The Powder River Basin (PRB) in Wyoming and Montana is the fastest-growing coalbed methane (CBM) play in the United States. The number of producing wells in the basin has increased from 270 in 1997 to 2,469 as of March 2000. The U.S. Bureau of Land Management estimates by 2011 the number of CBM wells in the PRB will increase to 60,000. Most of the earliest wells in the PRB CBM play were in the eastern portion of the basin, near Gillette, WY, where the coal seams are relatively shallow. The cost of gas extraction is cheaper here because the coal is less than 1000 ft deep in most places. Water must be pumped out of coal seams in order to release the pressure that keeps the methane gas adsorbed to the coal. The co-produced water from CBM wells in this part of the basin is low in total dissolved solids and is being discharged into infiltration ponds, stock tanks, or streams.

Since 1996, the trend of CBM development has been west and north into the deeper parts of the basin. This trend to the west and north is of great concern to irrigators in the area because as the coal gets deeper the associated water becomes more saline and sodic. Preliminary chemistry results from CBM-produced water in Wyoming indicate that sodium is the dominant cation and that sodium adsorption ratio (SAR) values increase toward the western and northern portions of the basin. SAR is a measure of the exchangeable sodium in water or soil as compared to calcium and magnesium; it is used to determine the tendency of calcium and magnesium ions to be replaced by sodium. Higher SAR values indicate a higher replacement tendency. Some CBM-produced water from this area of the basin ultimately ends up in the Tongue and Powder River drainages. Irrigators who are using water from these drainages are concerned about how the degraded quality of the CBM-produced water will affect their soil, and by extension their crops.

Fine-grained soils high in clay are more susceptible to damage from irrigation water with elevated SAR values, whereas sandy soils are unaffected. Most of the soils of the Tongue and Powder River basins have large amounts of clay, and montmorillonite clays are common. Montmorillonite clays are especially susceptible to damage from water with elevated SAR values due to their high swelling capacity. More specifically, sodium montmorillonite is characterized by a higher swelling capacity and lower permeability than calcium montmorillonite, but calcium montmorillonite is more susceptible to sodium replacement of calcium or magnesium. Conversely, kaolinite undergoes very little or no swelling when exposed to water. The plate-like structure of montmorillonite is held together by calcium and magnesium ions. When sodium ions replace the calcium or magnesium ions, the plates are broken apart, or dispersed, which causes a soil's infiltration rate to decrease. The soil then becomes almost impermeable to rain or irrigation water.

Spatial mapping of clay minerals at the resolution provided by airborne hyperspectral data has not been done in the PRB. Clay minerals have distinct spectral signatures and can be identified by using hyperspectral data. This research, in addition to producing maps of the location of montmorillonite in the Powder River Basin, will attempt to determine the possiblity of distinguishing calcium montmorillonite from sodium montmorillonite using hyperspectral sensors. The research will also compare the Natural Resources Conservation Service soil texture maps of the area and will highlight the advantages and disadvantages of the new methods developed. Such a map would provide users a straightforward reference for determining areas where irrigating with CBM-produced water may or may not succeed. This project will contribute to the interests and needs of irrigators, regulators, and land managers in the PRB of Montana by providing detailed spatial information on clay mineralogy, a key component of soil irrigation suitability. 
Project Lead: Jamie McBeth

For additional information, please visit:

• USGS Central Region Energy Resources Team: Coalbed Methane Studies
• Montana Department of Environmental Quality: Coalbed Methane
• Montana State University - Department of Land Resources and Environmental Sciences: Water Quality and Irrigation Management