures 4 and 5) and shows the final highest relief of the rudist shoal nature of the platform margin (0.15° angle clinoform). The modelled lower velocity and density rudist shoal facies (orange on Figure 5) cause a time ‘push down’ in the underlying reflection 3, and should show a distinct low-impedance zone on the seismic. These seismic characteristics are caused by geological features and can thus be used to locate a rudist shoal platform margin within a low-angle platform setting. Reflection 4 marks the most basinal limit of the rudist shoal platform margin of Sequence I, and thus helps to delimit the probable extent of the underlying intrashelf basin. On the 40 Hz section (Figures 7c and 8c) reflection 4 appears to be one continuous reflection with decreasing amplitude and a waveform change at the edge of the platform, but does not show the detailed stratigraphic relationships of the platform margin seen at higher frequencies (Figures 7a, 7b, 8a and 8b). The higher frequency sections give an interpreter more confidence in identifying these subtle features as a rudist shoal platform margin.

In the intrashelf basinal setting, the areas with organic-rich limestones in cycles I-1 and III-2 (Figure 5) can be characterised by an intrashelf basinal seismic facies of continuous, high amplitude reflections that tie to the source rock beds (reflections above reflection 8 on Figure 7). With decreasing frequency (Figures 7a-7c) the number of reflections in this seismic facies decreases from two to one, probably due to interference between the layers. This seismic facies by itself would be hard to identify, but since it is commonly basinal and deeper than the overlying prograding platform margin geometry, its potential location could be predicted once the platform margin was identified.

The platform interior seismic facies is due to the erosional channels on the platform (cycles I-4 and on Figures 4 and 5). If the channels are deep enough, they can be imaged as reflections (reflection 5 on Figures 7a and 9a), but they are more often characterised by subtle amplitude and reflection continuity changes associated with the erosions (Figures 7b and 9b), or by disruption of underlying reflections due to velocity and density changes in the channel infill. On lower frequency data, these very subtle changes could be missed, or dismissed as faults or ‘noise’ in the seismic data, unless the interpreter was aware that they could occur within the platform setting. Once again, the identification of a platform margin geometry would be used to predict that one should find the platform interior seismic facies landward, and slightly higher in the section, from the platform margin geometry.

Although these distinct seismic characteristics do exist for a platform to low-angle carbonate ramp setting, the best interpretation is achieved by identifying all of these seismic characteristics and combining them to predict a specific depositional setting (i.e. carbonate platform bordered by an organic-rich intrashelf basin (Sequence I and III) or a flat to low-angle mixed carbonate-clay ramp (Sequence II)) and thus predicting reservoir and trap location from the lower frequency seismic.

A low-angle carbonate ramp bordered by an organic-rich intrashelf basin starts off with very slight progradation and aggradation (cycle I-2 of Figures 4 and 5). This is seen on the seismic sections with reflections 2 and 3 of Figures 7 and 9. This ramp then becomes more progradational and finally builds up a relief of a rudist shoal platform margin (cycle I-3 of Figures 4 and 5; Homewood, 1996). The rudist shoal platform margin is seen by reflection 4 on Figures 7 and 8. The progradational nature would be imaged in different ways depending on the thickness of the prograding units and their angles of progradation. In this model (Figure 5) the clinoforms range from 0.15° to 0.04°, but other regional work in the Natih E documents clinoform angles ranging from 1° to 2° (Droste and van Steenwinkel, 2004). This progradation is then overlain by shallower platform facies and erosional channels due to subaerial exposure (cycles I-4 and II-2 of Figures 4 and 5). The channels within cycle I-4 can be seen as amplitude changes, if they are in an area where cycle II-1 is below the tuning thickness (double reflection 5 on Figures 7a, 7b, 9a and 9b), or by time and amplitude distortion effects on underlying reflections. With this depositional setting, it is clear that one first needs to identify the rudist shoal platform margin, after which the intrashelf basinal organic-rich seismic facies can be predicted, as can the platform interior seismic facies. This depositional setting suggests that reflections within the platform should be organized from slight progradation to aggradation and then progradation of the platform margin over the ramp, followed by possible subaerial exposure. The details of thickness and direction of progradation within this setting will vary depending on local sedimentation patterns and subsidence.

