Abstract

An evaluation of the source rock potential of the Lower Cretaceous (Albian) Bear River Formation in the Darby Plate of the western Wyoming Overthrust Belt is the subject of this investigation. Measurements of total organic carbon (T.O.C.); vitrinite reflectance (R₀); visual kerogen analysis, including determination of thermal alteration index (T.A.I.) and kerogen morphology; and pyrolysis, provide information concerning the amount, type and maturation levels of kerogen in the Bear River shales. This type of analysis shows that the Bear River shales are moderately rich in humic, gas-prone organic matter and that the organic matter in a portion of the Darby Thrust Plate is thermally mature. Tectonic reconstruction and thermal modelling indicate that the probable cause for the advanced thermal history is heat produced by burial beneath the Absaroka Thrust Plate.

In the Bear River Formation, the source rock analysis suggests that kerogen in the portion of the Darby Plate between T.27N and T.32N has probably been expended, whereas kerogen in the formation to the north and south may be generating hydrocarbons at present.