|Open-File Report 00-372: Geologic Setting|
Rice, C.A., Ellis, M.S., and Bullock, J.H., Jr.
Open File-Report 00-372
The Fort Union Formation contains conglomerate, sandstone, siltstone, and mudstone, with minor amounts of limestone, coal, and carbonaceous shale. Coal in the formation ranges from a few inches to over 200 ft thick, with an average thickness of 25 ft. The Fort Union was deposited in fluvial environments that consisted of braided, meandering, and anastomosed streams in the center of the basin, and alluvial plains along the basin margins (Flores and others, 1999). Coal developed from peat that accumulated in low-lying swamps and in raised or domed mires, in fluvial floodplains, abandoned fluvial channels, and interchannel environments. The thickest coal beds developed from peat that accumulated in raised mires, which formed above drainage level (Flores and others, 1999). The coal beds either split laterally or pinch out in areas where the peat was incised by fluvial channels, now represented by sandstone; or was inundated with overbank, floodplain, or floodplain-lake deposits, now represented by mudstone (Fig. 2).
The stratigraphic relationship of coal beds in the Fort
Union Formation is very complex. The beds merge, split, and pinch out within
short distances. Therefore, targeted coalbed methane beds vary across the
basin (Fig. 2 and Fig.
3). Much of the CBM development is concentrated in the Wyodak-Anderson
coal zone, although other beds and zones are locally being targeted as
well (Fig. 3). Because of the complex stratigraphy,
correlation and nomenclature problems have arisen in the basin. According
to operator completion reports filed with the WOGCC, the Tongue River Member
coal beds producing coalbed methane in the sampled wells include the Wyodak,
Anderson, Canyon, Cook, Big George, Wall, and Pawnee. Two of the reservoirs,
identified as the Cache and Moyer, are in the Lebo Shale Member. Also,
in the operator completion reports, if the name of the coal bed was not
known, the operator designated the producing unit as Fort Union. In an
effort to clarify some of the correlation and nomenclature problems, the
U.S. Geological Survey is currently working with the U.S. Bureau of Land
Management and the WOGCC to standardize coalbed nomenclature (Flores, R.M.,
U. S. Geological Survey Open File Report 00-372
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