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