Maturation History and Thermal Evolution of Cretaceous Source Rocks of Bighorn Basin, Wyoming and Montana: ABSTRACT

Abstract

The Laramide basins of the Rocky Mountain region are deep asymmetric structural depressions containing thick sequences of Upper Cretaceous and Tertiary sandstone 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. In the Bighorn basin of Wyoming and Montana, total organic carbon (TOC) values for samples from a 2,000-ft (610-m) thick interval, including the Thermopolis, Mowry, Frontier, and Cody Formations, average 1 wt. %. The hydrogen indices and elemental analyses suggest that most of the samples presently contain kerogen between types II and III. The genetic potential of these samples suggests that they are moderately good source rocks. Vitrinite reflectan e 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 2,000-3,000 ft (610-914 m) down to 11,000-12,000 ft (3,353-3,658 m). On the basis of these observations, plus graphical and numerical thermal models for the Bighorn basin, it is suggested that (1) the Cretaceous section has generated hydrocarbons and could have produced the hydrocarbon production in the Bighorn basin, particularly from Cretaceous reservoirs, (2) migration distances for hydrocarbons into Cretaceous reservoirs could be short, (3) the stratigraphic and lateral distribution of marine sandstones intercalated within the Cretaceous source rocks provide ample opportunity for stratigraphic and/or diagenetic traps over a wide depth interval in this basin, and (4) owing to variations in thermal gradients within this basin, or similar Laramide-type basins, the hydrocarbon liquid window is expanded over a particular stratigraphic interval with dept . End_of_Article - Last_Page 937------------