Facies Architecture of Spraberry Submarine Fan Reservoirs (Permian), Midland Basin, Texas

Edgar H. Guevara, Noel Tyler

Facies of mud-rich submarine fans of the Spraberry formation (Permian, Leonardian) form oil reservoirs in the central part of the Midland basin, west Texas. The principal reservoirs are submarine-channel and associated facies in the upper parts of generally upward-coarsening and upward-thickening sequences of the Jo Mill (lower Spraberry) and Driver and overlying Floyd (upper Spraberry) fans. They are naturally fractured, massive and laminated, very fine-grained, calcareous sandstones and siltstones usually occurring in beds 2 to approximately 12 ft thick.

Two main subdivisions, inner and middle to outer fan, are recognized using isoliths, log motifs, and cores of predominantly terrigenousclastic, mostly basin-wide genetic intervals. The boundary between the subdivisions approximately coincides with the location of the subjacent Horseshoe atoll (Pennsylvanian). The narrow range in grain size of the fans results in limited basinward variations in texture and sedimentary structures and paucity of Bouma sequences.

Facies architecture strongly influences hydrocarbon distribution and recovery. Wells having the best cumulative productions generally occur in sandstone depositional axes. Most accumulations in inner fan facies are scattered structural traps having relatively high recovery efficiencies (24%, Jo Mill field). The largest accumulations in mid to outer fan facies are stratigraphic traps in meandering to anastomosing channel sandstone belts 1-3 mi wide occurring in a widespread area. Recovery efficiencies range from 5% (Spraberry trend, stratigraphic trap) to 15% (Benedum field, combined stratigraphic-structural trap). Additional reserves can be recovered from partly drained and untapped reservoir compartments, especially in layered and compartmentalized middle to outer fan facies.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.