Abstract: Montana's Coal--Important Energy Resource

Robert E. Matson

The coal beds in the Tongue River Member of the Fort Union Formation in eastern Montana have been studied extensively over the last 10 years. Special emphasis has been placed on southeast Montana, in the Montana part of the Powder River basin, where the coal ranks as subbituminous. The increasing availability of drill data has enabled the correlation of economically important coal beds over large parts of the area. Correlation charts have been compiled and regional structure contour maps on top of the Anderson and Canyon coal beds have been prepared.

The principal coal beds of economic importance in the Montana part of the Powder River basin are, from top to bottom, the Anderson, Dietz No. 1, Dietz No. 2, Canyon, Wall, Knobloch, Rosebud, McKay, and Robinson coal beds. These coal beds occur at various levels throughout the 1,800 ± ft (550 m) sequence of Tongue River Member sedimentary rocks. The upper beds are near the surface in the vicinity of Decker near the Wyoming border and east to the Powder River, the middle beds are most prominent in the vicinity of Ashland, and the lower beds are prominent in the Coalstrip-Sarpy Creek area.

The best subbituminuous coal occurs in the Decker area in the Anderson, Dietz No. 1, and Dietz No. 2 coal beds, where it averages about 9,500 Btu per pound, 0.4% sulfur, and low ash values. Although the Canyon is not mined in the area, its quality is comparable. North and east of Decker, the Btu values are lower in all of the coal beds, averaging between 8,000 and 8,600 Btu, although the ash and sulfur values are comparable to those at Decker. The middle beds, the Wall and Knobloch, average 8,000 or 9,000 Btu and have sulfur and ash values comparable to those at Decker. The lowest beds average about 8,600 Btu and have sulfur and ash values of about 0.8 and 8% respectively.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma