Geochemical evaluation of the Mississippian Limestone

Abstract

A multi-proxy geochemical investigation of the Mississippian Limestone interval in the STACK resource play area of the Anadarko Shelf was undertaken, in order to evaluate environmental conditions during deposition of sediments of the Mississippian Limestone interval within the Anadarko Shelf. Evaluation of the source rock potential of the interval was also undertaken in order to determine if the unit is capable of self-sourcing oil within the STACK. From the evaluation of stable nitrogen isotopes and redox sensitive elements, we concluded that redox conditions at time of deposition of sediments of the Mississippian Limestone interval within the Anadarko Shelf were oxic to suboxic, thereby ruling out anoxia as a factor that drove preservation of organic matter within the interval on the Anadarko Shelf. Analysis of organic carbon stable isotopes together with elemental proxies of paleoproductivity and detrital input suggests that there was lack of enhanced detrital input into the Anadarko Shelf, as well as an absence of heightened productivity within the Anadarko Shelf at time of deposition of sediments of the Mississippian Limestone interval. Finally, results from evaluation of organic geochemical analyses (Rock-Eval, saturate biomarkers) undertaken on Anadarko Shelf Mississippian Limestone rock extracts indicate presence of biomarkers said to be diagnostic of sediments of the Mississippian Limestone interval by previous investigations. Furthermore, biomarker analysis results support the paleoenvironmental interpretations obtained from stable isotope and elemental proxy results for sediments of the Mississippian Limestone interval within the Anadarko Shelf. Results from this study does not explicitly rule out that sediments of the Mississippian Limestone interval are capable of generating oil within the Anadarko Shelf, although pyrolysis results from the few samples examined in this study have poor source rock characteristics.

Insights from this study validate the utility of combining multiple geochemical proxies to tell a more nuanced story regarding paleoenvironmental conditions during deposition of mixed clastic-carbonate sediments, such as the Mississippian Limestone interval of the Anadarko Shelf, as well as the utility of some of these geochemical techniques (e.g., stable isotopes of nitrogen, elemental abundance of elemental proxies of redox, paleoproductivity and detrital input) in deciphering paleodepositional conditions in mixed clastic-carbonate sediments.