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Approximately 70 million bbl of oil have been produced from the chert-dominated Thirtyone Formation at Dollarhide field. The Thirtyone Formation is Devonian in age and contains two reservoir intervals, an upper dolomite and a lower chert, separated by nonporous limestone. The chert reservoir contains approximately 83% of the original oil in place (OOIP) and consists of two different facies-laminated microporous chert and burrowed chert. The laminated microporous chert was apparently deposited as sponge spicule sands (grainstones) in channels and fans on the slope of the Tobosa basin. The burrowed chert facies was apparently deposited as burrowed mixtures of sponge spicules, siliceous mud, and carbonate mud in broad slope environments between fans and channels. Spicules are thought to be derived from disaggregation of siliceous sponges living on the slope. Early marine and/or meteoric diagenesis dissolved many sponge spicules and reprecipitated the silica as microcrystalline chalcedony and quartz, resulting in spicule-moldic porosity and microporosity in the originally siliceous strata. The burrowed chert underwent small-scale differential compaction that produced short, discontinuous fractures, 1-10 mm in length and 0.1-0.5 mm in width.
Production from the field has been related to amounts and characteristics of the pore systems. The thickness of the porous chert ranges laterally from 0 to 80 ft (0-24 m), and pore volume (φh) ranges from 0 to 20 pore ft (0-6 pore m). The laminated microporous chert is very homogeneous and has high porosity (25-35%) and uniform permeability (5-30 md). Areas dominated by laminated microporous chert had moderate primary recovery (approximately 200,000 bbl of oil per well), excellent waterflood production (1-2.5 million bbl of oil per well), and poor 20-ac (8.1 ha) infill production (<20,000 bbl of oil per well). The burrowed chert has more heterogeneous porosity (5-30%) and permeability (<1-100 md). The dolomite also has heterogeneous pore networks that have porosities of 3-15% and measured permeabilities of less than 1-200 md. Wells having substantial amounts of porous dolomite and/or burrowed chert had moderate to good primary recovery (200,000-300,000 bbl of oil per well), moderate waterflood recovery (300,000-1,100,000 bbl of oil per well), and moderate 20-ac (8.1 ha) infill recovery (50,000-100,000 bbl of oil per well). Oil recovery from the CO2 flood is expected to average approximately 250,000 bbl per well, having slightly higher recovery (300,000 bbl) in areas that have thick laminated microporous chert and/or burrowed chert. Oil recovery from the homogeneous, laminated microporous chert during the CO2 flood is improved by large amounts of residual oil and good sweep efficiency but decreased by the previously efficient sweeping of original mobile oil by the waterflood. The CO2 recovery from the heterogeneous, burrowed chert is improved by significant amounts of unswept mobile oil but decreased by poorer sweep efficiency. As a field, primary recovery is estimated at 13% of OOIP, waterflood at 30% of OOIP, infill at 3.5% of OOIP, and CO2 flood at 11% of OOIP.
Art Saller currently works as a sedimentologist and sequence stratigrapher for Unocal in Sugar Land, Texas. He did undergraduate studies at the University of Kansas (1974-1978) and received an M.S. degree from Stanford University in 1980 and a Ph.D. in geology from Louisiana State University in 1984. From 1984 to 1986 he worked as a research geologist with Cities Service Oil and Gas in Tulsa, Oklahoma, and joined Unocal in 1986.Brian C. Ball received his B.S. degree in geology from the University of Wisconsin-Madison in 1979 and an M.S. degree in geology from East Texas State University in 1981. He has worked as a petroleum geologist in the Permian basin since 1981. His experience includes exploration, exploitation, and development studies for both major and independent oil companies. These studies encompassed carbonate, sandstone, and chert reservoirs. He joined Unocal in 1987 and has been a part of the Dollarhide team since 1992.
Steve Robertson is a reservoir engineer with Pure Resources in Midland, Texas. He has 25 years of industry experience, specializing in enhanced oil recovery methods, geologic modeling, and reservoir simulation. His current work involves full field simulation of CO2 floods, water floods, and tight gas reservoirs in the Permian basin. Steve has a B.S. degree in chemical engineering from the University of Minnesota and is a registered professional engineer in the state of Texas.
Bruce McPherson currently works as senior development geologist in Unocal's Eastern Gulf Asset in Lafayette, Louisiana. Bruce received B.S. (1984) and M.S. (1989) degrees from the University of Tulsa and joined Unocal in Midland, Texas, in 1990.
Clay Wene joined Unocal Corporation in Midland, Texas, in 1992. His primary experience has involved the design and management of secondary and tertiary recovery projects. In 2000, he joined Pure Resources in Midland as a petroleum engineer. He has B.S. and M.S. degrees in petroleum engineering form the University of Missouri-Rolla and the University of Texas at Austin, respectively, and is a registered professional engineer in the state of Texas.
Robert C. Nims received a B.S. degree in geology from the University of Oklahoma. He currently works as a geophysicist for Pure Resources in Midland, Texas. He has worked 20 years as an exploration and development geophysicist for Unocal prior to the merger of Unocal's Permian basin assets into Pure Resources. He has worked on structural, carbonate, and clastic plays in the Rocky Mountains, midcontinent, Permian basin, and Gulf Coast regions.
John Gogas is currently a geophysical consultant with Digital Prospectors in Midland, Texas. He received his B.S. degree in mathematics (1981) from Montana State University, Billings, and his M.S. degree in geophysical engineering (1984) from Montana Tech. He was an exploration and development geophysicist with Unocal in the Permian basin from 1984 to 1995.