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A High-Resolution Source Rock Microanalysis Evaluation of the Saskatchewan Group (Duperow and Birdbear Fm), Three Forks Group (Torquay and Bakken Fm) and Lr Madison Group (Lodgepole): Northern Williston Basin

Abstract

This paper presents the results from a high resolution, integrated geochemical and petrographic analytical assessment of the source rock potential of the Devonian Duperow, Birdbear and Torquay Formations, the Bakken Formation and the Lower Carboniferous Souris Valley Formation (Lodgepole) as a continuous source rock profile based upon core. Over 400 individual samples were collected from selected core creating a detailed depth- wise profile that permitted the complete and comprehensive integrated assessment using RockEval analysis and petrographic analysis. Samples were typically obtained using a micro drill, thereby avoiding contamination or dilution of sample. The most significant findings were the extreme degree of variability with a single formational unit or sub unit. The greatest depth-wise variation in TOC was 30wt% within a single formational sub-unit, similar depth-wise variations were observed for S1, S2 and Hydrogen Index. Furthermore, the microanalysis technique found several thin, but organic rich horizons and also permitted the subdivision of several formational sub-units based upon source potential. Generally speaking the source throughout the Duperow Fm was low, the Birdbear Fm contained two horizons with TOC's of 7.3 wt% and 18.0 wt%, the Torquay Fm had several 1m zones with a TOC greater than 1.0wt%, the L and U Bakken up to 32wt% whereas the Souris Valley contains several source horizons with a TOC greater than 10 wt%. Tmax also exhibited several 'reversals in value' and the high degree of variation in Hydrogen Index was easily rationalised through detailed petrographic analysis. The data generated in this study is evidence that Rock-Eval pyrolysis can effectively work as a microanalysis technique, determining and documenting the amount, range and quality of organic matter on a millimeter to centimeter scale. This study confirms that a high resolution approach integrating Rock-Eval pyrolysis and petrographic analysis is effective for classifying the quantity of organic matter (TOC), the quality of organic matter (Type) and the level of thermal maturity, which are all factors when characterizing the source potential of a given formation or accurately characterising the hydrocarbon potential within both source rock or unconventional play. Furthermore, high-resolution geochemical analysis can identify sub-units within an argillaceous unit and shed light on the cyclic depositional nature of organic matter within a formation.