The mixed carbonate-clay ramp is made of mainly flat, thin alternating layers of carbonate-dominated deposits and clay-dominated layers. These are characterised by the main reflections associated with Sequence II (reflections 6, 7, and 8 on Figure 7). These continuous reflections generally show the flat aggradational nature of the mixed ramp at this time (cycle II-1 and II-3). The slight amplitude change and rise in time along reflection 6 is due to a thickness change within the gently wedging unit of northwesterly prograding carbonates (cycle II-2). The higher impedance carbonate layer also causes a slight time ‘pull up’ in the underlying reflections (5, 4 and 1 on Figure 7). The high velocity and density hardgrounds associated with this setting do have an effect on the seismic, especially if they are located within thick intervals of mudstone and wackestones (cycle III-1), although they cannot be individually resolved.

All of these seismic characteristics can be seen on the higher peak frequency synthetic seismic (Figures 7a, 8a, and 9a), but could be missed on lower frequency data, i.e. 40 Hz industry standard (Figures 7c, 8c and 9c), unless the associated facies were sufficiently thick, i.e. greater than 40 m. The varying peak frequency synthetic seismic sections of the outcrop (Figure 7) show that these subtle seismic characteristics can be identified within the overall isopach ‘railroad track’ interval of the Natih Formation if one is expecting and looking for them.

COMPARISON WITH AN ACTUAL SEISMIC LINE

Synthetic seismic sections of outcrop-calibrated models can be used to ‘push’ the interpretation of subtler lower frequency industry seismic features in areas where a similar type of depositional/stratigraphic model can be expected (Schwab and Eberli, 2000). Figure 10a is an illustration of industry seismic, average 45 Hz peak frequency in this interval, taken from a subsurface data set parallel to the Adam Foothills Transect in the Jabal Madmar to Jabal Madar area (refer to Figure 2 for location). This seismic line is located in the platform margin setting in the area of rudist shoal progradation. The seismic has been flattened on the top of the Natih E (blue line in Figure 10) to take out later structuring in the area. There are two clear areas in the upper Natih E (cycle I-4) where changes in amplitude and phase are caused by channels (refer to the interpretation line drawing in Figure 10b). Amplitude maps that show similar channel geometries in plan view support the channel interpretation of this seismic facies.

Below the channels there appear to be low-angle clinoforms, with a flatter reflection geometry below the clinoforms (Figure 10). If we apply our high-resolution sequence stratigraphic outcrop based seismic characteristics of a carbonate-dominated platform from the Natih E to this seismic line, we would predict that the lower reflections show the more aggradational lower Natih E (cycle then changes into a more progradational nature (cycles I-2 and I-3 (Figure 10b)). This is incised by the erosional channels of cycles I-4 and II-1 at the top of the Natih E. This one-line interpretation would need to be integrated with other subsurface data, but nonetheless our depositional setting concept from the outcrop-based model can be applied to this data.

The clinoform angles might be steeper in this area, due to local topography on the platform and sedimentation patterns, than those shown on the outcrop model. Our regional model simply shows where one would expect progradation, and this seismic line confirms the predictions made by our observations between the Jabal Madar and Jabal Madmar outcrops (Figure 4). Thus the application of the detailed high-resolution sequence stratigraphic outcrop model to conventional seismic supports a more detailed interpretation of the location of the best reservoir facies, and laterally where one might expect source rock facies within the system.

CONCLUSIONS AND IMPLICATIONS

The comparison of analogue outcrop synthetic seismic based on a high-resolution sequence stratigraphic study, with actual seismic increases confidence in the geological interpretation of lower frequency industry seismic. Enhanced interpretation is done through the recognition of the significance of subtle seismic features. This study has shown the existence of subtle seismic characteristics associated with low-angle platform margins, platform/ramp interior erosions, and intrashelf organic-rich basinal facies, all on high frequency synthetic seismic data. The documented changes of these seismic characteristics on progressively lower frequency synthetic seismic data allows for an increased confidence in their interpretation on lower frequency industry seismic. The location of the platform margin is key in determining reservoir location, and setting the regional picture of where hydrocarbon source intervals could be found in adjacent intrashelf basins.

This low cost technique does not give the ‘right answer’, but will reduce exploration risks by helping to localize potential source and reservoir areas that could be located using seismic data. Carbonate intervals on industry seismic are commonly described as isopach ‘railroad track’ reflections, but through flattening of the section, or changing the scale, one may enhance subtle but diagnostic features. This study has shown the importance of using the highest possible frequency bandwidth to identify subtle changes in reflection character, i.e. amplitude, phase, time distortion and geometry, which are associated with facies changes.

