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Abstract

Across the west Nile Delta, channel complex, channelized lobe, and channel-levee turbidite reservoir systems were deposited throughout the Pliocene following the Messinian salinity crisis and the reestablishment of a muddy depositional slope on the Nile Delta cone. Commercial gas discoveries driven by seismic amplitude anomalies in all of these different turbidite reservoir architectures (Ruby, Fayoum, and Giza fields) are dispersed around the modern day Rosetta Canyon, in water depths ranging from 300m to 900m.

Structurally, the west Nile Delta (WND) is characterized by steep, fault-bounded margins which exerted a fundamental control on the stratigraphic position and fill of slope canyon and channel systems in the Pliocene play fairway. Syndepositional slope collapse has had a significant impact on the development of these slope reservoir systems. The Giza Field gas accumulation is an upper-slope channel complex set characterized by a 160m erosional confinement within a 2.5km wide fairway draping a 20 X 10 km wide plunging anticline, setting up a combination structural-stratigraphic trap. Down slope, the Giza channel complex set can be tracked for a distance of >100 km into a constructional levee confined system on the lower slope.

Visualization of the internal geometry of the Giza channel complex set is based on 3D multiazimuth (MAZ) seismic data tied to extensive conventional core data recovered from both the exploration discovery well and a subsequent appraisal well. The high resolution seismic, combined with log- and corescale observations, provide spectacular insights into the gross seismic architecture, internal geometry, and stacking patterns of the Giza channel complex set.

This paper will demonstrate the facies change and channel geometry variation with the Giza channel reservoir fairway, from incision and bypass, to the initial backfill within a low sinuosity aggradational stacked channel phase, to a more sinuous constructional channel levee fill style having ‘levee lobes,’ to ultimate channel abandonment. Fundamental controls on sedimentation patterns are controlled by a combination of a basin-bounding tectonic control on sediment input points, mass transport processes on the slope generated (generation of accommodation space for precursor lobes), and a deep seated intrabasinal tectonic control that has episodically generated subtle, emergent topography, which has, in part, controlled channel element sinuosity and net to gross.

Geological Setting

The Nile Delta lies on the northern margin of the African plate and has evolved through multiple rift episodes in the Triassic-Early Cretaceous, through a shortlived passive margin phase in the Cretaceous, and into an active compressional margin since the Late Cretaceous. Today, the Nile Delta is surrounded by active compressional belts which have important implications for the types of structures that develop across the Nile Delta.

The development of the Pliocene slope systems in the west Nile Delta is largely controlled by both the Coastal and Rosetta fault systems (Fig. 1). Extensional movement on these faults is also considered responsible for the generation of the northeast-southwest oriented west Nile (Raven-Taurus) anticline that sets up the structural trapping component of many of the fields in this area.

Figure 1.

Top mid-Pliocene P80 structure map of the West Nile Delta (inset). The Coastal and Rosetta fault trends set up the structural focus for the input of clastics into the deep-water slope systems of the west Nile Delta. The P80 slope reservoirs drape the regional structural anticline from Fayoum to Polaris fields. Subregional strike seismic section referenced in Figure 2. Lines A-D are seismic strike reflectivity sections through the Giza channel complex set over a downdip distance of 70km (Fig. 3).

Figure 1.

Top mid-Pliocene P80 structure map of the West Nile Delta (inset). The Coastal and Rosetta fault trends set up the structural focus for the input of clastics into the deep-water slope systems of the west Nile Delta. The P80 slope reservoirs drape the regional structural anticline from Fayoum to Polaris fields. Subregional strike seismic section referenced in Figure 2. Lines A-D are seismic strike reflectivity sections through the Giza channel complex set over a downdip distance of 70km (Fig. 3).

During the early Tertiary, there was a major clastic influx from the proto-Nile system and the development of multiple feeder systems that developed offshore into slope turbidite channels and more distal basin-floor fans. During the late Miocene, a regional Mediterranean lowstand event resulted in the Messinian salinity crisis and widespread deposition of evaporite deposits and the development of deeply entrenched incised valleys.

After the Messinian salinity crisis, the Plio-Pleistocene represented a return to deep marine conditions. Thick deposits of Kafr El Sheikh marine shales preceded the progradation of the present day Nile Delta and the offshore turbidite sands that traversed the slope. These deposits manifest themselves as a series of compensationally stacked channel-levee, lobe, and channel complex sets within a progradational mud-dominated slope. The deeper seated Raven-Taurus anticline has grown episodically throughout Pliocene times, and the northwesterly draping turbidite channels that cross these structures form the three-way components to the trap. The combination of subtle structural growth, and regular, episodic slope collapse at a variety of different scales, are instrumental in controlling the slope position and internal fill of stacked Pliocene gas reservoirs (Fig. 2).

Figure 2.

Subregional seismic strike cross section through the Plio-Pleistocene of the west Nile Delta, illustrating the range in slope reservoir systems from channel complex sets, lobes, and levees separated by laterally extensive seismically mapable condensed sections. Note the prevalent occurrence of the deposits of slope collapse throughout the Plio-Pleistocene, ranging from km scale slide blocks to extensive muddy slides, mass transport complexes (MTC’s) and debris flows. The middle Pliocene P80 Giza Field channel complex set is highlighted, flanked by precursor lobes and capped by the compressional toe to a muddy slide that acts as the top seal.

Figure 2.

