Abstract

The Uinta Basin and the Green River Formation contained therein have historically been the focus of numerous geologic and organic geochemical studies characterizing the classic Type I lacustrine kerogen in source beds such as the mahogany shale. In contrast to most of these investigations, the objective of the present study was to examine select source facies from a range of stratigraphic units within the Tertiary section. Particular attention was placed on the basal unit of the Green River Formation, informally designated as the black shale facies. This basal unit is thought to be responsible for most oil generation in the Uinta Basin, yet source-rocks in this sequence have never been examined by modern organic geochemical techniques.

Key organic-rich units from the Green River Formation have been defined in terms of their stratigraphy, depositional environments and unique organic geochemical characteristics. Elemental analyses show that the basal black shale facies kerogen contains an unusually high organosulfur content and extractable hydrocarbons are relatively enriched in paraffins and depleted in β-carotane. A depositional model for this source-rock proposes development of a nearshore benthic algal mat with preservation of algal lipid components via reaction with inorganic sulfur species produced by bacterial sulfate reduction. In contrast, other oil shale facies (mahogany shale, carbonate marker) show geochemical characteristic consistent with an offshore open lacustrine depositional model involving preservation of degraded algal biopolymers and bacterial biomass in the anoxic bottom waters of a large perennially stratified lake. Additional source facies examined include a vitrinite-rich paludal coal with an extremely high oil generative potential and a sulfur-rich open lacustrine oil shale having distinctive geochemical characteristics associated with hypersaline deposition.