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The Hanging Rock area is the western extension of Oil Springs Field, located 12 miles southeast of and 2200 ft updip from Natural Buttes Field in the southeastern Uinta Basin. The primary reservoir target in the area is the Tertiary Wasatch Formation at depths of 2500 to 4000 ft. The Wasatch Formation in the Hanging Rock area consists of interbedded, discontinuous, lenticular sandstones and red to green siltstones and claystones deposited in a lower delta plain environment. Sandstones within the Wasatch at Hanging Rock are predominantly chertarenites, with porosity values from 12 to 17% and permeability values from 0.22 to 37 md. Wasatch reservoirs at Hanging Rock are normally pressured to slightly overpressured, and gas-charged sandstones are commonly interbedded with water-bearing zones. Isotopic analysis of gas produced from the field suggests that coals in the underlying Mesaverde Group are the source and that the gas is thermogenic in origin.
Due to the discontinuous nature of sandstones within the Wasatch, the lateral and vertical distribution of individual reservoir units is extremely difficult to predict. Wells drilled on 80-acre spacing have encountered sandstones that are not correlative to offset wells. Various subsurface mapping techniques have been employed in an attempt to predict the distribution of individual reservoir units but results have been inconclusive.
In October 2006, a 21.5-square-mile 3-D seismic survey was acquired over the Hanging Rock area, and coherency and curvature analysis were utilized to identify and map faults within the Wasatch-Mesaverde interval. The data revealed conjugate, nearly orthogonal, sets of normal faults, with one set trending generally NE-SW and the other trending NW-SE. The faults appear to influence productivity within the Wasatch by providing both conduits for the migration of gas from the Mesaverde and traps for individual reservoirs within the Wasatch.
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