Subregional seismic strike cross section through the Plio-Pleistocene of the west Nile Delta, illustrating the range in slope reservoir systems from channel complex sets, lobes, and levees separated by laterally extensive seismically mapable condensed sections. Note the prevalent occurrence of the deposits of slope collapse throughout the Plio-Pleistocene, ranging from km scale slide blocks to extensive muddy slides, mass transport complexes (MTC’s) and debris flows. The middle Pliocene P80 Giza Field channel complex set is highlighted, flanked by precursor lobes and capped by the compressional toe to a muddy slide that acts as the top seal.

Seismic Data

Over 9000 sq km of conventional 3D regional seismic data has been acquired over the west Nile Delta and processed to provide seamless cover across the main structures in the area. Pliocene sands exhibit typical Class 3 AVO anomalies in the presence of gas and manifest themselves as seismically distinct, contrasting-bright reflectivity packages. The rock properties of wet sands and shales renders them virtually indistinguishable from one another in full stack domain, though partial stacking of the near offset data can sometimes illuminate wet aquifer sands beneath the gas legs.

Multiazimuth (MAZ) seismic data (five new azimuths of streamer seismic data) were acquired in 2004 over the Giza field, covering an area of approximately 3000 km2. The existing vintage seismic data shot in 2000 was merged as a sixth azimuth to create an acquisition geometry across every 30 degree azimuth on a 25 m bin size grid (Rietveld et al., in press).

An additional MAZ seismic survey was acquired subsequently in 2006 over the southern extent of the Giza and Fayoum fields based on three azimuths merged with the original MAZ survey. In 2008, the existing data sets were reprocessed jointly through a prestack time migration work flow onto a common 12.5 x 12.5 m grid. The resulting seismic image provided much improved temporal and spatial resolution compared to the legacy data, having a seismic bandwidth of up to 90Hz at reservoir level (Keggin et al., 2006; 2009).

Giza Field

The Giza Field is located in the North Alexandra B concession, 50 km offshore and 10 km west of the modern day termination of the Rosetta channel, and comprises a stacked sequence of slope channel-levee, lobe and channel complex reservoir systems of P75-P80 mid Pliocene age (Fig. 2). The P80 sequence can be traced for >100km from a upper to mid slope setting within the west Nile Delta offshore and shows a clear evolution from an entrenched channel complex set to a levee bounded channel complex set downdip (Fig. 3). The southernmost updip limits of the channel complex set can be mapped to the confluence of two major fault systems that parallel the offshore delta; the Rosetta and the Coastal fault system (Fig. 1). These two fault trends converge in this region and create a point source from which the delta sediment feeder systems transported sediments to the offshore delta.

Figure 3.

Downslope evolution of the Giza channel complex system. Location of the MAZ seismic cross-sections are referenced in Figure 1. Section (A) from the mid Pliocene upper-to-mid slope at Giza South (estimated to be ~40km from the mid-Pliocene shelf break) where the Giza system is erosionally confined, through to section (D) on the mid-to-lower slope at Treacle (60km downslope from Giza South) where the system is largely constructionally confined within outer levees.

Figure 3.

Downslope evolution of the Giza channel complex system. Location of the MAZ seismic cross-sections are referenced in Figure 1. Section (A) from the mid Pliocene upper-to-mid slope at Giza South (estimated to be ~40km from the mid-Pliocene shelf break) where the Giza system is erosionally confined, through to section (D) on the mid-to-lower slope at Treacle (60km downslope from Giza South) where the system is largely constructionally confined within outer levees.

NAB-1, the first well penetration on the Giza structure was drilled in 1978 on 2D seismic data and targeted a deep Messinian structural closure. Although NAB-1 penetrated the axial heart of the P80 slope channel fairway, the well was unsuccessful at the P80 level, encountering a mud and silt-dominated interval. This interval was interpreted as an internal mud-rich channel terrace, and the major depositional fairway was to the west and east of the well penetration (Fig. 4).

Figure 4.

(A) RMS attribute extraction based on 2008 reprocessed MAZ data over Giza and Fayoum fields, showing the Giza channel complex set reservoir fairway and the Giza North-1, Giza South-1, and NAB-1 wells. (B) Composite logs for the Giza North-1 and Giza South-1 wells reflect the change in style of fill of the channel complex set over a distance of 15km downdip.

Figure 4.

(A) RMS attribute extraction based on 2008 reprocessed MAZ data over Giza and Fayoum fields, showing the Giza channel complex set reservoir fairway and the Giza North-1, Giza South-1, and NAB-1 wells. (B) Composite logs for the Giza North-1 and Giza South-1 wells reflect the change in style of fill of the channel complex set over a distance of 15km downdip.

The Giza P80 level discovery well (Giza North-1) was drilled in 2007 and encountered a gross interval of 160m containing excellent reservoir-quality sands (30% porosity, Darcy permeability, and 45% net to gross). In 2009 the Giza South-1 well drilled the P80 interval 15km up depositional dip from the discovery well on the downdip extension of the Fayoum crestal high. The Giza South-1 well formed the conclusion to an extensive appraisal program of the P80 Giza channel complex reservoir, including 350m of conventional core and the acquisition of an extensive wireline log suite.

Slope Channel Terminology

The hierarchy, scales, stacking patterns and naming conventions used in the description of the Giza field are similar to those of Sprague et al. (2002) (Figs. 5A, 5B).

  • Channel complex set: ~1.5 -2.5 km wide and ~ 150-190m thick.

  • Channel complex: <1km wide and ~50m thick.

  • Channel element: ~250m wide and 10-15m thick.

