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A Reinterpretation of the Lower Ferron Sandstone in the Drunkards Wash CBM Field, Central Utah

Abstract

Deposition of the Ferron Sandstone Member occurred during a widespread regression of the Western Interior Seaway during the Turonian. The Ferron is informally subdivided into two units. The Upper Ferron, ferronensis sequence is well exposed as a shallow marine and coastal plain clastic wedge along the edge of the Wasatch Plateau in central Utah. In contrast the Lower Ferron has previously only been documented in the subsurface in the Drunkards Wash, Buzzard Bench and Helper CBM fields around the town of Price, where it is represented by shallow marine facies overlain by coastal plain deposits including the gas producing coals. Outcrops to the south-east of the field include a series of heterolithic marine deposits called the Clawson and Washboard units which have previously been interpreted as “offshore sand ridges” derived from cross-shelf transport from the north. Correlation of 55 borehole logs from Drunkards Wash, coupled with outcrop studies have resulted in a new depositional model for the system. The correlations suggest that the Lower Ferron is comprised of a series of five progradational to aggradational shoreface parasequences which prograded in a south-easterly direction. A geocellular model which incorporates the subsurface data and the adjacent outcrops supports the hypothesis that the Clawson and Washboard are simply the down-dip expression (offshore transition zone) of these shorelines. This is also supported by correlation of a bentonite horizon that occurs in both the outcrops and in a core from the field. This model is a significant modification to the existing paleogeographic understanding of the region. The data have been incorporated into a very large scale geocellular model of the Cretaceous strata in the Books Cliffs and Wasatch Plateau. This model which is 100 x 120 x 5 km, highlights the very thick successions of offshore Mancos Shale in the area and suggests that that further, coal bearing coastal plain deposits exist to the west beneath the Wasatch Plateau. These may have further CBM potential.