ORGANIC GEOCHEMISTRY AND CRUDE OIL SOURCE ROCK CORRELATION OF THE MISSISSIPPIAN PETROLEUM SYSTEMS NORTHERN OKLAHOMA

The Mississippian limestone in northern Oklahoma and southern Kansas is a major oil play within the southern Midcontinent region. Mississippian carbonate reservoirs are known for their heterogeneity with respect to reservoir quality and produced fluids. Hydrocarbons from the Mississippian reservoirs are chemically heterogeneous, and cannot be explained solely by a single Woodford Shale source-rock model. New molecular geochemical data from east and west of the Nemaha uplift in north-central Oklahoma provides a new insight into the source of hydrocarbons in the Mississippian play, and attempts to provide a plausible scenario of the hydrocarbon charge history. Organic-rich zones within the Mississippian carbonate section were sampled and screened for total organic carbon (TOC), organic petrography, Rock-Eval pyrolysis and geochemical markers. Additionally, twelve oil samples were analyzed from Mississippian and Woodford producing wells. Rock extracts and oil samples were analyzed using gas-chromatography and gas-chromatography mass-spectrometry techniques for quantitative analysis of diamondoids, saturate and aromatic biomarkers. Results indicate that the Mississippian source-rock has good generation potential and reached the early oil window. Extracted bitumen from Mississippian rocks and related oils show unique biomarker signatures; such as the presence of extended tricyclic terpane, high input of C₂₇ relative to C₂₈ and C₂₉ in regular and rearranged steranes, together with high C₂₇ monoaromatic steroids relative to their C₂₈ and C₂₉ homologues. Moreover, on the basis of diamondoid compound class, the Mississippian samples showed abundance of 4,8- and 4,9-dimethyl dimantanes relative to the 3,4- isomer. The extent of cracking -as measured by diamondoids- reveal a dramatic change in diamondoids concentration over the areas east and west of the Nemaha uplift. A high concentration of diamondoids was observed west of the Nemaha uplift, thus indicating episodic hydrocarbon charge of uncracked followed by cracked oils migrating out of the Anadarko basin, which supports a long-distance migration model. In contrast, the Mississippian samples from east of the Nemaha uplift are depleted in diamondoids, suggesting a short migration and localized hydrocarbon kitchen under lower thermal stress.