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Character and Regional Correlation of Reservoir Units in the Mississippian Madison Formation (Wyoming and Utah)


Dolomitic mudstones from peritidal facies of the Mississippian Madison Formation form important reservoir units within the Greater Green River Basin of Wyoming, Utah, and Colorado. How these facies extend into the subsurface of southern Wyoming and into northeastern Utah is poorly understood. We use new outcrop data and carbon isotope chemostratigraphy from the Uinta Mountains, Utah and existing subsurface data from cores in southern Wyoming to extend correlations from northern Utah to northern Wyoming, facilitating mapping of the distribution and extent of key reservoir facies. Data were collected from outcrop locations on the southern flank of the east-west trending Uinta Mountains. Lithofacies within the Uinta sections range from shallow, peritidal mudstone-wackestone to outer ramp, spiculite-rich calcareous siltstone. Collectively, the lithofacies suggest deposition on a west-dipping homoclinal carbonate ramp. Fossil assemblages, sedimentary structures, and other characteristics indicate that lagoons were at times restricted. These restricted settings facilitated early dolomitization of peritidal and subtidal strata. This dolomitization enhanced porosity (up to 35%), creating reservoir quality lithologies within peritidal units. However, high porosity (up to 25%) also exists within some of the non-dolomitic peritidal mudstones-wackestones. In addition, at least two stratigraphic horizons with cavernous porosity are documented. The diagenetic processes governing reservoir quality will be assessed using standard petrographic techniques and stable isotope geochemistry. Regional correlation takes advantage of available sequence stratigraphic and biostratigraphic data as well as changes in the global carbon cycle during the Mississippian that imparted large fluctuations in the d C value of the oceans, which are preserved as systematic stratigraphic shifts in dC values through carbonate successions. The nature and timing of the isotopic shifts are well-understood and can be correlated to the Global Boundary Stratotype Section and Point (GSSP) for the Mid-Carboniferous boundary in Arrow Canyon, NV. This work provides new insights into the nature and regional extent of key production units within the Madison Formation.