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.