  • Leveed channel: ~250m wide, 10-15m thick, and posessing asymmetric levees up to 1km wide.

Figure 5A.

Strike reflectivity seismic section across the Giza North channel complex set. Note the different scales and geometries of the two large scale erosion surfaces: an early precursor slide scar (red), the deep entrenchment of the channel complex set (blue), bounded by the P78 and P80 mid Pliocene condensed sections (CS). The terminology for the scale of “channels” within the fill used in this paper are highlighted: channel complex set – channel complexes – channel elements. Giza North 1 well logs: green = gamma ray; red = resistivity.

Figure 5A.

Strike reflectivity seismic section across the Giza North channel complex set. Note the different scales and geometries of the two large scale erosion surfaces: an early precursor slide scar (red), the deep entrenchment of the channel complex set (blue), bounded by the P78 and P80 mid Pliocene condensed sections (CS). The terminology for the scale of “channels” within the fill used in this paper are highlighted: channel complex set – channel complexes – channel elements. Giza North 1 well logs: green = gamma ray; red = resistivity.

Figure 5B.

Generic geoseismic channel complex set fill interpretation based on the Giza Field at Giza North 1 well. There are four main stages in the evolution of the fill of each channel complex in the west Nile Delta mid slope setting:

Stage I: Erosion and sediment bypass, characterised by channel lags, debrites and slide blocks|

Stage II: Aggradational Fill dominated by sinuous channel elements (200m wide; 15 m deep) stacked within channel complexes (~1km wide; 50m deep).

Stage III: “Switch-off” is always associated with numerous bypass/erosion surfaces, and the broadening out of the accommodation space within the channel complex set

Stage IV: Constructional Fill of channel complex dominated by lobes, sand back-filled channel elements and (mud back-filled) leveed channels.

Figure 5B.

Generic geoseismic channel complex set fill interpretation based on the Giza Field at Giza North 1 well. There are four main stages in the evolution of the fill of each channel complex in the west Nile Delta mid slope setting:

Stage I: Erosion and sediment bypass, characterised by channel lags, debrites and slide blocks|

Stage II: Aggradational Fill dominated by sinuous channel elements (200m wide; 15 m deep) stacked within channel complexes (~1km wide; 50m deep).

Stage III: “Switch-off” is always associated with numerous bypass/erosion surfaces, and the broadening out of the accommodation space within the channel complex set

Stage IV: Constructional Fill of channel complex dominated by lobes, sand back-filled channel elements and (mud back-filled) leveed channels.

Evolution of the Giza Slope Channel Complex Set

Across the Giza Field area, the P80 reservoir interval is up to 160m thick and bounded by condensed sections clearly visible on seismic and logs (Fig. 5A). Within this parasequence there are two key erosional surfaces at different scales and geometries. The evolution of the fill of the P80 channel complex set is discussed below, and is subdivided into four key stages (Fig. 5B):

Stage 0._Reestablishment of a progradational muddy slope

Following the mid-Pliocene P78 deep-water condensation, a mud-silt dominated slope sequence was reestablished in the area of the west Nile Delta. The initial incursion of P80-aged system onto the slope is thought to have utilized the preexisting topography, as a small scale leveed channel, which has been documented across the Nile Delta slope (Felt et al., 2004). It is asserted that the initial relative sea-level fall and the resulting changes in pore pressures at the sea floor, coupled with an episodic deeper seated tectonic control, generated subtle, shallow detached slides (~15-20m deep) on the downdip side of the crest of the Giza structure. The development of the slides in turn generated the accommodation space for the deposition of mid-slope lobes (the so-called “precursor lobes”). The nature of the low relief truncation and the seismic expression of the lobe bounded by the extensional head hinged at the deeper seated structural crest is a common feature of the these slope systems from the Pliocene to the present day (Fig. 6) (Butterworth et al., 2006).

Figure 6.

The Autonomous Underwater Vehicle (AUV) imaged present day seafloor topography defines a modern analogue for the generation of accommodation space on the upper slope by shallow detached slides, where the extensional head is hinged at the structural crest. The surface expression of the deeper seated anticline can be seen in the dip reversal on the modern day slope terrace towards the back of the image. The geological model is that early leveed channels supplied sand into these paleotopographic lows to create the “precursor” lobes imaged on the seismic. The RMS seismic amplitude of the P80 Giza Field highlighting the deposition of a precursor lobe, immediately downdip of the structural crest of the field, clearly defined by the propagation of faults at the deeper seated structural crest.

Figure 6.

The Autonomous Underwater Vehicle (AUV) imaged present day seafloor topography defines a modern analogue for the generation of accommodation space on the upper slope by shallow detached slides, where the extensional head is hinged at the structural crest. The surface expression of the deeper seated anticline can be seen in the dip reversal on the modern day slope terrace towards the back of the image. The geological model is that early leveed channels supplied sand into these paleotopographic lows to create the “precursor” lobes imaged on the seismic. The RMS seismic amplitude of the P80 Giza Field highlighting the deposition of a precursor lobe, immediately downdip of the structural crest of the field, clearly defined by the propagation of faults at the deeper seated structural crest.

Across the upper to middle west Nile Delta slope, the impact of these shallow slides on the development of accommodation space has resulted in the deposition of a series of these lobes of similar dimensions (5km2 by 15m thick) pinned at a similar relative slope position. The expression of these processes is also observed on the present day slope and indicates that the creation of accommodation space through slope failure is a long lived feature of slope evolution irrevocably tied to a deeper seated tectonic control. The resulting “terminal lobes” and their leveed-channel feeder systems are the precursor to the development of the deeply incised channel complex set.

