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The lower Frontier Formation, Moxa arch area, southwestern Wyoming, is one of the most prolific gas-producing formations in the Rocky Mountain region. Lower Frontier sediments were deposited as strandplains and coalescing wave-dominated deltas that prograded into the western margin of the Cretaceous interior seaway during the Cenomanian.
In this study, sedimentologic, petrologic, and stratigraphic analyses were conducted on cores and logs of Frontier wells from the Whiskey Buttes and Moxa fields. Twelve sedimentary facies have been identified. The most common sequence consists of burrowed to cross-bedded nearshore
marine (delta-front and inner-shelf) sandstones disconformably overlain by crossbedded (active) to deformed (abandoned) distributary-channel sandstones and conglomerates. The sequence is capped by delta-plain mudstones and silty sandstones.
Tight-gas sandstone reservoir facies are nonhomogenous and include crevasse splay, abandoned and active distributary channel, shoreface, foreshore, and inner shelf sandstones. Distributary-channel facies represent 80% of perforated intervals in wells in the southern part of the Moxa area, but only 50% to the north. Channel sandstone bodies are occasionally stacked, occur on the same stratigraphic horizon, and are laterally discontinuous with numerous permeability barriers. Percentage of perforated intervals in upper shoreface and foreshore facies increases from 20% in the south to 50% in the north. These sandstones thicken to the north and east and are more laterally continuous than channel facies. The lower Frontier contains strike-oriented shoreface (delta front) and dip-oriented di tributary channel sand bodies in approximately equivalent amounts. Delta-plain mudstones thin to the north and east and are an important stratigraphic seal. Highest gas production rates are from distributary-channel sandstones closer to the axis of Moxa arch. However, there appears to be little correlation between the thickness of any reservoir facies and net production.
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