Source and Time of Generation of Hydrocarbons in Fossil Basin, Western Wyoming Thrust Belt: ABSTRACT

Abstract

Oil trapped in Triassic-Jurassic reservoir rocks in fields along the Ryckman Creek-Pineview structural trend on the Absaroka thrust plate was generated in source rocks in the footwall Cretaceous sequence which was overridden by the thrust plate. This conclusion is supported by data from a variety of analytic techniques used to make oil-to-oil and oil-to-source rock correlations. The source of hydrocarbons in Paleozoic reservoirs in the Whitney Canyon-Carter Creek trend on the Absaroka plate is more difficult to identify with certainty. The accumulations in the Paleozoic rocks are dominantly gas with some condensate, and a significant amount of H2S is present in contrast to the low-sulfur sweet oil and gas in the Ryckman Creek-Pineview trend. The identical sul ur content and chromatographic character of condensate from the Paleozoic and the Triassic-Jurassic reservoirs suggest that both are from the same source. Reconstruction of the maturation history and measurement of present levels of thermal maturation of source rocks in different structural settings in the Fossil basin demonstrate that peak generation and migration of hydrocarbons from Paleozoic source rocks predated the Absaroka fault which was formed about 75 m.y.B.P. Cretaceous source rocks beneath the Absaroka plate were immature when overridden by the fault and have reached a high level of maturity since that time. Most of the hydrocarbons were generated and expelled from source rocks in the footwall sequence during the last 60± m.y., after the hanging-wall structural traps involving both Mesozoic and Paleozoic reservoirs were formed. End_of_Article - Last_Page 800------------