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Intermontane basins of the northern United States Overthrust belt have received only minor attention from petroleum geologists and geochemists in the modern literature, despite the common occurrence of oil shales and thick sediment columns in such basins. The present study investigates the petroleum source rock potential of the freshwater lacustrine sediments, including mudstones, oil shales, and lignites, of the Kishenehn basin of northwestern Montana and southeast British Columbia. Outcrop samples of the Tertiary Kishenehn Formation include excellent, albeit immature, petroleum source rocks. Average total organic carbon contents and hydrogen indices exceed 6% and 500 mg/g, respectively, and numerous samples contain type I kerogen. All samples studied are thermally immat re to marginally mature, having vitrinite reflectance values of 0.28 to 0.51% Ro.
Organic geochemical and petrographic analyses of the Kishenehn sediments allow estimates of detailed depositional setting and specific biological input to the sediment column. The southernmost sediments in the basin contain the least land-plant organic matter input and possess the richest petroleum source potential. Although vitrinite reflectance values are not particularly sensitive to maturity differences in this sample set, molecular geochemical data were found to be useful in evaluating fine maturity distinctions and are used here to establish the order of Kishenehn outcrops on the basis of maturity. This use of molecular markers as maturation and depositional setting indicators for freshwater lacustrine environments has widespread application in thrust belts. The results of this tudy will be useful in exploration of over two dozen intermontane basins throughout the northern extent of the United States Overthrust belt.
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