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Upper Pennsylvanian and Lower Permian slope sandstones of the Eastern shelf (Midland basin), Permian Spraberry and Dean fan sandstones and siltstones (Midland basin), and Delaware Group basinal sandstones (Delaware basin) were deposited by slope-accelerated density currents and illustrate a spectrum of intracratonic submarine slope and basin depositional styles. Each depositional system also contains several large reservoirs (cumulative production exceeding 10 million bbl) and numerous smaller reservoirs constituting three prolific oil plays.
The Eastern shelf submarine-slope fan system was deposited along the margin of an actively prograding clastic shelf. Down-slope sediment transport was by turbidity currents, and deposits are scale-models of larger oceanic submarine fans. Spraberry/Dean reservoirs were largely deposited by relatively nonturbid, saline density currents flowing off of shallow, restricted shelves and platforms. Reservoir geometries reflect the increasing importance of channelized flow across the basin floor. Delaware sandstones were deposited by saline density currents originating on surrounding broad, evaporitic reef-barred platforms. Elongate, lenticular geometry and textural maturity characterize Delaware Sand reservoirs. Despite the variations in specific sand-body genesis, reservoirs of each system d splay numerous similarities. (1) Reservoir facies are embedded in organic, oil-prone source rock basinal mudrocks. (2) Entrapment is largely by the updip or uplap pinch-out of porous fan facies that is inherent in the depositional architecture of submarine slope systems. (3) Porosity and permeability range from fair to poor, and proximal-to-distal variations decrease as saline density currents assume the dominant role in sediment transport. (4) Water saturation and residual oil saturation are high. (5) Solution gas drives dominate reservoirs; consequently, waterflood and pressure maintenance are necessary for efficient recovery. (6) Great internal facies complexity of the slope channel and suprafan deposits, combined with low to moderate permeabilities, results in low recovery efficienci s. Because of the internal and external stratigraphic complexity of these slope system reservoirs, the potential remains for discovery of new reserves and significant improvement in recovery of the 12 billion bbl of known oil-in-place.
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