Anna, L., 2009, Geologic assessment of undiscovered oil and gas in the Powder River Basin Province, Wyoming and Montana: U.S. Geological Survey Digital Data Series DDS–69–U, p. 93. | Anna, L., 2009, Geologic assessment of undiscovered oil and gas in the Powder River Basin Province, Wyoming and Montana: U.S. Geological Survey Digital Data Series DDS–69–U, p. 93. |