The Fort Union Formation of the Powder River Basin, Campbell County, Wyoming: The Methane Gas Potential of the Upper Fort Union Formation Sandstones

Michael J. Blackstone, Geology and Geological Engineering, University of North Dakota, P.O. Box 8358, Grand Forks, ND 58202, phone: (307) 267-3426, [email protected]

A portion of the Powder River Basin (PRB) 96 miles long and 18 miles wide was researched in order to establish the relationships between the major upper Fort Union coal beds and stratigraphically equivalent sand bodies. The area included a south to north divide between thick coal zones as delineated by thickest coal maps of the PRB. Previous authors have written about east to west “deltas” within the coal zone that appeared to coalesce along this south-north thin coal trend. Oldham points out the commercial value in studying the sands associated with these coals in The Mountain Geologist January 1997 in his report on the Oedekoven and Chan shallow gas fields which produced approximately 2 BCF water-free methane.

Several cross sections were created across the project area. Due to the shallow depth to the sands, mainly gamma ray logs were available to construct the cross sections. Sandstone isopach maps were then created using these cross sections. These maps strongly suggest the presence of a six mile wide, sand-rich paleofluvial system that drained the basin north across the study area, parallel to the strike of the PRB. The “deltas” from the east may be tributary fluvial channels.

Differential compaction and up dip meanders in this south to north trending system set up potential traps for gas generated by the coal beds. Discovery will depend upon running the correct logs to detect gas within the sand bodies, or rig fires, which was the first indication of gas at the Oedekoven Field.