ABSTRACT: 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 latestage 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 sonic travel time, neutron porosity, gamma ray, and bulk density log suite. nalysis 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 lithofacies. 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 #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990