Heterogeneous Deep-Sea Fan Reservoirs, Shackelford and Preston Waterflood Units, Spraberry Trend, West Texas

The Permian Spraberry Trend, once regarded as the world's largest uneconomic oil field, is a prime candidate for reserve growth through extended conventional recovery. More than 9.4 billion barrels (Bbbl) of oil was discovered in the trend, which is part of a giant oil play (10.5 Bbbl of in-place oil) that produces from submarine fan reservoirs of the Spraberry and Dean Formations in the Midland Basin, West Texas. Historically, development of the naturally fractured, very fine grained sandstone and siltstone reservoirs was based on the assumption that reservoirs were laterally continuous and that fractures would foster efficient drainage at 160-acre well spacings. Yet ultimate recovery of oil is estimated to average 6 percent of the original oil in place. Almost 10 Bbbl of oil will remain in Spraberry-Dean reservoirs at abandonment. Of this volume, 4 Bbbl is nonresidual mobile oil that awaits extended conventional recovery in untapped or poorly drained reservoir compartments. This unproduced mobile oil forms a major component of the target for Spraberry reserve growth.Geological characterization of the Shackelford and Preston waterflood units (SPWU) in the central Spraberry Trend established that three separate submarine fans (two in the upper and one in the lower Spraberry), each characterized by a variety of midfan facies, are productive. Paleodip-oriented channels display a transition from braided in the updip parts of the SPWU to meandering downdip. Laterally, channels are flanked by levees that grade into upward-coarsening, unconfined distal-fan sediment. Investigation of facies-defined hydrocarbon saturations clearly shows that facies boundaries compartmentalize the reservoir, providing for interwell, stratigraphic entrapment of oil.Fieldwide 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 as much as six times the amount of oil produced from wells located off of the depositional axis. This swath of superior wells trends obliquely to the natural fracture orientation, suggesting that although fractures are important in early production, the contribution of matrix porosity is critical throughout the life of the reservoir.Current economics dictates that reserve growth might best be attained by siting new strategic infill wells in the depositional axis and by selectively recompleting existing wells to produce bypassed oil in undrained reservoir compartments in areas of poorer reservoir quality.
Noel Tyler
J. Chris Gholston

Tyler, Noel, and Gholston, J. C., 1988, Heterogeneous Deep-Sea Fan Reservoirs, Shackelford and Preston Waterflood Units, Spraberry Trend, West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 171, 38 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation