Abstract

Uinta Basin rocks include thick sequences of low-permeability (tight) sandstones that contain a large volume of gas. Most known gas accumulations are in the eastern part of the basin in the Upper Cretaceous Mesaverde Group, the uppermost Cretaceous to lower Eocene North Horn Formation, and the Paleocene and Eocene Wasatch, Colton, and Green River Formations. Although most gas completions are in Tertiary strata, rocks in the underlying, sparsely drilled Mesaverde Group in the north-central part of the basin are potential targets as well.

Vitrinite reflectance (R_m) for each of the previously mentioned low-permeability sequences increases to the north. Thermal maturity for the Mesaverde Group is sufficient for gas generation over a large area of the basin. R_m at the base of the Mesaverde increases from 0.65 percent at shallow depths along the south edge of the basin to 1.5 percent in the central basin at depths of about 11,000 ft (3,350 m). At the top of the Mesaverde, R_m increases from 0.50 percent at outcrops along the south edge of the basin to 2.2 percent near the town of Altamont, at depths of approximately 18,000 ft (5,500 m).

Porosity-versus-R_m plots allow prediction of porosity and, thus, hydrocarbon potential for Mesaverde sandstones in unexplored areas of the basin. Porosity values of nonmarine Mesaverde sandstones with thermal maturity less than about 0.70 percent R_m or greater than about 2.0 percent R_m decrease as thermal maturity increases, and follow “normal” sandstone trends. However, between 0.70 percent R_m and 2.0 percent R_m, in the window of hydrocarbon generation, porosities for Mesaverde sandstones do not decrease as thermal maturity increases. Deeply buried, overpressured, gas-saturated Mesaverde sandstones are likely to have porosities in the 5-9 percent range where R_m values do not exceed 1.8-2.0 percent.

Projecting fluid-pressure and thermal maturity data from drilled areas of the basin into undrilled areas reveals the possibility of a regional, overpressured, basin-centered gas accumulation associated with organic-rich rocks, where gas generation is probably occurring at present. Wells drilled in the Mesaverde and lower part of the Tertiary, in the areas where R_m at the base of the Mesaverde is greater than 1.1 percent, should have the best potential for gas production. Overpressured gas reservoirs (R_m > 1.1 percent) are likely to have no free water and be surrounded by successive zones of mixed water and gas (R_m 1.1-0.75 percent), and of water only (R_m <0.75 percent).

Reservoir quality is directly related to lithofacies in the tight-gas sequences of Uinta Basin. In Cretaceous and Tertiary formations, by far the best quality reservoir rock is diagenetically altered fluvial sandstone. A very late (10 Ma) fracturing episode also enhanced reservoir quality. Other good-quality and economically important reservoir rocks are Cretaceous lenticular fluvial and blanketlike marine sandstones and Tertiary open-lacustrine sandstones.