Assessment of coal geology, resources, and reserves in the Montana Powder River Basin

Abstract

The purpose of this report is to summarize geology, coal resources, and coal reserves in the Montana Powder River Basin (MTPRB) assessment area in southeastern Montana. This report represents the fourth assessment area within the Powder River Basin to be evaluated in the continuing U.S. Geological Survey regional coal assessment program. There are four active coal mines in the MTPRB assessment area: the Spring Creek and Decker Mines, both near Decker; the Rosebud Mine, near Colstrip; and the Absaloka Mine, west of Colstrip. During 2011, coal production from these four mines totaled approximately 36 million short tons (MST). A fifth mine, the Big Sky, had significant production from 1969-2003; however, it is no longer in production and has since been reclaimed. Total coal production from all five mines in the MTPRB assessment area from 1968 to 2011 was approximately 1.4 billion short tons (BST). The Rosebud/ Knobloch coal bed near Colstrip and the Anderson, Dietz 2, and Dietz 3 coal beds near Decker contain the largest deposits of surface minable, low-sulfur, subbituminous coal currently being mined in the assessment area. A total of 26 coal beds were identified during this assessment, 18 of which were modeled and evaluated to determine in-place coal resources. The total original coal resource in the MTPRB assessment area for the 18 coal beds assessed was calculated to be 215 BST. Available coal resources, which are part of the original coal resource remaining after subtracting restrictions and areas of burned coal, are about 162 BST. Restrictions included railroads, Federal interstate highways, urban areas, alluvial valley floors, state parks, national forests, and mined-out areas. It was determined that 10 of the 18 coal beds had sufficient areal extent and thickness to be evaluated for recoverable surface resources ([Roland (Baker), Smith, Anderson, Dietz 2, Dietz 3, Canyon, Werner/Cook, Pawnee, Rosebud/Knobloch, and Flowers-Goodale]). These 10 coal beds total about 151 BST of the 162 BST of available resource; however, after applying a strip ratio of 10:1 or less, only 39 BST remains of the 151 BST. After mining and processing losses are subtracted from the 39 BST, 35 BST of coal were considered as a recoverable resource. Coal reserves (economically recoverable coal) are the portion of the recoverable coal resource that can be mined, processed, and marketed at a profit at the time of the economic evaluation. The surface coal reserve estimate for the 10 coal beds evaluated for the Montana Powder River assessment area is 13 BST. It was also determined that about 42 BST of underground coal resource exists in the MTPRB assessment area; about 34 BST (80 percent) are within 500-1,000 ft of the land surface and another 8 BST are 1,000-2,000 ft beneath the land surface.