--> --> Evaluation of Cretaceous petroleum systems using produced oils and gases, Powder River Basin, USA

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Evaluation of Cretaceous petroleum systems using produced oils and gases, Powder River Basin, USA

Abstract

GeoMark Research has completed an applied geochemical assessment of Cretaceous-aged petroleum systems in the Powder River Basin. The study utilized detailed analyses of produced oils and gases to identify and evaluate Cretaceous petroleum systems in the basin, and to provide a more thorough understanding of source and migration relationships of existing and potential plays. Using biomarker (i.e., chemical fossils) and stable carbon isotope analyses, the source, thermal maturity (calculated as Vitrinite Reflectance Equivalent, %VRE) and geologic age characteristics of 134 oil samples were determined. A proprietary analytical technique, GC-Triple Quad Mass Spectrometry (GC-MSMS or simply, QQQ) allowed us to obtain source, age and maturity information for the first time from an additional 30 high maturity oils that were essentially devoid of biomarkers. Alkyl aromatic, C40 carotenoid and diamondoid compounds were characterized for these oils in addition to terpane and sterane biomarkers. This dual analytical approach allowed prediction of the rock unit from which oils were generated and expelled and probable migration directions. Produced gas sample pairs for 24 of the oils provided complementary thermal maturity data and further insight into migration. At least three and perhaps four distinct Cretaceous source rock units are responsible for generating oils contained in the various Cretaceous (and older) reservoir units. Two major oil families, corresponding to Upper and Lower Cretaceous source units were identified, each with three sub-families. Source rock, gas and oil thermal maturities vary in a complex way across the basin, particularly for Upper Cretaceous oil families. Mixing of Upper and Lower Cretaceous oils and secondary charging episodes by more mature fluids were identified. These observations provide a new, high-resolution insight into the 4-dimensional nature of Cretaceous petroleum systems in the Powder River Basin.