Source-rock analysis was performed on shales from the Lower Cretaceous (Albian) Bear River Formation in the Darby thrust plate of the western Wyoming overthrust belt. Measurements of total organic carbon (TOC); vitrinite reflectance (RO); visual kerogen analysis, including determination of thermal alteration index (TAI) and kerogen morphology; and pyrolysis provide information concerning the amount, type, and maturation levels of kerogen in the Bear River shales. TOC analysis indicated that the shales are moderately rich in organic matter (1.0 to 1.5 wt. % TOC). Kerogen morphology (structured) and pyrolysis data (suggestive of type III organic matter) indicated that organic matter in the Bear River Formation is humic and gas prone. TAI and RO values uggest that Bear River shales are in the oil-early gas-generating range (0.7 to 1.1% RO) in the northern and southern parts of the Darby plate whereas organic maturities are substantially more advanced (1.8 to 2.0% RO) in the central part of the plate.
The levels of thermal maturation, as defined by TAI and RO values, were used as constraints on a Lopatin-type time temperature index (TTI) thermal model. The TTI modeling suggests that normal depositional burial could account for the levels of thermal maturation observed in the northern and southern parts of the Darby plate, whereas an additional heat source, possibly burial of the Bear River Formation beneath the Absaroka thrust plate, is necessary to account for the relatively advanced thermal maturation measured in the central part of the Darby plate.