--> --> Outcrop and core characterization of the Uteland Butte member and Castle Peak interval, lower Green River Formation, southwestern Uinta Basin, Utah

AAPG Rocky Mountain Section Meeting

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Outcrop and core characterization of the Uteland Butte member and Castle Peak interval, lower Green River Formation, southwestern Uinta Basin, Utah

Abstract

The Uteland Butte member and Castle Peak interval of the Eocene Green River Formation have emerged as leading horizontal tight oil targets in the Uinta Basin of northeastern Utah. To date, 136 horizontal wells have targeted the Uteland Butte member with cumulative production of 22.0 million barrels of oil equivalent (BOE), whereas only 33 wells have targeted the more prospective Castle Peak interval, recording 4.2 million BOE of production. The carbonate-dominated Uteland Butte member represents the first widespread transgression of freshwater Lake Uinta, and the overlying siliciclastic-dominated Castle Peak interval corresponds with a period of overall lake regression and increased fluvial input. This study focuses on the proximal and immature expressions of the Uteland Butte member and Castle Peak interval using the newly acquired Willow Creek No. 1 core and outcrops along the southwestern margin of the Uinta Basin. Centimeter-scale sedimentological descriptions, mineralogical analyses, and geophysical data were combined to create a facies model used to elucidate depositional cyclicity, relative paleo-lake levels, and package continuity across the study area. The Uteland Butte member is dominated by lacustrine and marginal lacustrine facies including bedded dolomite, beach-type packstone deposits, fossiliferous limestone, organic-rich mudstone, and thin coal beds. The Castle Peak interval exhibits a wide variety of facies in alternating packages of terrestrial (paleosols and fluvial sandstone channels) and lacustrine (fossiliferous limestone, organic-rich mudstone, and siltstone) deposits. Meterscale shallowing upward cycles are superimposed on larger decameter-scale depositional cycles that can be correlated across the southwestern Uinta Basin, although facies vary based on location relative to the ancient shoreline. To understand the proximal study area with respect to the distal oil-producing area, observations are compared with the Petes Wash U 13-06 GR core from the south-central basin, which exhibits similar depositional cyclicity, but has more distal facies.