This seismic forward modelling case study shows that there are changes in seismic geometries and facies identifiable along reflections on 2-D lines. Similar patterns should be visible on amplitude maps of the horizon. Since amplitude maps are in 3-D, similar patterns that would fit with the platform margin or platform interior channelling would improve confidence in the interpretation of these subtle seismic features seen on lower frequency seismic data. Seismic characteristics similar to the outcrop-based synthetic seismic modelling of this study have been recently published for a 3-D seismic study in the Natih E (Masaferro et al., 2003).

Along the Adam Foothills Transect, Sequence I within the Natih E and F exhibits the full suite of stratal geometries and facies encountered within the whole Natih Formation studied here. Sequence I begins as a flat to low-angle mixed carbonate/clay ramp and then evolves into a carbonate platform with an adjacent organic-rich intrashelf basin. These characteristics and geometries were modelled on the synthetic seismic sections of the outcrop. The results of this study may be applied to similar settings within the Natih Formation, or throughout the Middle East where similar depositional styles of carbonate platforms occurred (i.e. the Mishrif and the Shu’aiba Formations). When applying the results of this study, it is important to recognize that local sedimentation patterns and subsidence can affect the details within the depositional phases of platform and ramp construction.

ACKNOWLEDGEMENTS

This work was part of a joint industry project by ELF EP, Institut Français du Pétrole (IFP) and Bureau de Recherches Géologiques et Minières (BRGM), sponsored by the FSH/COPREP. Publication of this article is by the kind permission of Total, IFP, BRGM, Petroleum Development Oman and the Ministry of Oil and Gas of the Sultanate of Oman. We thank V. Bigault-de-Cazanove for running the model, J.B. Fulton and C. Tiltman for drafting support, H. Sayer for reduction of synthetic seismic, J.P. Leduc, O. Ridet and J. Roger for aid in sample collection, and M. Mettraux for thin section analyses. We also thank GeoArabia’s two anonymous reviewers for their thorough and constructive comments, which have improved the manuscript.

ABOUT THE AUTHORS

Anne M. Schwab has been a Geophysics Lecturer at the University of Aberdeen, Scotland since 2000. Previous to that she was a Senior Geophysical Advisor with Elf UK in Aberdeen (1997-2000), a Seismic Stratigrapher with Elf EP in Pau (1990-1997), and a Senior Geophysicist with Exxon USA in Denver (1982-1990). Anne’s research interests include seismic forward modelling of outcrops, seismic stratigraphy in carbonates and clastics, and regional paleogeographic studies. She received a PhD in Geology in 2003 from the University of Aberdeen, Scotland.

a.schwab@abdn.ac.uk

Peter W. Homewood is Professor of Carbonate Geology and Director of the Shell-endowed Carbonate Research Centre at Sultan Qaboos University, Oman. Previous to that he was Senior Advisor for Sedimentology at Elf EP and then TotalFinaElf. Peter has taught at universities in Switzerland and France. He served as Editor of Sedimentology (1986-1990), International Association of Sedimentologists Publications secretary (1990-1994), and AAPG European Distinguished Lecturer (1998-1999). In 1995 Peter received the Elf Science Prize and the TotalFinaElf communications award in 2000.

homewood@squ.edu.om

Frans S.P. van Buchem is a Senior Research Scientist at the Geology-Geochemistry Division of the Institut Français du Pétrole (IFP). Since 1990 he has studied the sedimentology, (organic) geochemistry, and sequence stratigraphy of carbonate systems and source rocks through time in outcrop analogs and the subsurface of the petroleum provinces of western Canada, the western United States, northern Africa, and the Middle East. He received a PhD in Geology in 1990 from the University of Cambridge, United Kingdom.

frans.van-buchem@ifp.fr

Philippe Razin is Professor of Geology at the University of Bordeaux, France. He received his Doctoral of Science degree from the University of Bordeaux in 1989 and joined the Bureau de Recherches Géologiques et Minières as an expert in sedimentology and basin synthesis. Philippe was involved in various projects (mapping, water and mineral exploration, geotechnics, 3-D modelling). He moved to the University of Bordeaux in 1997, where he teaches sedimentary and structural geology, geodynamics and field mapping. His research activities concern relations between tectonic and sedimentation, in collaboration with BRGM, IFP, IFREMER and oil companies.

razin@egid.u-bordeaux.fr