BOOTH, J.R., and A.E. DUVERNAY III, Shell Deepwater Development, Inc.

Abstract: Sequence Stratigraphic Framework, Depositional Models, and Stacking Patterns of Ponded and Slope Fan Systems in the Greater Auger Basin: Central Gulf of Mexico Slope

Introduction

The Greater Auger Basin on the central Gulf of Mexico slope is a prolific hydrocarbon basin with roughly 1 billion barrels STOOIP proven to date. Multiple well penetrations at Shell's Auger Field and the nearby "Macaroni" oil discovery test time-equivalent stratigraphy in proximal and distal settings of this contiguous intraslope basin. Integration of well data with a 250-square mile 3D seismic grid presents a unique opportunity to examine ancient turbidite systems in the context of a calibrated basin-scale fill model. Our study utilizes genetic seismic sequence and facies analysis (e.g. Prather et al., 1998) to relate the character of Pliocene condensed section-bounded sequences to depositional processes and the evolution of accommodation space. We furthermore describe cyclic reservoir stacking patterns from ponded and slope accommodation space and relate our observations to depositional processes largely influenced by changing basin physiography (e.g., Ross et al., 1994).

Basin Setting and Configuration

The Auger Basin is a salt-bounded intraslope basin comprising an area of approximately 250 square miles on the central Gulf of Mexico upper slope. Plio-Pleistocene turbidite delivery into the basin occurred from the northeast through a long-lived sediment corridor and from the north and west through a series of constrictions between basin-bounding salt masses (Figure 1). After filling available accommodation space, turbidite bypass and delivery to outboard basins occurred through openings between the southern and eastern basin-bounding salt masses.

Two primary types of accommodation space are recognized in the Auger Basin, (1) ponded accommodation space and (2) slope accommodation space (e.g., Prather et al., 1998). Palinspastic reconstruction of a Late Pliocene bathymetric surface demonstrates the relative paleo-basin settings of Auger Field and the Macaroni Discovery. Auger reservoirs occur within slope accommodation space - roughly 1000 feet structurally higher than the basin-bounding sill that controls the upper limit of ponded accommodation space - whereas Macaroni sands occur within low-relief ponded accommodation space immediately adjacent to the basin-bounding sill.

Sequence Stratigraphic Framework

Quantitative biostratigraphic analysis demonstrates that the primary reservoir section in the Auger Basin occurs in the Pliocene G. nepenthes — Cristellaria "S" stratigraphic interval, in which Styzen (1996) recognizes five major glacio-eustatic cycles.Turbidite reservoirs at Auger Field and the Macaroni discovery occur within lowstand and highstand portions of these cycles. Seismic stratigraphic analysis subdivides the interval into twelve condensed section bounded sequences, demonstrating a higher-order cyclicity superimposed upon the five major cycles.The seismically-defined sequences occur in the uppermost ponded seismic facies assem-blage (e.g., Prather et al. 1998), and display significant internal discordance, proximal-to-distal.seismic facies variability, and a repetitive seismic facies succession.The facies succession consists of basal convergent onlapping packages (Cbh facies) overlain by chaotic zones (B facies) and capped by continuous draping events (D facies). Onlap and draping facies that are best developed in the distal portion of the basin are frequently removed in the proximal portion of the basin by low-angle erosional surfaces beneath chaotic zones.

Reservoir Architectures and Stacking Patterns

Reservoirs at Shell's Auger Field have been reinterpreted from McGee et al. (1993) as northeasterly point-sourced submarine fans deposited on the flank of a NW-SE trending paleoridge (see Figure 1). Producing zones consist of amalgamated sheet sands and hybrid sheet/channel sands interpreted as proximal base-of-slope and channel-lobe transitional fan deposits. Depositional sequences display fining-upward trends of basal sheet-sands or hybrid sheet/channels overlain by channel overbank and mass-flow deposits. Condensed zones form the caps to these sequences (Figure 2a). Several of the depositional sequences are comprised of higher-order, fining-upward parasequences that are typically channelized or thin-bedded.

Time-equivalent turbidite reservoirs penetrated at Shell's "Macaroni" discovery represent line-sourced fan deposits in a basin "backstop" setting. Primary reservoir architectures are layered and amalgamated sheet sands that occur within high frequency, parasequence-scale depositional cycles. Individual cycles consist of basal layered sheets overlain by amalgamated sheets and amalgamated channels capped by thin, condensed intervals (Figure 2b).

