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Core and Log Based Stratigraphic Framework of Mississippian Limestone in Portions of North-Central Oklahoma: Implications for Reservoir Evolution and Seal Genesis

Abstract

The Mississippian play in the southern Midcontinent has demonstrated a lack of predictability in initial production volumes as a result of several variables including changes in rock fabric, intersection of different lithofacies with regional and localized unconformities and a lack of understanding of the stratigraphic architecture. In this study, lithostratigraphy, biostratigraphy, and sequence stratigraphy are integrated to address these issues. The first fundamental concept to be addressed is the effect of cyclicity on the distribution and evolution of reservoir and seal facies. The Mississippian Limestone (Tournaisian to Viséan) contains third and fourth order depositional cycles that are recognized in core and on well log signatures. These cycles generally shallow upward to a higher-energy boundary bed, which is superceded by clay-rich and dark-colored limestone that represents flooding and initiation of the next cycle. Using core-calibrated wireline logs, a series of cross-sections were constructed to help differentiate the cyclic subunits within the Mississippian limestone interval in north-central Oklahoma. Fabric and texture of these cycles were established using core descriptions and thin section petrography, which resulted in the identification of reservoir and seal facies. Cores from wells across several counties in northern Oklahoma were correlated to log signatures to provide a regional framework. Prominent flooding surfaces in core are expressed on logs as increased gamma-ray values above the cleaner, less clayey beds. Subdividing the Mississippian limestone in both core and wire line logs resulted in the recognition of characteristics of reservoirs and seals within specific intervals. Understanding the genesis and spatial distribution of these reservoirs and seals within the context of a reliable sequence stratigraphic framework enhances exploration and development strategies for oil and gas production in the heterogeneous Mississippian limestone.