Depositional Systems and Oil Reservoirs in Spraberry Formation (Permian), Midland Basin, Texas: ABSTRACT

Terrigenous clastic submarine-fan facies comprise the upper and lower parts of the Spraberry Formation (Permian, Leonardian) of west Texas. The Jo Mill submarine-fan system makes up the lower Spraberry. The upper Spraberry comprises the Driver and the overlying Floyd submarine-fan systems. The Midland basin plain system, composed mainly of shales and carbonates with thinner, interbedded sandstones, makes up the middle Spraberry and vertically separates the Jo Mill and Driver fans. Sand-rich inner fan facies of all three fan systems extend 50 mi into the Midland basin from the northern shelf edge. The abrupt transition from inner to midfan facies coincides with the underlying Horseshoe atoll, suggesting this bathymetrically positive feature influenced sedimentation patterns in younger sediments. Midfan to outer fan facies extend downdip from the subjacent atoll 100 mi farther into the basin. Sand contents decrease and interbedded mudstones thicken basinward. Recovery efficiencies are strongly related to submarine-fan facies. Recoveries in inner fan, incised-channel reservoirs (24% in Jo Mill field) are higher than in midfan and outer fan reservoirs (5% in the Spraberry trend to 15% in Benedum field). These more distal reservoirs consist of anastomosing to meandering channel-fill and interchannel facies associations. Stacking of channels through time resulted inmore » dipelongate depositional axes in which wells have produced from two to as much as six times more oil than wells tapping interaxial areas. Pronounced stratigraphic heterogeneity and current well completion practices result in partly drained reservoir compartments, providing abundant opportunities for additional oil recovery.« less