Abstract: Characteristics of Abnormally Pressured Compartments and a High Temperature Carbon Dioxide Reservoir in the Southern Piceance Basin, Colorado

Michael S. Wilson, Bret G. Gunneson, Royale Honore, Roy Enrico

A wildcat drilled during 1990-1992 to evaluate a deep gas prospect in the southern Piceance basin, Colorado, encountered a gas charged overpressured compartment in the Lower Cretaceous Niobrara and Frontier Formations, overpressured salt water flows in the Pennsylvanian Minturn Formation, and a subnormally pressured reservoir of high temperature carbon dioxide in the Mississippian Leadville Formation.

The gas charged Niobrara overpressured compartment coincides with depleted source rocks with vitrinite Ro of 2.8 to 3.5%. It occupies the basin center, but has dissipated along uplifted basin margins and is diminished where Tertiary sediments have been removed by erosion. Calcareous shales and bentonite beds are probable vertical seals.

Overpressured salt water occurs in permeable quartzitic sandstones of the Minturn Formation and is vertically sealed by shales and argillaceous limestones. Isotope and salinity data indicate that the salt water is original formation water. Inflows of 400-degree F overpressured water caused significant drilling complications.

Carbon dioxide with minor methane and hydrogen sulfide is trapped in the Leadville Formation at 18,000 feet on a tilted fault block structure. Vuggy, moldic and sucrosic porosity zones occur in dolomitized wackestones and grainstones. Vitrinite Ro near the reservoir is 4.4 to 6.1%. Bottom hole temperature was measured at 440-463°F. The reservoir is evidently subnormally pressured and is separated from the Minturn salt water by limestone and calcareous shale beds. Isotope data indicate that the carbon dioxide was derived from alteration of carbonates, probably by Tertiary igneous intrusions.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994