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The Laramide Basins of the Rocky Mountain region are deep asymmetric structural depressions containing thick sequences of Upper Cretaceous and Tertiary strata. The combined effects of tectonics and sedimentation have contributed to the thermal evolution of the basins and to the maturation history of the source rocks. Thermal and mechanical models were used to quantify the effects of Laramide uplifts and subsequent synorogenic deposition on the hydrocarbon maturation of Cretaceous source rocks in the Bighorn basin. Laramide deformation and resultant sedimentation has clearly affected hydrocarbon maturation of Cretaceous source rocks. (Thermopolis, Mowry, Frontier, Cody).
In the Bighorn Basin of Wyoming and Montana, total organic carbon values for samples from a 2000+ ft thick interval including the Thermopolis, Mowry, Frontier, and Cody Formations average 1 wt% T.O.C. The hydrogen indices (H.I.) and elemental analyses suggest that most of the samples presently contain kerogen between type II and III. The genetic potential (G.P.) of these samples suggest that they are moderately good source rocks. Vitrinite reflectance values, production indices, elemental analyses and the distribution of extractable hydrocarbons suggest that these Cretaceous source rocks can be within the liquid hydrocarbon window from a present day depth of 2000–3000 ft (610–915 m) down to 11,000–12,000 ft (3354–3658 m).
Both inorganic (clay diagenesis) and organic (vitrinite, pyrolysis, elemental analysis of kerogen) maturation parameters were used to define the thermal history of this Laramide-style basin. A finite difference numerical model was employed which accounts for 1) basinwide variations in thermal conductivity for stratigraphic sequences 2) structural geometry of the basin, and 3) heat flow. This model allowed us to define thermal gradients throughout the basin at maximum burial of these Cretaceous shales. Modified Lopatin-type reconstructions suggest that a significant region containing Cretaceous source rocks has been within the liquid hydrocarbon window. The earliest onset of hydrocarbon maturation in the northern Bighorn basin was Latest Paleocene, with some regions still containing immature Cretaceous source rocks as a consequence of Cenozoic erosion, uplift of the Pryor Mountains, and lack of burial.
© Wyoming Geological Association, 2015