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Petroleum Geochemistry of Ordovician-Sourced Oils From the Williston Basin, USA


Over 350 Williston Basin oils were grouped into families which share common sources based on genetic-specific biomarkers and stable carbon isotope compositions using multivariate statistics. Over ninety (90) Middle Ordovician-sourced oils were identified by their unique n-paraffin distributions (odd over even carbon number preference around n-C17 & n-C19 and greatly diminished isoprenoids) due to the microorganism G. prisca. These oils are also characterized by relatively high C19 tricyclic terpane and C24 tetracyclic to C23 tricyclic terpane ratios as well as having the most positive (enriched) stable carbon isotope compositions of oils from the Williston Basin. Most of these oils were produced from the Ordovician Red River Formation, and source rock evaluation of Red River cores and cuttings (e.g. TOC and Rock-Eval data) suggests that the Red River is also the principal source rock unit. Estimates of oil thermal maturity based on biomarkers show that the most mature oils exist within the basin depocenter while the least mature oils are located along the Cedar Creek Anticline. Furthermore, another smaller set of oils (<10) contain G. prisca biomarkers (albeit diminished) but have very negative (depleted) stable carbon isotopic compositions; these oils may have been generated from shales of the Ordovician Winnipeg Group. Finally, a few oils with predominantly Mississippian carbonate biomarkers (i.e. Lodgepole Formation source) appear to have an Ordovician component as revealed by principal component analysis.