Channel Complex Set Stage I. “Erosion and sediment bypass”

Subsequently, as a result of slope progradation and lowering sea-level (and the probable exposure of the shelf-slope break), the preexisting channel-levee sediment fairway was reactivated and deep entrenchment of the slope channel complex set into the predominantly muddy slope occurred (Fig. 5A).

During the initial phase of erosion and sediment by-pass, the channel base is defined by rotational slide blocks outside of the channel complex set margins, representing outer bank slope instability and collapse. The slide blocks consist of laminated silts and muddy turbidites of the precursor slope system (Fig. 7). The initial basal fill is composed of muddy debrites and slide blocks that possess seismically dim, chaotic reflectivity (Figs. 8A, 8B). These debrites may be locally or entirely eroded and therefore not present in all parts of the channel. Episodic, high energy conditions are characterized by basal conglomeratic lag deposits, the remnants of channel elements supplying sediment downdip (Fig. 9). The lag deposits may be preserved as erosional remnants within the debrite envelope, and formation pressure data from several wells indicate these may or may not be in pressure isolation from the main sandstone fill of the channel. Basal lag deposits are also commonly the habitat for perched water.

Figure 7.

Seismic colored inversion cross section of the Giza channel complex set and the laterally adjacent precursor lobe. Collapse features in the lobe are analogous with the slope failure observed on the banks of the modern day Rosetta Channel.

Figure 7.

Seismic colored inversion cross section of the Giza channel complex set and the laterally adjacent precursor lobe. Collapse features in the lobe are analogous with the slope failure observed on the banks of the modern day Rosetta Channel.

Figure 8A.

Seismic colored inversion strike section through Giza North illustrating the larger scale stacking patterns and geological interpretation of the channel complex set fill.

Figure 8A.

Seismic colored inversion strike section through Giza North illustrating the larger scale stacking patterns and geological interpretation of the channel complex set fill.

Figure 8B.

CI-flattened time slices from the P80 flooding surface through the Giza Field channel complex set illustrating the evolution of the channel element stacking patterns throughout the successive stages of the fill. Note the increasing sinuosity in the younger channel elements.

Figure 8B.

CI-flattened time slices from the P80 flooding surface through the Giza Field channel complex set illustrating the evolution of the channel element stacking patterns throughout the successive stages of the fill. Note the increasing sinuosity in the younger channel elements.

Figure 9.

Stage I. Incision and Bypass. Conventional core images of sharp-based channel axis lag (A), sandstone injection (B), syndepositional deformation (C) and ‘external’ slope mudstone slumps within the basal confinement (D). Perched water is common in these basal sandstone lags. Seismic cross-section indicates relative position within the fill of the channel complex set.

Figure 9.

Stage I. Incision and Bypass. Conventional core images of sharp-based channel axis lag (A), sandstone injection (B), syndepositional deformation (C) and ‘external’ slope mudstone slumps within the basal confinement (D). Perched water is common in these basal sandstone lags. Seismic cross-section indicates relative position within the fill of the channel complex set.

Channel Complex Set Stage II. “Aggradational Fill”

Stage II of the channel complex set evolution is seismically most conspicuous on the northern downdip flank of the Giza structure, and for this reason, can often be misinterpreted as the ultimate base of the channel fill. This unit is made up of amalgamated fining-up intervals of pebbly sandstones/conglomerates and coarse- to fine-grained sands as channel complexes (~30-40m thick), confined by internal erosion surfaces within the basal fill of the channel complex set (Fig. 5B). These channel complexes reflect the vertical stack and fill of erosional channel elements interbedded with channel margin thin bedded fine grained sandstones and siltstones (Figs. 10A, 10B).

Figure 10A.

Stage II. Aggradational Fill. Conventional core images from Giza North of the backfill facies of channel elements, interpreted as the preservation of channel axis sandstones (A,B) and abandonment facies (C).

Figure 10A.

Stage II. Aggradational Fill. Conventional core images from Giza North of the backfill facies of channel elements, interpreted as the preservation of channel axis sandstones (A,B) and abandonment facies (C).

Figure 10B.

Stage II. Aggradational Fill. Conventional core images from Giza South of the backfill facies of channel elements, comprising frequent bypass surfaces, sandy debrites (A’), syndepositional deformation and injection (B’) and local climbing ripple cross-laminated internal levees (C’). Much of this interval is interpreted as the preservation of channel margin sandstones.

Figure 10B.

Stage II. Aggradational Fill. Conventional core images from Giza South of the backfill facies of channel elements, comprising frequent bypass surfaces, sandy debrites (A’), syndepositional deformation and injection (B’) and local climbing ripple cross-laminated internal levees (C’). Much of this interval is interpreted as the preservation of channel margin sandstones.

Stage II is made up of a series of laterally migrating thalwegs of the channel element, controlled in part by the larger scale channel complex. The channel elements show evidence of repeated incision and clinoforms that dip into the main axis. Seismic attribute extractions suggest deposition as sinuous ribbons of channel elements 10-15m thick typically on the outer bend of both the channel complex and channel complex set (Fig. 8A). The channel margin sediments are seismically dim facies and are composed of thin bedded sands and silts.

The high permeability units offer the best reservoir potential within the overall channel complex set; but their significantly higher Kh values suggest they may be potential high permeability drains that require careful reservoir management.

