Vitrinite Reflectance Data for Cretaceous Marine Shales and Coals in the Bighorn Basin, North-Central Wyoming and South-Central Montana.

摘要

The Bighorn Basin is a large Laramide (Late Cretaceous through Eocene) structural and sedimentary basin that encompasses about 10,400 square miles in north-central Wyoming and south-central Montana. The basin is bounded on the northeast by the Pryor Mountains, on the east by the Bighorn Mountains, and on the south by the Owl Creek Mountains. The north boundary, as defined in this report, includes a zone of faulting and folding referred to as the Nye-Bowler lineament (Wilson, 1936). The northwest and west margins are formed by the Beartooth Mountains and Absaroka Range, respectively. Important conventional oil and gas resources have been discovered and produced from reservoirs ranging in age from Cambrian through Tertiary (Fox and Dolton, 1989; 1996a, b; De Bruin, 1993). In addition, a potential unconventional basin-centered gas accumulation may be present in Cretaceous reservoirs (Johnson and Finn, 1998; Johnson and others, 1999; Surdam and others, 1997; Finn and others, 2010). In recent years, advances and success in horizontal drilling and multi-stage fracture stimulation have led to an increase in exploration and completion of wells in marine shales in other Rocky Mountain Laramide basins that were traditionally thought of as hydrocarbon source rocks. Important parameters that control hydrocarbon production from these shales include: reservoir thickness, amount and type of organic matter, and thermal maturity (Milici, 1993; Curtis, 2002). The purpose of this report is to present new vitrinite reflectance data collected from Cretaceous marine shales and coals in the Bighorn Basin to better characterize the thermal maturity and petroleum potential of these rocks. These new data supplement previously published data by Hagen (1986), Nuccio and Finn (1998), Yin (1997), and Finn and Pawlewicz (2007). Ninety-eight samples from Lower Cretaceous and lowermost Upper Cretaceous strata were collected from well cuttings from wells stored at the U.S. Geological Survey (USGS) Core Research Center in Lakewood, All samples were analyzed by vitrinite reflectance to determine levels of thermal maturation. Preparation of samples for reflectance analysis required (1) crushing the larger pieces into 0.25- to 1-millimeter pieces, (2) casting the pieces with epoxy in pre-cut and drilled plugs, and (3) curing the samples overnight. Subsequently, a four-step grinding and polishing process was implemented that included sanding with progressively finer sandpaper (60 to 600 grit) followed by a two-step polishing process (0.3 and 0.05 micron).