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

Abstract

Oil trapped in Triassic-Jurassic age 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 overridden by the thrust plate. This conclusion is supported by a preponderance of data from a variety of analytical techniques used to make oil to oil and oil to source rock correlations. The source of the 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 Paleozoic rocks are dominantly gas with some condensate, and a significant amount of H2S is present in contrast to the low-sulphur sweet oil and gas in the Ryckman Creek-Pineview trend. The similarity in chromatographic character of liquids from both the Paleozoic and the Triassic-Jurassic reservoirs indicates that all are from the same Cretaceous 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 78 MYBP. Cretaceous source rocks beneath the Absaroka fault 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 Cretaceous source rocks in the footwall sequence during the last 55 million years, after the hanging wall structural traps involving both Mesozoic and Paleozoic reservoirs were formed. REGIONAL SETTING The Fossil basin is a somewhat loosely defined structural basin that covers a portion of the western Wyoming thrust belt and extends southward a short distance into Utah (Fig. 1). The name is derived from the richly fossiliferous lacustrine rocks of the Eocene Green River Formation which were deposited in a depression on the Absaroka thrust plate. The projected surface traces of the four major thrust faults recognized in the southwest Wyoming - northern Utah portion of the thrust belt are shown in Figure 1. Faulting was episodic during a period of compressional deformation that lasted for approximately 90 million years from latest Jurassic to early Eocene time. The westernmost thrust (Willard) is the oldest of the four major thrusts and the faults become progressively younger to the east. A prolonged period of extensional deformation began shortly after the youngest major thrust (Hogsback) was formed in early Eocene time. Crustal extension created a number of listric normal faults and structural depressions which are largely confined to the western margin of the Fossil basin. Tertiary deposits post-date most of the period of compressional folding and effectively conceal the majority of the older structures in the Fossil basin area. HYDROCARBON ACCUMULATIONS