Basin Morphological Controls on Submarine-Fan Depositional Trends: Spraberry Sandstone, Permian Basin, Texas

The Leonardian Spraberry Formation 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 cone fed only by relatively small rivers and windblown sediment in a tectonically inactive setting. Because the Spraberry is not exposed, the study utilized a rich subsurface data base of more than 1,200 well logs developed in the pursuit of the 10 billion barrels of oil discovered in the fan cone. Entrapment of this oil is almost entirely stratigraphic, most of the resource being captured in mid-fan to distal fan sediments in the distal Midland Basin more than 100 mi basinward of the shelf edge. The analysis of controls on depositional trends in the Spraberry had an immediate goal of providing a framework for continued Spraberry exploration. Taken in broader context, however, the Spraberry provides a superb analog for fine-grained, mud-rich offshore fan systems, which are a high-priority, global target for exploration and development. Patterns of sedimentation were strongly influenced by the paleobathymetry of the north-south- elongate Midland Basin. Approximately 200 mi long and a maximum of 80 mi wide, this long but narrow basin was fringed by prograding carbonate shelves. Siliciclastic sediment entered the basin through multiple canyons incised mostly across its north rim. Focus of sediment influx shifted temporally, and sediment transport through canyons was intermittent. Temporarily abandoned canyons were reoccupied during later depositional cycles. The basin consisted of a smaller, proximal subbasin landward of the subjacent Horseshoe Atoll, beyond which, down slope, was a more extensive basin composing the distal basin floor. 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 was initially ponded landward of the atoll then cascaded into the deeper and more distal basin plain. Flanking basin slopes laterally restricted prograding fan lobes. Predominant sediment influx to the proximal subbasin was from the northwest. As sediments spilled across the subjacent atoll, fan deposits graded basinward from proximal fan to mid-fan, and transport was redirected to the southwest by a prominent shelf-slope promontory (Glasscock narrows) along the eastern margin of the subbasin. Distal fan deposits lie south of the promontory. Almost all of the production from the Spraberry is from mid-fan to distal fan deposits basinward of the Horseshoe Atoll.