Lithofacies and related hydrocarbon accumulations in Tertiary strata of the western and central Uinta Basin, Utah

Strata of Tertiary age in the Uinta Basin were deposited in an internally drained depression. They can be divided into (1) a central core of organic-rich open lacustrine claystone and mud-supported carbonate surrounded by (2) marginal lacustrine sandstone, claystone and mud- and grain-supported carbonate deposited in deltaic, interdeltaic, and lake-margin carbonate-flat environments, and (3) alluvial claystone, conglomerate, and sandstone units deposited peripheral to the lacustrine sediments. The alluvial facies is not a major producer of hydrocarbons. However, overbank and channel sandstone units are gas-bearing on the south flank of the basin and are bitumen-bearing at Asphalt Ridge on the northeast margin of the basin. Marginal lacustrine strata including delta-front, overbank, and channel sandstone units contain the principal reservoirs for hydrocarbons in the Uinta Basin. Oil and gas appears to have been generated in open lacustrine source rock and trapped in marginal lacustrine reservoir units between rocks of the central open lacustrine facies and the peripheral alluvial facies. Several correlation markers within the Green River Formation can be traced throughout the subsurface of the basin. From base to top they are the (1) lower marker, (2) Paleocene-Eocene boundary, (3) top of the carbonate marker unit, (4) middle marker, (5) Mahogany oil-shale bed, and 6) the upper marker. The oldest Tertiary units that yield large volumes of oil or gas are near the lower marker. Most production is from overpressured open and marginal lacustrine facies in the Altamont-Bluebell area. Overpressured marginal lacustrine units in the north central part of the Uinta Basin are the most prolific strata adjacent to the Paleocene- Eocene boundary. Oil and gas from rocks in the zone midway between the carbonate marker and Paleocene-Eocene boundary is generally overpressured and occurs primarily in the Altamont-Bluebell area. Rocks in the carbonate marker zone contain the youngest widely distributed overpressured beds. Overpressured fields are present along the north-central margin of the basin; however, fields near the south flank of the basin have normal pressures. The carbonate marker unit underlies a thick sequence of oil-stained marginal lacustrine facies extending from the center of the basin to the south flank. It has provided access to the outcrop of pressure release and fluid migration from the overpressured zone. Production of oil and gas from strata occurring near the middle marker is normally pressured. Marginal lacustrine units are the principal producing sequences in the middle marker zone, including those at Redwash field.