Strippable Coal Reserves of Montana, North Dakota, and Wyoming
Joseph Blake Smith
A recent U.S. Bureau of Mines investigation utilized coal-resource data from published sources and company files to determine the location and extent of large blocks of strippable coal in Montana, North Dakota, and Wyoming. Major coal deposits, containing strippable reserves of 14 billion tons in Montana, 4.2 billion tons in North Dakota, and 13 billion tons in Wyoming, were delineated. Limits used to define a strippable deposit were: (1) minimum coal-bed thickness of 5 ft; (2) overburden-to-coal ratio of less than 10:1; and (3) total overburden thickness of less than 120 ft except where reserves occur in multiple beds or a thick single bed.
Deposits in Montana are lignite and subbituminous coal in the eastern part of the state in the Tongue River Member of the Tertiary Fort Union Formation. Reserves include coal beds as thick as 85 ft. However, most reserves are in beds ranging in thickness from 20 to 30 ft. The deposits range in size from 10 million tons to nearly 2 billion tons and four deposits each contain reserves exceeding 1 billion tons.
Western North Dakota deposits are 5- to 25-ft-thick lignite beds in the Tongue River Member. About 2 billion tons of the reserve are in two large deposits in southwestern North Dakota and 1 billion tons of the reserve are in 30- to 200-million-ton deposits in Mercer, Oliver, and McLean Counties.
Tertiary rocks along the margins of the Powder River basin contain most of the strippable reserve in Wyoming. Ranging in thickness from 30 to 100 ft, the Roland bed crops out on the east flank of the basin and contains several billion tons of strippable subbituminous C coal with an overburden ratio of less than 2:1. On the other margins of the basin, the 100- to 200-ft-thick Healy bed on the western flank and the 35-ft School and 20-ft Badger beds on the south flank contain large strippable deposits. Elsewhere in the state, strippable deposits are subbituminous coal of Late Cretaceous and Tertiary ages in the Hanna and Great Divide basins in south-central Wyoming and the Hams Fork region in the southwestern part of the state.
AAPG Search and Discovery Article #91051©2012 AAPG Rocky Mountain Section Meeting, Albuquerque, New Mexico, 23-26 February 1969