Mining Paleocene Fort Union Formation Coals of the Red Desert-Great Divide Basin, South Central Wyoming, Over the Next 100 Years

Abstract

Coal companies have been mining thick coal beds in the Great Divide Basin over the past 120 years. This large synclinal feature has shallow, mineable coal near Point of Rocks on the west side and on the east side near Rawlins. Over 8.9 million short tons of coal were mined in Sweetwater County, WY in 2013, nearly half of that by underground methods. The mines supply fuel to the local Bridger Power Plant. At a modest one (1) percent growth rate per year for the next 100 years, over 1.53 billion short tons of coal could be mined from the Fort Union, Lance, and Almond formations in this area. The axis is north-south in the southern part of the basin, and trends N60W in the northwest part of the basin. It is one of two eastern sub-basins of the Greater Green River Basin in Wyoming. It is geologically distinct from the Washakie Basin to the south by the Wamsutter arch in the subsurface. The deepest part of the basin lies along the steep eastern flank of the Rawlins uplift, but the depocenter for latest Cretaceous and Paleocene strata lie in the northern part of the basin just south of the Wind River Mountains. Across the Great Divide Basin the Fort Union Formation thickens and increases in organic-rich bedding. The lower unit contains thick coals of mineable thickness near Point of Rocks, WY, on the east side of the Rock Springs Uplift. This coal-bearing interval thins out 10 miles south of the Black Butte Coal Mine. Paleocene Fort Union Formation strata at T25N R95W reach a maximum thickness of 4,720 ft. Net coal is usually less than 50 ft. The deepest Fort Union coals in T23N R94W are 6,605 ft, near the basin depo-center. The thinnest Fort Union Formation at 1,480 ft thick occurs near Wamsutter Field at T20S R93W. These syn-depositional alluvial continental sediments were deposited from nearby tectonic uplifts during Laramide time that formed the original Green River Basin. Only the lower part of the formation is organic rich containing subbituminous coal. WSGS coal geologists have correlated over 50 individual Fort Union coal beds from 1,992 petroleum wells and 3,562 coal exploration wells across the basin. These coals were mapped in the subsurface and correlated to known surface exposures. Shallow coal less than 3,500 ft was mapped for mining purposes, and coal calculations were determined in terms of thickness and depth. Significant minable coal resources were determined down dip from the active Bridger and Black Butte coal mines in Sweetwater County.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015