New source rock data for the Mowry and Thermopolis Shales in the Wind River Basin, Wyoming

In 2020, the U.S. Geological Survey (USGS) assessed the potential for undiscovered, technically recoverable continuous (unconventional) oil and gas resources in the Mowry Shale in the Wind River Basin Province (Finn and others, 2021). To better characterize the source rock potential of the Mowry Shale and associated strata, 129 samples were collected from 45 wells from the well cuttings collection stored at the USGS Core Research Center in Lakewood, Colorado. The sampled wells are located along the margins of the basin in order to obtain samples that were not subjected to the effects of deep burial and subsequent organic carbon loss due to thermal maturation (Daly and Edman, 1987) (fig. 1). One hundred samples are from the upper siliceous part of the Mowry Shale (Finn, 2021), 18 from the lower part of the Mowry Shale (Shell Creek Shale equivalent, Eicher, 1962; Finn, 2021), and 11 from the Thermopolis Shale (fig. 2). The sample intervals were determined by examining the cuttings under a binocular microscope and the darkest chips were selected for analysis based on observations by Hosterman and Whitlow (1981), Charpentier and Schmoker (1982), Hunt (1996), and Landon and others (2001) that total organic carbon (TOC) content generally increases as color goes from gray to black and therefore is a rough (although not always reliable) indicator of organic richness. Obvious material from carvings and contamination, such as wood chips, metal, and plastic were removed. The cuttings were composited into samples from thickness intervals that were generally 10 to 20 ft thick but ranged from 5 to 80 ft depending on how much material was available for a proper analysis. The samples were analyzed by the USGS Central Energy Resources Science Centers Petroleum Geochemistry Research Laboratory. Total carbon and total organic carbon content were determined using a LECO® Carbon Analyzer, Model C744 (LECO Corporation, St. Joseph MI) by the combustion method after carbonate removal (see Jarvie, 1991; and Oliver and Warden, 2020, for details). The programmed pyrolysis analysis was performed using a Hydrocarbon Analyzer with Kinetics (HAWK) instrument (Wildcat Technologies, Humble TX) (see Espitalié and others, 1977; Tissot and Welte, 1978; Peters, 1986; Hunt, 1996; and Dreier and Warden, 2021, for discussions of the pyrolysis method). Values directly measured by the HAWK are S1, S2, S3, and Tmax. The values HI, OI, PI, S2/S3, G.P., and S1/TOC were calculated by the author.