Resolution of Complex Minnelusa Stratigraphy, Powder River Basin, Wyoming, Using an Interactive Workstation-Based Interpretation Environment

Lyndon H. Bell

The Permian Minnelusa Formation of the Powder River basin is a complex of cyclic shoreline eolian sands and nearshore reworked sands, evaporites, and carbonates. Hydrocarbons occur in secondary porosity created by late-stage facies-dependent dissolution of anhydrite cement. Trapping is by lateral facies changes and by paleotopographically formed structural anomalies. Stratigraphic trends change abruptly and, therefore, create serious geologic interpretation and mapping problems. Correlation of rock units using only raw petrophysical curves is difficult. However, correlations based on lithology are easier to make and have a firmer stratigraphic basis. Lithology was determined by numerical analysis of the sonic travel time, neutron porosity, gamma ray, and bulk-density log uite. Analysis results were digitally stored as log curves representing percentages of shale, anhydrite, dolomite, quartz, and porosity. Displays of these curves were used for correlation. Summary statistics from specific depth intervals were used for mapping lithologies. Lithofacies cross sections and maps, conventional cross sections and structure contour maps, and fluid production maps were used to interpret the petroleum geology of the study area.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.