Abstract

Oil and gas accumulations in the Uinta Basin occur within rocks of the Campanian and Maastrichtian? Mesaverde Group, Maastrichtian to lower Eocene North Horn Formation, and the Paleocene and Eocene Wasatch, Colton, and Green River Formations, and the upper Eocene Uinta Formation. The basin’s largest oil and associated gas fields produce from fluvial and lacustrine reservoirs of the Green River Formation and/or its lateral and temporal equivalents in the Wasatch, Colton, and North Horn Formations. Nonassociated gas is produced from fields developed near the surface trace of subsurface faults and fractures. Some faults that cut the Cretaceous and Tertiary units of the basin represent reactivation of covered faults associated with the ancestral Uncompahgre structural element.

Currently productive oil and gas-bearing rocks can be divided into three groups of common character. Group I is composed of oil- and associated gas-bearing deeply buried overpressured Tertiary rocks that are characterized by reservoirs whose in situ matrix permeability values are near, and are commonly below, 0.1 md and whose porosity values (most porosity being secondary) average 5 percent, ranging from 3 to 10 percent. These strata contain natural open fracture networks and transmissivity (T = permeability × height) values through producing intervals that are commonly high. Group II rocks are characterized by combined primary and secondary porosity values of 10 to 16 percent in normally pressured Tertiary oil and associated gas reservoirs whose matrix permeability values may be as high as 1 d. Transmissivity values for such sequences can be relatively high because of their high matrix permeability. Group III rocks include normally pressurred? Tertiary and Cretaceous sandstone reservoirs that commonly contain nonassociated gas and have porosity values ranging from 8 to 16 percent, but whose in situ permeability throughout the pay or gas producing section is 0.1 md or less to gas (exclusive of fracture permeability). Transmissivity values for low-permeability but productive tight-gas intervals are very low because of relatively few natural open fractures.

Channel sandstone units are the principal reservoirs for both oil and gas in the Uinta Basin although both are produced from lacustrine sandstone turbidites and inversely graded siliciclastic and carbonate bars as well. The size of individual channel sandstone bodies (and therefore reservoir units) on the basin’s south margin is largely dependent upon induration of the substrate across which streams flowed.

Many open fractures within overpressured Tertiary strata are probably the result of the rapid and ongoing generation of hydrocarbons within the largely impermeable subsurface rock cell. Projection of maturity values and fluid-pressure data to undrilled parts of the basin, and the current subsurface temperatures indicate the probability of regional, overpressured, gas accumulation in Cretaceous strata of the north-central part of the basin, where gas generation is likely to be occurring at present. Geochemical indicators indicate an eastward direction of oil and gas migration which is the same as that for fluid flow as interpreted from fluid-pressure data. In the eastern and southern parts of the basin, some nonassociated gas has migrated upward from Cretaceous source rocks through a permeable network of faults and fractures into the Mesaverde Group and Wasatch Formation.