Abstract

A total coal thickness of at least 100 ft, with single beds as much as 23 ft thick, occurs locally in the Paleocene Fort Union Formation in the Wind River Reservation. Most of the coal beds are in the upper Paleocene Shotgun Member of the Fort Union Formation; this member is largely equivalent to the Waltman Shale Member of the Fort Union Formation farther east in the Wind River Basin. The thickest accumulations of coal are in two areas, in the syncline south of Little Dome anticline and in a roughly circular area slightly south of the axis of the Wind River Basin, east of the Pavillion and Muddy Ridge gas fields. The depositional setting of the thick accumulation south of Little Dome anticline is largely unknown because that coaly interval does not crop out and there is little relevant subsurface data. The syncline was actively subsiding during the Paleocene suggesting that peat accumulation could have been controlled by subsidence rates. The thick coal accumulation east of the Pavillion and Muddy Ridge fields was deposited near the shoreline of Paleocene Lake Waltman between two major delta systems. Thick peats began to accumulate in low-lying and raised mires shortly after the initial incursion of Lake Waltman into the area. Stream systems in the area changed from braided to meandering and anastomosed at about the same time. This environment seems similar to the environment in the Powder River Basin in which thick coal beds of the Fort Union Formation were deposited.

Thick coals are at depths favorable for coalbed gas production (> 6,000 ft) throughout the syncline south of Little Dome anticline and in the Pavillion and Muddy Ridge fields area. Gas was encountered while drilling through these coals, indicating that they might be gas reservoirs. Thus far, no gas wells have been completed in these coal beds so their production characteristics are unknown. Coal beds exposed to surface water recharge in other areas have proven difficult to dewater, and therefore are not favorable for gas production. Because most of these coals do not crop out, it is unlikely that they are being recharged by surface waters.