Abnormal Pressures Encountered in a Deep Wildcat Well, Southern Piceance Basin, Colorado
Michael S. Wilson, Bret G. Gunneson, Kristine Peterson, Royale Honore, Matthew M. Laughland, 1998. "Abnormal Pressures Encountered in a Deep Wildcat Well, Southern Piceance Basin, Colorado", Abnormal Pressures in Hydrocarbon Environments, B.E. Law, G.F. Ulmishek, V.I. Slavin
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A deep wildcat well in the southern Piceance Basin of western Colorado (Mobil O’Connell F11X-34P) encountered three separate and distinct overpressured zones. The shallowest overpressured zone occurs within the Upper Cretaceous Mesaverde Group and coincides with gas-charged, thermally mature coal beds with vitrinite reflectance (RO) ranging from 0.8 to 2.0%. This overpressured zone is located in the center of the basin where Mesaverde coal beds are thickest and where burial beneath Tertiary sediments is deepest. The overpressured zone is surrounded by subnormally pressured zones, and normally pressured, water-saturated strata occur along the basin margins.
A deeper overpressured zone occurs within marine shales of the Upper Cretaceous Niobrara and Frontier Formations. These gas-charged, depleted source rocks have RO values ranging from 2.8 to 3.5% in the O’Connell well. The overpressured zone occupies the center of the basin. Subnormal pressures apparently occur in a ring around this overpressured zone. Low pressure gas has also been found below the Niobrara-Frontier section in fractured quartzitic sandstones of the Lower Cretaceous Dakota Group.
Highly overpressured salt water flows were encountered in the Pennsylvanian Minturn Formation and caused severe drilling and casing complications in the O’Connell well. This overpressure zone occurs in fractured quartzitic sandstones and is sealed by overlying argillaceous limestone beds. Salinity and isotope data indicate that the water is probably original formation water. The lateral extent of this pressure system is not known.
A reservoir with excellent porosity was discovered in dolomite beds of the Mississippian Leadville Formation. Measured borehole temperatures ranged from 441° to 464°F (239°C), indicating a present-day geothermal gradient of 2.2°F/100 ft (40°C/km). Measured RO values above the reservoir are as high as 4.4 to 6.1%. During testing, the Leadville reservoir flowed carbon dioxide gas, traces of methane, nitrogen, hydrogen sulfide, and salt water to the surface at approximately normal pressure. Isotope data indicate that the carbon dioxide gas was derived from alteration of carbonate rocks, probably due to hydrothermal activity associated with Tertiary igneous intrusions along the southeastern margin of the basin.
Burial and thermal history reconstructions indicate that maximum paleogeothermal gradient was 3.0°F/100 ft (52°C/km). Maximum paleotemperature in the Leadville Formation was as high as 644°F (340°C), based on estimates from vitrinite and fluid inclusion data from core samples. Maximum paleotemperature occurred at approximately 27 Ma, based on argon thermochronology of cuttings from the Pennsylvanian Maroon Formation. Analyses of apatite fission tracks in cuttings from the Wasatch Formation and upper Mesaverde Group indicate that significant erosion and cooling have occurred since 5 Ma. Cooling, removal of overburden and gas leakage along faults and fractures have contributed to gradual pressure decline in the Niobrara and Mesaverde overpressured zones.
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Abnormal pressures, pressures above or below hydrostatic pressures, occur on all continents in a wide range of geological conditions. According to a survey of published literature on abnormal pressures, compaction disequilibrium and hydrocarbon generation are the two most commonly cited causes of abnormally high pressure in petroleum provinces. In young (Tertiary) deltaic sequences, compaction disequilibrium is the dominant cause of abnormal pressure. In older (pre-Tertiary) lithified rocks, hydrocarbon generation, aquathermal expansion, and tectonics are most often cited as the causes of abnormal pressure.
The association of abnormal pressures with hydrocarbon accumulations is statistically significant. Within abnormally pressured reservoirs, empirical evidence indicates that the bulk of economically recoverable oil and gas occurs in reservoirs with pressure gradients less than 0.75 psi/ft (17.4 kPa/m) and there is very little production potential from reservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Abnormally pressured rocks are also commonly associated with unconventional gas accumulations where the pressuring phase is gas of either a thermal or microbial origin. In underpressured, thermally mature rocks, the affected reservoirs have most often experienced a significant cooling history and probably evolved from an originally overpressured system.