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Organic and Inorganic Geochemistry of the Mississippian Limestone in the Anadarko Shelf


To date, over 900 million barrels of oil and over 6 tcf of natural gas have been produced from the mixed carbonate-siliciclastic Mississippian Limestone of the Anadarko Shelf. Recent exploration and production efforts in the Anadarko Shelf have focused on the STACK (Sooner Trend Anadarko Canadian Kingfisher Counties) play in central Oklahoma. There is hydrocarbon production from multiple intervals within the STACK play, including production from the heterogeneous Mississippian Limestone interval. Depositional factors such as redox and detrital input impact organic richness, mineralogy and thermal maturity which in turn influence hydrocarbon producibility. Recent studies indicate that the Mississippian Limestone interval is self sourcing in addition to containing hydrocarbons sourced from the underlying Woodford Shale. Consequently, reconstructing depositional paleoenvironmental conditions is crucial to understanding elements that impact organic matter production and preservation and which ultimately will aid in delineating areas where there is enrichment of organic matter, which could be production sweet spots. In this study, we investigate depositional redox and detrital input and their effects on organic matter enrichment using a combination of organic and inorganic geochemical techniques including biomarkers, stable isotope geochemistry and trace element geochemistry. Trace element proxies of redox and detrital input as well as stable nitrogen isotopes were measured on rock samples from cored wells in the STACK play. Rock-Eval pyrolysis was carried out on samples to evaluate hydrocarbon generation potential while bitumen was extracted from rocks using an Accelerated Solvent Extraction (ASE) apparatus. Sterane and Terpane biomarkers were examined in recovered bitumen via gas chromatography triple quadrupole mass spectrometry (GC-QQQ/MS). In addition, taxon-specific biomarkers, such as tricyclic diterpanes, were analyzed in order to understand processes controlling organic matter origins. Preliminary results suggest that redox conditions at time of deposition were oxic to suboxic and depositional redox conditions do not appear to influence organic richness whereas areas with higher detrital input appear to be enriched in organic matter. Our results support previous studies which imply that the Mississippian Limestone interval in the STACK play are capable of sourcing hydrocarbons, and that the generated hydrocarbons possess unique geochemical fingerprints which are different from those of Woodford Shale derived hydrocarbons.