--> Tight-Oil Resource Potential of the Mahogany Zone Within the Eocene Green River Formation, Uinta Basin

AAPG ACE 2018

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Tight-Oil Resource Potential of the Mahogany Zone Within the Eocene Green River Formation, Uinta Basin

Abstract

The Mahogany zone in the Eocene Green River Formation was deposited about 49 Ma during the maximum transgressive stage of Lake Uinta in the Piceance and Uinta Basins of Colorado and Utah. This interval contains some of the most organic-rich source rocks in the world, often up to 35 wt. % total organic carbon (TOC). Lake Uinta had deep depocenters in both basins; in the Uinta Basin depocenter, the Mahogany zone is deeply buried, and various investigators have proposed it to be a source of asphaltic oil as well as bitumen (now incorporated into oil sands) and the solid hydrocarbon gilsonite. In this study, the geochemistry and mineralogy of the Mahogany zone was examined in core and cuttings samples collected from across the Uinta Basin. Analyses include major and trace element chemistry, infrared spectroscopy, TOC content, programmed pyrolysis, and X-ray diffraction mineralogy. Samples were taken in one-foot intervals in two cores from the Mahogany zone. The first core, Petes Wash U 13-06 GR, was collected from between the southern margins and the depocenter (Mahogany zone ~2500 ft deep), and the second, Norling 1-9B1, was taken from the deep basin center (Mahogany zone ~7500 ft deep). The samples were characterized to establish stratigraphic correlations between the cores and to estimate kerogen transformation ratios for the Mahogany zone organic matter. This was done based on programmed pyrolysis parameters, including hydrogen (S2/TOC × 100) and production (S1/S1+S2) indices, with and without chloroform soxhlet extraction. Results show that some conversion of kerogen to bitumen and oil has occurred in the Mahogany zone in the Norling core and that it is just within the oil window at this depth and location. Analysis of the extracted organic matter by gas chromatography shows the presence of normal alkanes in higher abundances than typically observed in immature bitumen, which further indicates that cracking of kerogen has occurred. These results provide additional evidence that the Mahogany zone is a source for some of the petroleum accumulations in the Uinta Basin.