Abstract

Ashley Valley oil 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 along the south flank of the Uinta Mountains. The primary reservoir is the eolian Pennsylvanian/Permian Weber Sandstone in a northwest-southeast-trending, faulted and fractured anticline which developed as a subtle subsidiary feature along the west-plunging axis of the Section Ridge anticline. The Ashley Valley anticline and other structures in the area were created during the Laramide orogeny and are associated with blind, basement-involved thrust faults trending marginal to the Uinta Basin. Nearby field-scale outcrops serve as analogs of the reservoir-facies characteristics and boundaries that determine the overall Weber heterogeneity. Eolian deposits of the Weber Sandstone include dunes, interdunes, and sand sheets.

The oil produced from the Weber Sandstone represents a mixture from source rocks in both the Permian Phosphoria Formation and Cretaceous Mancos Shale. Pay thickness of the Weber is about 60 ft (20 m) with an average porosity of 13 percent and permeability ranging from 27 to 161 millidarcies in subarkose to quartz arenite. The field productive area is 780 acres (320 ha) where there are currently 17 producing wells. Cumulative production is over 20 million barrels (3 million m³) of oil. A significant amount of oil remains in the reservoir. Injection of carbon dioxide gas may greatly improve oil recovery from the field based on the successful carbon dioxide-flood program at Rangely field in western Colorado.

Since 1948, over 60 Weber exploratory wells have been drilled in the region to find additional fields like Ashley Valley field. Targets include subtle anticlines on trend with Ashley Valley and major surface structures such as the Split Mountain and Section Ridge anticlines. Other wells have also tested the Weber potential beneath basement-involved thrusts such as the Miners Draw fault. None have been successful.

The Weber Sandstone also serves as a ground-water aquifer for the region. Recharge occurs in high-elevation areas where the Weber crops out, such as the nearby Blue and Split Mountains and the Uinta Mountains. These hydrodynamic conditions suggest that Permian-sourced oil in the Weber may have been flushed to the south by fresh ground water moving from the north and northwest, thus leaving the best oil potential closest to the Uinta Basin Boundary fault where Cretaceous-sourced oil contributed to the hydrocarbon system.