ABSTRACT: Controls on Reservoir Distribution and Architecture in Slope Settings: Implications for the Global Deepwater Play

PRATHER, BRADFORD E. , Shell International E&P, The Hague, Netherlands

Realization that high-performance turbidite reservoirs exist in continental slope environments has substantially changed industry’s perception of the profitability of the global deep-water play. Lessons learned from developing and producing turbidite fields show that thick, high net-to-gross sheet sands with areally extensive, well-connected aquifers typify the architecture of high performance turbidite reservoirs. The highest performing turbidite reservoirs (rates 10,000 BOPD and EURs >20 MMBO) are found in intraslope basins on above-grade slopes and at the base of graded slopes. This pattern of distribution suggests there is a link between evolution of slope systems, and the occurrence of high performance turbidite reservoirs.

Turbidite reservoir distribution and architecture across slope environments varies as a function of accommodation space. The degree of slope substratum mobility, sediment flux, and sand-mud content control the type and distribution of accommodation space across slope and base-of-slope systems. Presence of ponded-basin accommodation space and large amounts of mid- to upper-slope healed-slope accommodation space distinguish above-grade slope systems from graded-slope systems. Large amounts of healed-slope accommodation space in basin floor and toe-of-slope positions and absence of ponded-basin accommodation space distinguish graded-slope systems from above-grade slope systems.

Sheet-sand deposition on above-grade slopes results from ponded basin "fill-and-spill" processes. Spill-and-fill dominates early phases of deposition in above-grade slopes underlain by highly mobile substrates prior to progradation of graded (unconfined) slopes. Slopes with lower substrate mobility tend to have an early graded slope that evolves with time into an above-grade slope. Late onset of above-grade slope conditions on these slopes results in sheet-sand deposition in ponded basins that are too shallow to be prospective for hydrocarbons. Sheet sands are also found in basin-floor positions and  at the toes of graded (unconfined) slopes associated with stable substrates.

Many recent turbidite discoveries, principally on the continental slope of west Africa, and a great deal of the remaining deepwater potential in the global play are associated with stepped or terraced above-grade slopes that lack intraslope basins with ponded-basin accommodation space. Since reservoirs in these settings have yet to be developed, their performance characteristics are poorly understood. Many of them are associated with belts of highly sinuous ribbon and shoestring channel sands with locally scattered, thin, ponded fans. These sinuous channel belts and small fans occur across lower gradient portions or steps on the slope whereas straight to lower sinuosity channels form across ridges between the steps where seafloor gradients are higher. Highly discontinuous external and internal (subseismic) architectures associated with these reservoir types present development challenges not encountered with sheet-sand reservoirs due to poorer reservoir connectivity resulting in reservoir compartmentalization and limited aquifer support.

AAPG Search and Discovery Article #90916@2001-2002 AAPG Distinguished Lectures