NELSON, CHARLES R., Gas Research Institute, Chicago, Illinois 60631; and TIMOTHY J. PRATT, TICORA Geosciences, Inc., Arvada, Colorado 80002
The Tongue River Member of the late Paleocene Fort Union Formation in the Powder River Basin, Wyoming and Montana contains thick beds of subbituminous coal which are reservoirs for an estimated 24 Tcf of coalbed methane resources. In 1999, coalbed methane production from this play totaled 55.4 Bcf from about 1,380 wells. The average gas reserve finding cost in this coalbed methane play is about $0.25 per Mcf. The major play area in the Powder River Basin for the commercial production of coalbed methane is along the eastern flank of the basin, downdip from large surface coal mines in Campbell County, Wyoming. This coalbed methane play is unique because several coal properties are strikingly different from those of coalbed methane plays in other U.S. basins. In particular, the coal rank and gas content are much lower whereas the absolute water permeability is typically much greater. This paper presents results from evaluations of geochemical, petrographic, and reservoir properties of the Paleocene Fort Union Formation Canyon Seam subbituminous coal at the Trition Federal coalbed methane well in Campbell County, Wyoming. The core analysis data reveal that at this site the Canyon seam coal is multi-layered reservoir with each layer have different bulk compositional and fluid mobility properties. A very unique reservoir property of this subbituminous coal is its very fast gas desorption rate which indicates the presence of significant macropore volume in the coal matrix. The core analysis data revealed that the gas desorption rate varies as a function of stratigraphic position within the Canyon seam and is dependent on the inertinite maceral content of the coal.
AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico