Abstract

The upper portion (Permian) of the Minnelusa Formation, a petroleum reservoir in the Powder River Basin, is well exposed in Crook County, Wyoming, in the core of a northerly trending anticline, herein called the Simons' Ranch anticline. In addition to the Minnelusa Formation, the Permian Opeche Formation, Minnekahta Limestone, and Spearfish Formation are exposed in the asymmetric, open, northerly plunging anticline. The outcrops of the upper Minnelusa Formation (Wolfcampian) at this locality on the northwestern flank of the Black Hills uplift contain oil residues and thick evaporites which have not been greatly altered by solution. Most other Minnelusa outcrops in the Black Hills are brecciated due to solution and collapse. The unaltered section at Simons' Ranch is ideal for determining primary sedimentary environments of the upper Minnelusa Formation.

During the investigation, the upper Minnelusa Formation was divided into the "Red Marker," averagely interbedded carbonates and evaporites; the lower sandstone, thickly interbedded sandstones and evaporites; a massive gypsum; and an upper yellow sandstone. The "Red Marker" indicates cyclic saline and intertidal environments; the lower sandstone indicates dunal and intertidal upper- and lower-flat environments; gypsum suggests a sea marginal pool setting; and the upper sandstone appears to represent dune migration onto the coastline.

Hydrocarbon occurrences in the upper Minnelusa Formation in the Powder River Basin are discontinuous. The upper sandstone is the primary reservoir and has been described as a paleo-dune field, a channel system, or remnant sandstones due to differential erosion. Both aeolian dunes within the uppermost sandstone and differential erosion at the Minnelusa-Opeche contact were encountered during this study. Therefore, both factors should be considered as controls on reservoir distribution during hydrocarbon exploration.