Sequence Stratigraphic Framework, Depositional Models, and Stacking Patterns of Ponded and Slope Fan Systems in the Auger Basin: Central Gulf of Mexico Slope
J.R. Booth, A.E. DuVernay, III, D.S. Pfeiffer, M.J. Styzen, 2000. "Sequence Stratigraphic Framework, Depositional Models, and Stacking Patterns of Ponded and Slope Fan Systems in the Auger Basin: Central Gulf of Mexico Slope", Deep-Water Reservoirs of the World, Paul Weimer
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Exploratory and development drilling at Shell’s Auger and Macaroni Fields has provided well control in time-equivalent stratigraphy from proximal and distal settings of a contiguous intraslope basin. The oil and gas reservoirs in the basin occur in the Pliocene Discoaster brouweri–Reticulofenestra pseudoumbilica interval (3.64 Ma–1.95 Ma) that spans six glacio-eustatic cycles. Higher order cyclicity is demonstrated by the presence of twelve condensed section bounded sequences in the interval, as well as a repetitive vertical facies succession internal to the sequences. The vertical succession consists of basal onlapping facies having good internal continuity overlain by chaotic to transparent facies that has erosional basal contacts and is capped by highly continuous, single-loop reflections. Well data from Auger and Macaroni fields reveals the basal onlap packages to contain high net/gross, thickly-bedded sheet sands and highly amalgamated channel-fill deposits, whereas the overlying chaotic facies contains laterally discontinuous channel-fill and low net/gross overbank deposits. A combination of erosional downcutting by the chaotic zones and stratigraphic pinch outs within onlap packages result in a high degree of proximal to distal seismic facies variation in the basin.
Accommodation in the basin is classified using terminology of Prather et al. (1998) into two primary types, slope accommodation and ponded accommodation. Auger Field reservoirs have been deposited as point-sourced submarine fans in the proximal portion of the basin within slope accommodation, whereas reservoirs at Macaroni Field have been deposited in the distal portion of the basin within ponded accommodation. The observed stacking patterns and the proximal to distal facies changes in the basin are attributed to a repetitive fill sequence driven by decreasing accommodation during intervals of turbidite deposition and renewal of accommodation during sequence-bounding intervals of condensed deposition. Two stages of turbidite fill are identified within slope accommodation: (1) an initial ‘healing phase’ and (2) a subsequent ‘bypass phase.’ Healing-phase deposition occurs adjacent to seafloor escarpments at the onset of turbidite deposition and is dominated by high net/gross sheet sands and amalgamated channel sands. The bypass phase begins upon establishment of graded slopes and is characterized by channel-fill sands and lower net/gross overbank deposits. Coeval with deposition within slope accommodation, turbidite fill within ponded accommodation also occurs in two stages: (1) an initial ‘ponded phase’ and (2) a subsequent ‘spill phase.’ The ponded phase is dominated by high net/gross sheet sands and occurs while deposition is constrained by a basin-bounding bathymetric sill. The spill phase occurs when the basin sill is overtopped and turbidite delivery progresses to the outboard basin. Underlying ponded phase sheet sands are frequently cannibalized by erosional channel systems during the spill phase.
This study establishes that world-class turbidite sheet sands occur not only within ponded accommodation of intraslope basins, but also within slope accommodation. The study also reveals that sheet sands within ponded accommodation may occur stacked at higher frequency than within slope accommodation. The observed proximal-to-distal facies changes in the basin, the vertical stacking patterns, and the spatial distribution and timing of cut and fill events suggest that decreasing accommodation in the basin—not rising eustatic sea level—drives the transition from sheet sand dominated fans to overlying channel and overbank dominated fans.