Chapter 4 New Source Rock Data for the Thermopolis and Mowry Shales in the Wyoming Part of the Bighorn Basin Click here to return to Volume Title Page By Thomas M. Finn Chapter 4 of Petroleum Systems and Geologic Assessment of Oil and Gas in the Bighorn Basin Province, Wyoming and Montana By U.S. Geological Survey Bighorn Basin Province Assessment Team U.S. Geological Survey Digital Data Series DDS–69–V U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2010 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Finn, T. M., 2010, New source rock data for the Thermopolis and Mowry Shales in the Wyoming part of the Bighorn Basin: U.S. Geological Survey Digital Data Series DDS–69–V, 16 p. ISBN: 1–4113–2667–5 iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Acknowledgments .........................................................................................................................................1 Depositional Setting ......................................................................................................................................1 Stratigraphy ....................................................................................................................................................6 Thermopolis Shale ................................................................................................................................6 Muddy Sandstone .................................................................................................................................6 Mowry Shale..........................................................................................................................................6 Methods...........................................................................................................................................................7 Results .............................................................................................................................................................7 Quantity of Organic Matter .................................................................................................................7 Types of Organic Matter ......................................................................................................................8 Distribution of Organic Matter in the Mowry Shale ........................................................................9 Summary........................................................................................................................................................13 References ....................................................................................................................................................13 Figures 1. Rocky Mountain region showing locations of Laramide basins ...........................................2 2. Index map of the Bighorn Basin Province ................................................................................3 3. Extent of the Western Interior Seaway during Campanian time ..........................................4 4. Regional stratigraphic cross section of Cretaceous and Paleocene rocks .......................5 5. Stratigraphic chart and type log of Lower and lowermost Upper Cretaceous rocks .......6 6. Source rock sample localities ....................................................................................................8 7. Tmax versus depth plot ................................................................................................................9 8. Total organic carbon content and S2 .......................................................................................11 9. HI vs OI diagrams ........................................................................................................................11 10. The ratio S2 /S3 .............................................................................................................................12 11. Variations of total organic carbon and kerogen types for the Mowry Shale ...................12 Table 1. Results of Rock-Eval and total organic carbon analysis .....................................................10 Plate 1. Subsurface stratigraphic cross section of the Thermopolis and Mowry Shales and assocated rocks in the Bighorn Basin .............................................. link New Source Rock Data for the Thermopolis and Mowry Shales in the Wyoming Part of the Bighorn Basin By Thomas M. Finn and others, 1999; Finn and others, Chapter 3, this CD–ROM). Abstract It has been suggested that various Cretaceous marine shales are the principal hydrocarbon source rocks for these accumula- Thirty samples of well cuttings, collected from marine tions in the Bighorn Basin (Burtner and Warner, 1984; Hagen shales from the Cretaceous Thermopolis and Mowry Shales in and Surdam, 1984; Meissner and others, 1984). the Bighorn Basin, were analyzed using Rock-Eval and total The purpose of this report is to present new data from organic carbon analysis to determine their source rock poten- results of Rock-Eval and total organic carbon (TOC) analysis tial. The samples were collected from wells located near the for samples collected from immature Thermopolis and Mowry outcrop belt along the margins of the basin and are generally Shales in the Bighorn Basin and characterize their source immature with respect to hydrocarbon generation. Results for rock potential. These new data supplement previously pub- the Thermopolis Shale indicate that it is composed of mainly lished reports by Schrayer and Zarrella (1963, 1966, 1968), Type-III gas-prone kerogen, however S2/S3 ratios indicate that the Thermopolis is capable of generating both oil and gas. Nixon (1973), Burtner and Warner (1984), Hagen and Surdam Organic richness levels for the Thermopolis Shale are consid- (1984), Hagen (1986), Davis (1986), and Davis and others ered poor to fair. Results indicate that the Mowry Shale has a (1989). This study was conducted to support the U.S. Geologi- generative potential based on organic richness that is consid- cal Survey’s assessment of undiscovered oil and gas resources in the Bighorn Basin in 2007–08. ered to be poor to very good, and hydrogen indices and S2/S3 ratios indicate it is capable of generating both oil and gas. Acknowledgments Introduction The author thanks Paul Lillis and Mike Pribil for numer- The Bighorn Basin is one of many structural and sedi- ous helpful discussions regarding organic geochemistry and mentary basins that formed in the Rocky Mountain foreland source rocks, and for their assistance in interpreting the Rock- during the Laramide orogeny (Late Cretaceous through early Eval and TOC data presented in this report. Important geologic Eocene). The basin is nearly 180 mi long, 100 mi wide, and input was provided by Mark Kirschbaum and Tom Judkins. encompasses about 10,400 mi2 in north-central Wyoming The manuscript benefited from reviews by Larry Anna, Paul and south-central Montana (fig. 1).The basin is structurally Lillis, Dick Keefer, Doug Nichols, and Gayle Dumonceaux bounded on the northeast by the Pryor Mountains, on the and their suggestions and comments are greatly appreciated. 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- Depositional Setting Bowler lineament (Wilson, 1936). The northwest and west margins are formed by the Beartooth Mountains and Absaroka During much of Cretaceous time, the part of central Range, respectively (fig. 2). Wyoming that is now the Bighorn Basin was located near the The first commercial hydrocarbon production from west edge of the Rocky Mountain foreland basin, an elongate Cretaceous reservoirs in the basin was established at Garland north-south structural depression that developed to the east and Greybull fields in 1906 and 1907, respectively (Fox and of the tectonically active Western Cordilleran highlands prior Dolton, 1996) (fig. 2). Since then cumulative production from to the Laramide orogeny. Throughout much of its history the Cretaceous and Tertiary reservoirs is about 94 million barrels foreland basin was flooded by a broad epicontinental sea, of oil and 830 billion cubic feet of gas (IHS Energy Group, referred to as the Western Interior Seaway (WIS) that devel- 2007). In addition, a potential unconventional basin-centered oped