2014 Rocky Mountain Section AAPG Annual Meeting

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Geochemical Characterization of the Cow Ridge Member, a Freshwater Lacustrine Stage of the Eocene Green River Formation, Piceance Basin, Colorado


The Cow Ridge Member of the Eocene Green River Formation was deposited in a freshwater lake that preceded saline Lake Uinta in the Piceance Basin, Colorado. The thickest Cow Ridge deposition is in the central and northwestern portion of the Piceance Basin where it immediately underlies the Garden Gulch Member of the Green River Formation. Elsewhere in the basin, a tongue of fluvial Wasatch Formation separates the Cow Ridge Member from Lake Uinta sediments. The Cow Ridge Member is equivalent to the informal Uteland Butte member of the Green River Formation in the Uinta Basin and has been described as similar to the Luman Tongue of the Green River Formation in the Greater Green River Basin. The Cow Ridge Member is likely to be too thermally immature to have generated oil in the Piceance Basin, but it can serve as an analog for ongoing shale oil development in Uteland Butte. Outcrop samples were collected from several sections in northwestern Colorado and core samples were obtained from the U.S. Geological Survey's Core Research Facility in Denver. Samples were screened using infrared spectroscopy to assess mineral and organic content and further analyzed to determine programmed pyrolysis parameters, total organic carbon content, total carbonate, and stable isotopic composition of unaltered carbonates. Shale is the predominant lithology with interspersed marlstones, coquinas, sandstones, and some thin coal beds. Differences between the Cow Ridge Member oil shale and overlying shales of the Garden Gulch and Parachute Creek Member include the nearly ubiquitous presence of kaolinite, lower total organic carbon content, and lower hydrogen indices.