Abstract: Depositional Geometry of the Permian Minnelusa Formation, Powder River Basin, Wyoming

Daniel W. Bean

The eolian sandstone reservoirs and sabkha carbonate traps of the Permian Minnelusa Formation within the Powder River basin of northeastern Wyoming provide a well preserved example of a complex arid depositional

system. Over 90,000 interformational tops were picked from 6200 wells to build a model of the depositional geometry of the Minnelusa system. Cyclic carbonate and evaporite deposition allowed excellent preservation of the geometry of these eolian sandstone reservoirs. Three major transgressive-regressive cycles are recognized in the upper Minnelusa. The orientation of successive eolian dune complexes changed as much as ninety degrees as the dominant dune type changed from transverse to longitudinal, even though paleo-wind direction varied little. Both compactional and paleostructural factors influenced eolian sand deposition and preservation. Differential compaction of underlying carbonate muds due to loading by eolian sands localized areas of sand preservation. A predictive model for reas of reservoir deposition, preservation, and hydrocarbon trapping was created by careful mapping of the relatively continuous carbonate and evaporite trap facies. Four primary reservoir-trap geometries are recognized, although most Minnelusa oil fields combine several of these elements. The orientation of the preserved eolian deposits combined with regional structure result in a different spatial frequency and field size for the stratigraphic traps of each cycle. This highly penetrated eolian system serves as a well documented analog for exploration in other eolian systems.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994