On the northern flank of the Giza structure, these channel elements are often repeatedly incised into one another; there is preferentional preservation of channel axis facies, which assists vertical permeability and connectivity. To the south, the sedimentary record is preserved as predominantly thin bedded channel margin facies, suggestive of episodic erosion and bypass without the backfill of channel element axial deposits.

Channel Complex Set Stage III. “Channel “Switch-off”

The switch from Phase II to IV marks a change in depositional style reflected in the increased sinuosity and backfill of channel elements. This unit is characterized by a 15-30m thick interval dominated by debris flow deposits, sandy lags, extensive bioturbation, silty turbidites and well developed Glossinfungites surfaces (Fig. 11). This interval is interpreted as a phase of “bypass” and “switch-off” of the backfill to the channel complex set. This is in part controlled by the accommodation space within the channel complex set from erosionally confined to more levee confined accommodation space (Figs. 5A, 5B); however, there is almost certainly an overprint from an allocyclic control associated with the initiation of sea-level rise and the delivery of sediment into the basin.

Figure 11.

Stage III. “Switch-off”. Conventional core images of the facies that subdivide the aggradational from the constructional channel complex set fill. Note the presence of sharp topped sandstones (A), synsedimentary deformation (B), sandstone injection (C) and extensive bioturbation (D).

Figure 11.

Stage III. “Switch-off”. Conventional core images of the facies that subdivide the aggradational from the constructional channel complex set fill. Note the presence of sharp topped sandstones (A), synsedimentary deformation (B), sandstone injection (C) and extensive bioturbation (D).

The widespread development of the “switch-off stage” has important implications for connectivity. Geochemical data from gases indicate that the constructional component is filled independently of the aggradational component of the channel complex set. This is supported by the residual salt analysis data from the cores, as the constructional and aggradational fill are on different gradients. Over geological time, the reservoir pressures now lie on a single gradient, but the evidence of differential gas charge into the channel complex set at Giza Field suggests that the internal baffle of the “switch-off” stage will act as as a barrier during production. stage.

Channel Complex Set Stage IV. “Constructional Fill” Abandonment phase (Channel Mud-filled; Levees Sandy)

Stage IV of the fill of the channel complex set is characterized by sinuous channel element and leveed channel abandonment elements, and a distinctly finer grained (very fine- to medium-grained sand) backfill. The latter are comprised of mud back-filled channel and finely interbedded laminated sands and shales as sandy aggradational levees. Core (Fig. 12) and pressure data indicate that vertical connectivity is enhanced by burrows and injected sandstones. The MAZ seismic defines very characteristic wedge-shaped geometry; the topographic relief at the channel margins is indicative of a leveed system (Figs. 8A, 8B). These strongly outer bend asymmetric levees typically have a topographic relief in excess of 10m and, although thinly bedded, are characterized by high net to gross (>60%) proximal to the leveed crest (within 100-200m). These leveed channel systems represent the final stages of channel abandonment. However, in plan view there are some interesting geometries developed in these late stage “levees” downdip of the deeper seated structural crest, which are developed as coalesced terminal lobes to channel elements. These have been deposited at the terminus of a channel element, and successively backfilled and avulsed (Figs. 13, 14A, 14B).

Figure 12.

Stage IV Constructional Stage. Conventional core images of the facies stacking patterns in the levee and lobe deposits of the late stage constructional fill to channel complex sets. Note the fine grained low density turbidites with synsedimentary deformation and vertical burrows (A,B), mud-flake breccia bypass surfaces (C) and climbing ripple cross-laminated proximal levee deposits (D).

Figure 12.

Stage IV Constructional Stage. Conventional core images of the facies stacking patterns in the levee and lobe deposits of the late stage constructional fill to channel complex sets. Note the fine grained low density turbidites with synsedimentary deformation and vertical burrows (A,B), mud-flake breccia bypass surfaces (C) and climbing ripple cross-laminated proximal levee deposits (D).

Figure 13.

P80 structure map overlain by the vertical stacked channel element thalwegs mapped on the MAZ seismic. Along the 30 km downdip evolution of the Giza channel complex set, three zones are separated according to the relative position of the deeper seated structure (the green oval representing the middle area between present day structural highs). These channel elements are the basic building blocks of the fill of the channel complex set. Note the relatively straight channel elements immediately updip of the Giza anticline, thought to reflect local down cutting and sediment bypass resulting from subtle syndepositional structural uplift.

Figure 13.

P80 structure map overlain by the vertical stacked channel element thalwegs mapped on the MAZ seismic. Along the 30 km downdip evolution of the Giza channel complex set, three zones are separated according to the relative position of the deeper seated structure (the green oval representing the middle area between present day structural highs). These channel elements are the basic building blocks of the fill of the channel complex set. Note the relatively straight channel elements immediately updip of the Giza anticline, thought to reflect local down cutting and sediment bypass resulting from subtle syndepositional structural uplift.

Figure 14A.

3D visualisation of the aggradational (A) and constructional (B) components of the fill of the Giza channel complex set, viewed from the north looking south up depositional dip within the channel complex set. (A). Relatively low sinuosity channel elements with a clear on and off axis within the aggradational fill of the channel complex set.

Figure 14A.

3D visualisation of the aggradational (A) and constructional (B) components of the fill of the Giza channel complex set, viewed from the north looking south up depositional dip within the channel complex set. (A). Relatively low sinuosity channel elements with a clear on and off axis within the aggradational fill of the channel complex set.

Figure 14B.

