The Waltman Shale Total Petroleum System: Does it have a favorable future?

Stephen B. Roberts¹, Ronald C. Johnson¹, and Paul G. Lillis². (1) USGS, Box 25046 MS 939, Denver Federal Center, Denver, CO 80225, phone: 303-236-7788, [email protected], (2) U.S. Geological Survey, Box 25046, MS 977, Denver Federal Center, Denver, CO 80225

The 2005 USGS assessment of undiscovered petroleum resources in the Wind River Basin, Wyoming, includes an evaluation of Paleogene rocks in the Waltman Shale Total Petroleum System (TPS). Source rocks in this TPS are organic-rich lacustrine shales that charged conventional accumulations in stratigraphic or structural/stratigraphic traps within the Shotgun Member of the Fort Union Formation. To date, more than 2.5 million barrels of oil and 6 billion cubic feet of associated gas have been produced from this petroleum system in the Fuller Reservoir, Haybarn, and Greater Madden fields.

Previous studies indicate that the Waltman Shale Member of the Fort Union Formation contains a mix of Type-II and Type-III organic matter; total organic carbon averages about 2.7 weight percent. Waltman oil is high gravity (>40˚API) with a high paraffin content; associated gas is isotopically lighter than gas produced from Upper Cretaceous and Tertiary reservoirs underlying the Waltman Shale Member. Thermal maturity of the Waltman in the deep Wind River Basin generally ranges from 0.75–0.80 percent Ro, and approaches 1.10 percent Ro in areas west of the Madden anticline.

Sandstone and conglomerate deposited in fluvial, shoreface, and deltaic systems prograding from western and southern margins of Lake Waltman, and fan-delta complexes present along the northern lake-basin margin are potential reservoirs for additional Waltman petroleum accumulations. Future exploration might target areas where these contiguous depositional systems are interbedded with thermally mature Waltman source rocks, and areas where faults or fractures provide pathways for petroleum migration to shallow reservoirs from mature source rocks at depth.