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Petroleum Geology of Ashley Valley Field and Hydrocarbon Potential of the Surrounding Area, Uintah County, Utah

Thomas C. Chidsey, Jr, Utah Geological Survey, P.O. Box 146100, Salt Lake City, UT 84114-6100, phone: 801-537-3364, [email protected] and Douglas A. Sprinkel, Utah Geological Survey, 1594 W. North Temple, Suite 3110, Salt Lake City, UT 84116.

Ashley Valley field was discovered in 1948 and represents the first commercial oil production in Utah. The field is located southeast of Vernal, Utah, north of the Uinta Basin boundary fault. The primary reservoir is the eolian Pennsylvanian/Permian Weber Sandstone in a northwest-southeast-trending faulted anticline. Pay thickness of the Weber is about 60 ft with an average porosity of 13% and permeability ranging from 27-161 mD. Nearby production-scale outcrops provide analogs of the subsurface reservoir-facies characteristics and boundaries that determine the overall Weber heterogeneity. The productive field area is 780 acres and there are currently 17 producing wells. Cumulative production is over 20 million bbls of oil. The oil represents a mixture of hydrocarbons from source rocks in both the Permian Phosphoria Formation and Cretaceous Mancos Shale.

Over 50 unsuccessful exploratory wells have been drilled to search the 300-square-mile area of the eastern Uinta uplift for another field like Ashley Valley. Targets include subtle anticlines on trend with Ashley Valley field, other major surface structures, and areas hidden beneath basement-involved faults such as the Cliff Ridge fault.

The Weber Sandstone also serves as a ground-water aquifer for the region. Recharge occurs where the Weber outcrops in high-elevation areas such as the nearby Blue and Split Mountains and the Uinta Mountains. Hydrodynamic conditions suggest that oil in the Weber may have been flushed southward by the flow of fresh ground water moving from outcrops to the north of Ashley Valley, thus creating the best oil potential closest to the Uinta Basin boundary fault.