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Abstract

The lower Frontier Formation in the Moxa arch area of southwestern Wyoming is one of the most prolific gas–producing formations in the Rocky Mountain region. In this study, sedimentologic and stratigraphic analyses were conducted on cores and logs of Frontier wells from the Whiskey Butte and Moxa fields. Lower Frontier sediments were deposited as strandplains and coalescing wave–dominated deltas that prograded into the western margin of the Cretaceous interior seaway during Cenomanian time. Twelve sedimentary facies have been identified. The most common sequence consists of burrowed to cross–bedded nearshore marine (delta front, shore–face and inner–shelf) sandstones disconformably overlain by cross–bedded (active) to soft–sediment deformed (abandoned) distributary–channel sandstones and conglomerates. The sequence is generally capped by delta plain mudstones and silty sandstones.

Low permeability sandstone reservoir facies are nonhomoge– neous and include crevasse splay, abandoned and active distributary channel, shoreface, foreshore, and inner shelf sandstones. Distributary–channel facies form 80% of perforated intervals in wells in the southern part of the Moxa area but only 50% to the north. Channel sandstone bodies occur on the same stratigraphic horizon, are laterally discontinuous with numerous permeability barriers, and are occasionally stacked. Upper shoreface and foreshore sandstones thicken to the north and east and are more laterally continuous than channel facies. The percentage of perforated intervals in upper shoreface and foreshore facies increases from 20% in the south to 50% in the north. The lower Frontier sandstones contain strike–oriented shoreface (delta front) and dip–oriented distributary 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 the Moxa arch. However, there appears to be little correlation between the thickness of any reservior facies and net production.

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