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Abstract: Construction of a Full Field Reservoir Model for the Intra Qua lboe Formation, Asasa Field, Offshore Nigeria: Characterization of Stacked Deepwater Sediment Gravity Flow Reservoirs

Paul, Joe; Jurick, Dana; and Stewart, Wayne - Mobil Technology Company

The Asasa Field (Nigerian National Petroleum Corporation 60%Wl, Mobil Producing Nigeria, 40% WI) in offshore Nigeria produces over 110,000 barrels of oil per day from Pliocene, passive margin slope valley reservoirs. Relationships between conventional core, log response, depositional environment, vertical successions, and sequence stratigraphic framework provide the foundation for detailed reservoir characterization of this submarine slope valley field.

Sediment gravity-flows were focused in a depositional fairway due to the collapse of a portion of the deltaic platform and the formation of a broad embayment of the coastline. During a major sea-level lowstand, the slope valley was eroded and filled by delta-derived, coarse-grained sediment-gravity flow sands that are dominantly turbidites.

The initial Asasa slope valley was created by erosion during a Pliocene sea level lowstand that occurred at about 3.9 Ma. Relief within this submarine trough was as great as 315-425 ft. This slope valley focused sediments along a depositional fairway that filled early, but remained a sediment fairway for at least four successive higher order erosion and fill lowstand cycles during a major third order lowstand. Each successive cycle demonstrates a change in sediment grain size, sorting and sand body geometry up-section. Overall the cycles become more fine-grained through time, reflecting a changing sediment supply during a major third order sea level lowstand.

Reservoir pressure data and production information indicates the existence of four main pressure compartments in the field. These data indicate that the field behaves as a layered system with strongly buffered vertical communication, but strong lateral communication. Reservoir compartmentalization is complex as non- synchronous stratigraphic units are juxtaposed and in pressure communication due to erosion and reworking of underlying units. Consequently, some reservoir compartments consist of depositionally unrelated stratigraphic units.

The slope-valley fill is a lowstand deposit characterized by at least four cycles of erosion and fill. Sand body geometry, sediment grain size and sorting change up-section within this lowstand. Understanding these relationships is possible through integration of log response, conventional core, and the sequence stratigraphic framework. The integration methodology and resulting reservoir model are critical to a field depletion strategy, sound reservoir management decisions, and is the focus of this presentation.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil