Hydrocarbon Maturation, Source Rock Potential, and Thermal Evolution of Late Cretaceous and Early Tertiary Rocks of Hanna Basin, Southeastern Wyoming: ABSTRACT

Abstract

The Hanna basin is a small (1000 mi/sup 2/ or 2600 km/sup 2/), anomalously deep (38,000 ft or 11.5 km), intermontane basin in the Rocky Mountain foreland province that contains an extremely thick Late Cretaceous through early Tertiary sedimentary sequence. This sequence resulted from a large influx of sediments during abrupt basin subsidence associated with Laramide tectonism. Active-basin-subsidence curves (backstripping) suggest that at least 18,000 ft (5.5 km) of Late Cretaceous and early Tertiary sediments was deposited within 15 m.y., creating a thermally mature setting for potential hydrocarbon source rocks. Geochemical analyses of numerous coals and organic-rich shales, distributed in the Late Cretaceous and early Tertiary sequence within the central portions of the basin, indicate that these are moderate to good hydrocarbon source rocks with varying degrees of thermal maturity. Modified Lopatin-type reconstructions, using basin-wide thermal conductivities and heat flow, suggest a present-day entrance to the hydrocarbon liquid window at 8000 ft in the central portions of the basin, the onset of hydrocarbon generation taking place in earliest Paleocene time. In addition, vitrinite reflectance and pyrolysis profiles, combined with kerogen elemental analysis, suggest generation of gas and perhaps consequent overpressuring in the deeper reaches of the basin. The integration ofmore » thermal, mechanical, and geochemical models, along with stratigraphic reconstructions, results in the delineation of hydrocarbon generative zones in the Hanna basin.« less