3D visualisation of the aggradational (A) and constructional (B) components of the fill of the Giza channel complex set, viewed from the north looking south up depositional dip within the channel complex set. (B). Note the late stage development of lobes fed from channel elements that backfill, avulse and switch over the opposite side of the channel complex set, generating a characteristic late stage leveed-lobe fill within a weakly confined system.

Figure 14B.

3D visualisation of the aggradational (A) and constructional (B) components of the fill of the Giza channel complex set, viewed from the north looking south up depositional dip within the channel complex set. (B). Note the late stage development of lobes fed from channel elements that backfill, avulse and switch over the opposite side of the channel complex set, generating a characteristic late stage leveed-lobe fill within a weakly confined system.

Stage V. Slope Abandonment

Cessation of the channel complex set backfill is an abrupt cessation of clastic supply within this relatively long-lived sediment fairway into a mud-dominated slope. The partial top seal to the Giza Field is formed by a well-developed large scale (10’s km2) muddy slide deposit (Fig. 5A). This in turn is overlain by a single leveed channel system that continued to utilize the older channel complex set fairway on the upper slope. However, this system abruptly avulses to the west in a position on the updip side of the deeper seated Giza structure, suggestive again of a subtle deeper seated tectonic control on the evolution of these constructional leveed channel reservoirs on an unconfined mid to upper slope.

Evolution of Channel Element-Scale Stacking Patterns Through the Evolutional Backfill of a Channel Complex Set

The Giza channel complex set fill is similar to that documented elsewhere in the basin (Samuel et al., 2003; Katamish et al., 2005). However, the influence of paleotopography shaped by underlying tectonic controls is one the most striking features in the make-up of the Giza channel complex set. Marked changes in the channel element stacking patterns are clearly imaged on the high-resolution MAZ seismic and tied to the well control points of Giza North-1 and Giza South-1 as the system evolves over a moderately uneven upper-slope location.

The description of the downdip evolution of the upper slope Giza channel complex set backfill history is subdivided into three ‘zones’: updip of the Giza anticline; over the Giza anticline; and downdip of the Giza anticline (Fig. 13).

Proximal “Updip” Channel Complex Set Fill Geometries (Giza South-1 Well)

In the southern area covered by the 2008 reprocessed MAZ seismic data, the Giza channel complex set runs along the eastern flank of the Fayoum structure, a subtle paleo-high thought to be present at the time of deposition. Throughout the successive fill stages, channel elements are fairly straight, have wavelengths typically around 0.8 to 1.8km,and amplitudes of 0.6-1.1km (Fig. 13). They show a tendency of vertical aggradation so that the pathway of a previous channel element controls that of the next. This behavior is suggestive of relatively mud-prone depositional conditions whereby muddy overbank deposits of the channel are able to confine the throughput and hence strongly control the pathway of successive channels (McHargue et al., 2011), or alternatively that there was successive channel element cut and fill with preservation of channel margin facies only.

Only in a few instances are clear switch-overs observed, after which the process of vertical aggradation reestablishes itself. Throughout the fill of the channel complex set, a moderate increase in the degree of sinuosity of the channel elements is observed. This is paired with a stacking pattern that continues to be very organized. The Giza South-1 well penetrated the channel complex set in this setting, slightly offset from the main axis of channel aggradation. If not completely absent, the fill characteristics described in stage II of the model mainly reflect the preservation of channel margin deposits at Giza South-1 well.

Channel Complex Set Fill Geometries on Southern Limb of Giza Anticline

Throughout the central area, situated within a structural saddle between the Giza and Fayoum anticlines having gentler slope gradients, the pattern of vertical aggradational and organized stacking remains in place. In contrast to the stacking patterns observed in the south however, the channels are characterized by significantly longer wavelengths ranging from 2.5-3.5 km and much larger amplitudes in the order of 1.4-2.4 km. The channel amplitudes further increase in the later stages of the channel fill (Fig. 13).

The change in wavelength and amplitude of the channel elements is thought to be a response to the subtle change in slope topography as the channel complex set approaches the deeper seated structural crest. The straightness of the channel elements is interpreted to reflect kick point migration as the channel elements kept pace with structural growth on the Giza anticline and represent an area of sediment bypass.

Distal “Downdip” Channel Complex Set Fill Geometries (Giza North-1 Well)

On the north flank of the Giza structure, the initiation of the Giza P80 slope system is marked by the development of a precursor splay. The position of the precursor splay and further evolution of the channel complex set geometry are clear indicators of the evolving relief of the Giza anticline and the constant reshaping of paleoslope topography.

Structural growth continues to express its influence throughout the fill of the channel complex set. Further upslope between the Fayoum and Giza structures, a stacking pattern controlled by vertical aggradation and long wavelength, large amplitude channel elements unfolded. On the north flank of the structure however, repeated cut and fill of highly sinuous, short wavelength channel elements are observed in the earlier stages of the channel complex set fill, which have preferential preservation of channel axis facies. On the MAZ seismic, channel elements can therefore only occasionally be traced out and only over short distance (Fig. 13).

The repeated cut and fill behavior of the channel elements is thought to be symptomatic of a relatively sandier depositional setting where overbank deposits do not get fully established to control the pathway of successive channel elements. This rapid evolution in the depositional setting as the system makes its way across the Giza structure and onto the north flank can be traced back to the abrupt changes in backfill and flow segregation conditions. Whereas south from the Giza structure the system is governed by bypassing sand in the long wavelength sinuous channel elements, thicker bedded sandy axial fills in highly sinuous incising channels are observed over on the north flank of the actively growing Giza structure (Fig. 6).

