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Geochemistry and Mineralogy of the Eocene Green River Formation Petroleum System, Uinta Basin, Utah

Abstract

An extensive dataset of major and trace element chemistry, mineralogy, and organic geochemistry has been assembled for core and drill cuttings samples from wells across the Uinta Basin through different lacustrine facies within the Eocene Green River Formation. Data were collected to examine geochemical and mineralogical trends within the Green River Formation in the Uinta Basin and to compare to those in the more extensively studied and less deeply buried lacustrine intervals in the Piceance Basin in northwestern Colorado. Major element chemistry and X-ray diffraction results show that the major mineralogical intervals in the Uinta are generally consistent across the basin within the organic-rich offshore lacustrine facies. Total organic carbon (TOC) and programmed pyrolysis data indicate that hydrogen-rich kerogen is ubiquitous across the basin and dominates the organic-rich intervals defined previously using the Fischer assay method. Cuttings samples from the structurally deepest part of the Uinta Basin have a different mineralogical suite than samples from the Piceance Basin sedimentary depocenter and are most notably lacking saline carbonate minerals dawsonite and nahcolite, possibly due to thermal decomposition. Organic richness, as indicated by TOC values and Fischer assay oil yields, is lower in the deepest part of the Uinta Basin relative to the Piceance Basin, which may be a result of depletion of petroleum-generating potential due to thermal maturation. However, differences in organic productivity, organic matter preservation, and organic matter dilution by detrital inputs in the western Uinta Basin could also have played a role in reducing the organic matter content of these rocks. Other trends examined include trace element ratios, which highlight redox differences between marginal and offshore facies within the Uinta Basin and differences in detrital provenance between the Piceance and Uinta Basins.