The “Big George” coal bed, 30 mi (48 km) west of Gillette, Wyoming, is the thickest part of a large Anderson coal deposit. The coal resources of this central core, essentially a single bed of coal up to 202 ft (62 m) thick, were previously estimated at 113 billion short tons. This deposit is in the Paleocene Tongue River Member of the Fort Union Formation; overburden ranges from 700 to 2,400 ft (213 to 732 m).
The “Big George” bed was initially outlined using geophysical logs from nearly 300 oil and gas drill holes. More logs were studied in the northern portion of “Big George” and as far north as the Montana state line to examine the entire system of coal beds that includes this thick bed. We interpreted geophysical logs primarily for coal and sandstone, digitized lithologic intervals, and generated strip logs of lithologic sequences using a microcomputer. These computer-generated logs were generated in lines of sections, on matching elevations, to reconstruct the stratigraphic framework of subsurface coal in this part of the Powder River basin.
The framework was used to trace the interval containing the Anderson deposit into the Decker, Montana, and Recluse, Wyoming, areas. This interval appears to be confined by the Smith coal bed above; the bottom of the interval is less well defined. Lithologic patterns of the framework suggest that a major fluvial channel system defined part of the northwestern boundary of the “Big George” coal bed. The locations of these channels may have been controlled primarily by Laramide deformation in the Powder River basin.