As a consequence, the Stage II aggradational fill is well developed as a high net to gross section in the Giza North-1 well in comparison to the Giza South-1 well. It forms the clearest demonstration of the tectonic control on the stacking patterns observed along the Giza channel complex set.

Throughout the later stages of the fill, stacking patterns characterized by vertical aggradation (Stage IV) return. Disorganized highly sinuous channel elements conclude the final stages of the channel complex set fill (Fig. 6).

Farther down on the northern flank of the Giza structure and below the gas-water contact, the internal architecture is less clearly imaged on seismic due to the limited acoustic contrast between the brine-filled sands and shales. Nonetheless, further changes in the stacking patterns are apparent as the system reverts back to the highly sinuous organized stacking patterns.

Deep-Seated Syndepositional Structural Growth and the Impact on Channel Complex Set Fill Geometries and Net Sand Distribution

Although, the Giza channel complex set gross thickness remains relatively constant along the length of the system over the Giza-Fayoum structures covered by MAZ seismic, the net sand distribution shows marked changes. The discrepancy between the lower net encountered by the Giza South-1 well and the high net observed in the Giza North-1 well on the north flank of the structure is in part a function of the location of the well relative to the axis where channel aggradation is concentrated. Nonetheless, it is thought the trend in net distribution observed between Giza North-1 and Giza South-1 to a great extend reflects the dramatic change in stacking patterns with the stage II aggradational fill only fully developed on the northern limb of the structure. This pattern is not restricted to Giza alone and has been observed in several other channel complex sets in the west Nile Delta (Fig. 15). Consequently, this downdip facies variation documented in spatial relation to the position of subtly growing deeper seated structural highs is not restricted to the Giza-Fayoum area during the evolution and backfill of channel complex sets throughout the Pliocene on the upper to midslope of the west Nile Delta.

Figure 15.

Well logs from Pliocene slope reservoir channel complex sets in relationship to their relative downdip position to the deep-seated structural crest (using the Giza fairway as a background). Note the systematic change in vertical sedimentation patterns and lithofacies breakdown. Updip, the aggradational fill is dominated by the preservation of channel element margins, whereas downdip there is a predominance of channel element axial fill, as defined by the GR well log profiles. This is attributed to the control on the aggradation and backfill of the channel complex set by a subtle, deeper seated structural control. The pie charts clearly define the shift in depositional facies from the preservation of thin bedded low density turbidite sandstones in the updip channel complex set fill. This contrasts markedly to the preservation of high density turbidite sandstones immediately downdip of the structural crest.

Figure 15.

Well logs from Pliocene slope reservoir channel complex sets in relationship to their relative downdip position to the deep-seated structural crest (using the Giza fairway as a background). Note the systematic change in vertical sedimentation patterns and lithofacies breakdown. Updip, the aggradational fill is dominated by the preservation of channel element margins, whereas downdip there is a predominance of channel element axial fill, as defined by the GR well log profiles. This is attributed to the control on the aggradation and backfill of the channel complex set by a subtle, deeper seated structural control. The pie charts clearly define the shift in depositional facies from the preservation of thin bedded low density turbidite sandstones in the updip channel complex set fill. This contrasts markedly to the preservation of high density turbidite sandstones immediately downdip of the structural crest.

To summarize, the northern segments have channel complex sets characterized by higher net to gross packages (40-55%). The average net to gross for the well penetrations in the northern segments reflects a depositional evolution with the initial erosive phase is followed by a period of lateral and then vertical aggradation as the channels are progressively backfilled. This gives rise to the characteristic mixed erosional/depositional channel fills described by Clark and Pickering (1996). The seismic portrays the infill characteristics as a series of nested individual channels that exhibit high degree of lateral and vertical connectivity (Figs. 8A, 8B).

There are fewer wells to calibrate the southern segments of the west Nile Delta channel complex sets, though seismic characterization from the MAZ data suggests they are comprised of lower net to gross packages. Sand bypass in the upslope positions of these channels and accommodation changes in slope gradient are believed to be the most obvious explanations for this. The sands may be locally preserved as basal lag deposits though channel abandonment and preservation of debrite deposits provide a significant component to the channel fill and reflect the lower net to gross component of these packages. Stacking patterns recognized in the southern channel segments include ‘axial core’ concentrations of sand in parts of the channel, as observed in Giza South-1, preferential preservation of channel element margins, or a highly layered fill (Mayall et al., 2006) as observed in South Fayoum. Again, a significant component of muddy deposits by debris flows and channel abandonment reduce net to gross and potentially reduce reservoir connectivity.

Implications for Field Depletion

High quality multiazimuth data has provided spectacular insights into the channel morphology of the slope turbidite channel complex set of the Giza Field, west Nile Delta. They reveal complex reservoir architectures and an intrinsic link to deep seated structural controls on the development of accommodation space in an upper -slope setting.

Integration of the multiazimuth seismic data and the sedimentological observations of the conventional core provide a basis to build a robust understanding of the complex interplay of channel stacking patterns and distribution of sand within the reservoir. Intraformational reservoir layers can be calibrated to petrophysical and core data and mapped as 3-dimensional ‘geobodies.’ When integrated with the structural model, they provide a detailed static reservoir description that allows a description of potential compartments and layers that form the basis for predicting reservoir connectivity in a dynamic reservoir modeling package.

