Abstract

The Leonardian Spraberry Formation, West Texas, is a major oil-producing formation of the Permian Basin, accounting for more than 700 million barrels of oil produced from heterogeneous submarine-fan reservoirs. Extensive exploration and development of the Spraberry and deeper reservoirs provide the opportunity to establish the sedimentary evolution of a fan fed only by relatively small rivers in a tectonically inactive setting.

Patterns of sedimentation were strongly influenced by the paleobathymetry of the Midland Basin. Approximately 250 miles long and a maximum of 80 miles wide, this long but narrow basin was fringed by prograding carbonate shelves. Siliciclastic sediment entered the basin through multiple canyons incised across its northern rim. Focus of sediment influx shifted laterally, and sediment transport through canyons was intermittent. Temporarily abandoned canyons were reoccupied during later depositional cycles. The basin floor consisted of a smaller proximal subbasin landward of the subjacent Horseshoe Atoll, beyond which was a more extensive basin plain. Differential compaction of pre-Spraberry sediment overlying the peak-and-saddle morphology of the Horseshoe Atoll profoundly influenced Spraberry sedimentation patterns. Bathymetrically low saddles between atoll mounds became conduits for sediment transport into the deeper basin. Proximal submarine-fan sediment initially ponded landward of the atoll then cascaded into the deeper, more distal, basin plain. The most proximal, significant Spraberry field (Jo Mill) produces from sandstones and siltstones deposited in one such saddle.

Flanking basin slopes laterally restricted prograding fan lobes. Predominant sediment influx in the proximal subbasin was from the northwest. As sediments spilled across the subjacent atoll, fan deposits graded from proximal to mid-fan and transport was redirected to the southwest by a prominent shelf-slope promontory along the eastern margin of the subbasin. Distal fan deposits lie south of the promontory. Almost all production from the Spraberry is from mid-to-distal fan deposits basin ward of the subjacent Horseshoe Atoll. Recovery efficiency of these highly stratified and laterally discontinuous mid-to-distal fan reservoirs in the Spraberry Trend is less than 10 percent, substantially lower than the 25 percent recovery efficiency of the proximal Jo Mill field.