Discussion

Apart from bounding intervals of condensed deposition that are likely eustatically-controlled, observed turbidite fill sequences are largely controlled by changes in basin physiography induced by depositional rates that exceed the rate of formation of accommodation space (e.g., Ross et al., 1994) (Figure 3). Deposition within slope accommodation space occurs in two stages: (1) an initial "healing phase" and (2) a subsequent "bypass phase". Healing-phase deposition is sheet-sand dominated, characterized by rapid aggradation at local slope breaks, and represented by onlap-fill seismic facies. Graded slope profiles are quickly re-established during the healing phase, and turbidite systems evolve to become bypassing. Bypass-phase deposition is dominated by channel/overbank or mass wasting processes on graded slopes. Sands associated with this phase are thinly-bedded or laterally-discontinuous and occur within chaotic seismic facies packages.Turbidite deposition within ponded accommodation space also occurs in two stages: (1) a ponded phase and (2) a spill phase. Similar to the healing phase, ponded deposition is also sheet-sand dominated and represented by onlap-fill seismic facies. Layered sheets of the ponded phase are deposits from low-concentration flows running up the distal basin margin, whereas overlying amalgamated sheets are deposits from highly-concentrated flows impinging directly upon the basin-bounding escarpment. The spill phase occurs when the basin boundary is overtopped and turbidite delivery progresses to the outboard basin. Erosional downcutting at the basin-bounding sill and subsequent backfill of the gorge system result in channelized reservoir architectures and chaotic seismic facies.

The proposed fill model illustrates that world-class turbidite sheet sands occur within ponded and slope accommodation space of intraslope basins.The model also suggests that the frequency of occurrence of sheet sands may be greater in ponded than in slope settings. High rates of subsidence in ponded settings allow accommodation space to rapidly re-form in response to high-frequency perturbations in the rate of sedimentation.As a result, sheet sands are deposited and preserved at parasequence-scale cyclicity. Conversely, lower subsidence rates in slope settings may not allow sufficient accommodation space to re-form in response to high-frequency perturbations, thereby hindering both deposition and preservation potential of higher-order healing phase deposits. Sheet sand reservoirs in slope settings may therefore be confined to the basal portions of major depositional sequences.

References:
McGee, D.T., P.W. Bilinski, P.S. Gary, D.S. Pfeiffer, and J.L. Scheiman, 1993, Geologic models and reservoir geometries of Auger Field, deepwater Gulf of Mexico, in P. Weimer, A.H. Bouma, and B.F. Perkins, eds., Submarine fans and turbidite systems — sequence stratigraphy, reservoir architecture, and production characteristics: GCAGS Fifteenth Annual Research Conference, p. 233-244.
Prather, B.E., J.R. Booth, G.S. Steffens, and P.A. Craig, 1998, Classification, lithologic calibration, and stratigraphic succession of seismic facies of intraslope basins, deep-water Gulf of Mexico,AAPG Bulletin, v. 82, no.5a, p. 701-728.
Ross,W.C., B.A. Halliwell, J.A. May, D.E.Watts, J.P.M. Syvitski, 1994, Slope readjustment:A new model for the development of submarine fans and aprons, Geology, v. 22 no. 6, p. 511-514.
Styzen, M.J., 1996,A chart in two sheets of the Late Cenezoic chronostratigraphy of the Gulf of Mexico, Gulf Coast Section SEPM Foundation, Houston,TX.

Figure 1: Late Pliocene paleogeography of the Greater Auger Basin.Turbidite delivery to the basin occurred through constrictions between salt masses bounding the northern and western portions of the basin, whereas bypass and delivery to outboard basins occurred through constrictions along the southern and eastern basin margins. Reservoirs from Shell's Auger Field have been re-interpreted from depositional models of McGee et al. (1993) as point-sourced submarine fans deposited within slope accommodation space adjacent to a northwest-plunging paleoridge. Reservoirs at the Macaroni Discovery, twelve miles south of Auger Field in the distal portion of the basin, represent time-equivalent fan deposits in a backstop setting within ponded accommodation space.

Figure 2: (A) Typical fining-upward reservoir stacking pattern displayed in depositional sequences at Shell's Auger Field. Basal amalgamated Sheets (AS) are overlain by amalgamated channels (AC) and capped by channel-overbank complexes (CO). (B) Typical reservoir stacking pattern at Shell's Macaroni discovery 12 miles south of Auger Field. Reservoirs are stacked in parasequence-scale cycles consisting of basal layered sheets (LS) overlain by amalgamated sheets (AS) and capped by amalgamated channels (AC).

Figure 3: Generalized model for coeval evolution of ponded and slope fill sequences within the Greater Auger Intraslope Basin. (1A) "Healing phase" sheet sands deposited at slope breaks within slope accommodation space. (1B) Coeval "ponded phase" sheet sands deposited in ponded accommodation space. (2A) "Bypass phase" channel & overbank deposition within slope accommodation space, with localized channel downcutting & removal of underlying healing phase sands. (2B) Sheet sand deposition fills remaining ponded accommodation space. (3A) "Bypass phase" channel & overbank deposition within slope accommodation space. Over-steepening of proximal slopes, channel downcutting, & mass wasting. (3B) Ponded basin-bounding sill overtopped. Downcutting & erosion by "spill phase" channels. Subsequent backfill of erosional gorge as profile of equilibrium adjusts.