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Reserve Growth Through Geological Characterization of Heterogeneous Reservoirs--An Example from Mud-Rich Submarine Fan Reservoirs of Permian Spraberry Trend, West Texas

Noel Tyler, J. Crispin Gholston

Tight, naturally fractured Permian submarine fan reservoirs in the Midland basin contained more than 10.5 billion bbl of oil at discovery. Ultimate recovery is estimated to average 7% of the original oil in place. At abandonment 4 billion bbl of nonresidual mobile oil will remain in untapped or poorly drained reservoir compartments. This unproduced mobile oil is the target for Spraberry reserve growth through strategic infill drilling.

Mid-fan facies of three separate submarine fans are productive in the Shackelford and Preston waterflood units (SPWU) in the central Spraberry Trend. Braided to meandering paleodip-oriented channels are flanked by levees which grade into upward-coarsening, unconfined distal fan sediment. Facies boundaries compartmentalize the reservoir, providing for interwell, stratigraphic entrapment of oil. Field-wide heterogeneity is pronounced. Stacking of channels in the upper Spraberry in the eastern half of the SPWU results in a dip-oriented belt of better reservoir quality. Wells completed in this axis have produced two to six times the amount of oil produced from wells located off of the depo-axis. Although fractures are important in early production, the contribution of matrix porosity is c itical throughout the life of the reservoir. Current economics dictate that reserve growth might best be attained by siting new strategic infill wells in depositional axes and by selective recompletions of existing wells in areas of poorer reservoir quality for bypassed oil in undrained reservoir compartments.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.