Combined with static pressure data from multiple wells in a single channel complex set, and integration of dynamic appraisal data including reservoir performance analogues and extended well test data, this provides further insights into potential connectivity. This has translated into the development of a dynamic reservoir model and integrated asset plan to optimally exploit the resource from these complex systems. The ability to interpret discrete stages of the channel-fill system from the MAZ data allows for a more complete picture of the distribution of flow units within these confined reservoirs, and the ability to provide a range of models on perched water and potential aquifer support.

The multiazimuth seismic and well data testify that these reservoirs are complex and that the risk for compartmentalization should be considered in the depletion plan. Isolated compartments, perched water at multiple reservoir levels, and residual gas accumulations have been encountered across the west Nile Delta and indicate that seal integrity is a common risk theme across all these architectural elements. Our predictions for connectivity on an individual reservoir basis must therefore be reported within the context of a regional geological model.

We also recognize that fine scale heterogeneities below seismic resolution can impede fluid flow and render dynamic reservoir predictions unrealistic. Analog reservoir performance data and extended dynamic test data are essential to gaining a better understanding of the level of potential production complexity during the appraisal of these fields.

Key Observations and Conclusions

The interplay on the slope between sediment supply, relative sea-level, mass transport processes and the deep seated structure have all played a significant role in the evolution of the Giza Field P80 slope reservoir systems. Indeed it is proposed that the deeper seated tectonic fabric of the West Nile Delta by mid Pliocene times was the primary controlling factor on the development of accommodation space on the slope,. This includes the development of precursor lobes, and the internal fill of the channel complex set with a well documented down-slope evolution of channel elements tied to backfill events and the relative position to the deep seated structure. This has significant implication for the distribution of net:gross, connectivity at a variety of scales, and the impact on the production timescale to the depletion of the Giza Field.

References

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Moursy
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Acknowledgments

The authors would like to thank all the current and former members of the West Nile Delta subsurface teams for interactive discussion on reservoir models. BP, RWE Dea, and EGAS are thanked for permission to publish the data.

Figures & Tables

Contents

References

References

Butterworth
,
P.J.
,
A.
Moursy
,
I.
Ramadan
,
B.
Austin
,
P.
Cook
,
D.
Cowper
,
T.
Dodd
,
R.
Moore
,
S.
Thomas
,
N.
Usher
, and
D.
Brunsden
,
2006
,
Pliocene and Holocene Mass Transport Deposits of the West Nile Delta, Egypt
:
AAPG Bulletin
 , vol.
89
, no.
13
(Supplement), Abstract.
Clark
,
J.D
, and
K.T.
Pickering
,
1996
,
Submarine channels: processes and architecture
 :
Vallis Press
,
London
,
231
p.
Felt
,
V. L.
,
D.
Easley
,
T.R.
Williams
,
R.
Nelson
,
M.
Reda
,
M.
Fathy
,
M.
, and
S.
Montasser
,
2004
,
Understanding Thin Bedded Reservoirs in Pliocene Slope Channel Levee Complexes offshore West Nile Delta AAPG Bulletin
 , vol.
88
, no.
13 (Supplement)
, Abstract.
Katamish
,
H.E.
,
N.
Steel
,
A.
Smith
, and
P.
Thompson
,
2005
, WDDM, The Jewel of the Nile:
SPE Europe/EAGE
 ,
Madrid, Spain
, Paper SPE
94123
.
Keggin
,
James
,
Ted
Manning
,
Walter
Rietveld
,
Chris Page, Eivind Fromyr, Roald van Borselen, and Mazin Farouki, 2006, Key aspects of Multi-Azimuth acquisition and processing 76th Mtg SEG, SS2.1
.
Keggin
,
J.
,
M.
Benson
,
W.
Rietveld
,
T.
Manning
,
P.
Cook
,
E.
Jones
and
C.
Page
,
2009
,
Multi-Azimuth 3D provides robust improvements in Nile Delta seismic imaging: IPTC-11270-PP
.
Mayall
,
M.
,
E.
Jones
, and
M.
Casey
,
M.
,
2006
,
Turbidite channel reservoirs – key elements in facies prediction and effective development
:
Marine and Petroleum Geology
 , vol
23
,
821
841
.
McHargue
,
T.
,
M.J.
Pyrcz
,
M.D.
Sullivan
,
J.D.
Clark
,
A.
Fildani
,
B.W.
Romans
,
J.A.
Covault
,
M.
Levy
,
H.W.
Posamentier
, and
N.J.
Drinkwater
,
2011
,
Architecture of turbidite channel systems on the continental slope: Patterns and predictions: Marine and Petroleum Geology
  v.
28
, p.
728
743
.
Samuel
,
A.
,
B.
Kneller
,
S.
Raslan
,
A.
Sharp
, and
C.
Parsons
,
2003
,
Prolific deep-marine slope channels of the Nile Delta, Egypt
:
AAPG Bulletin
 , vol.
87
, p.
541
560
.
Sprague
,
A.R.
,
M.D.
Sullivan
,
K.M.
Campion
,
G.N.
Jensen
,
F.J.
Goulding
,
T.R.
Garfield
,
D.K.
Sickafoose
,
C.
Rossen
,
D.C.
Jennette
,
R.T.
Beaubouef
,
V.
Abreu
,
J.
Ardill
,
M.L.
Porter
, and
F. B.
Zelt
,
2002
,
The physical stratigraphy of deep-water strata: a hierarchical approach to the analysis of genetically related elements for improved reservoir prediction (abs)
:
AAPG Annual Meeting
,
Houston, TX
.

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