CHEMICAL AND ISOTOPIC COMPOSITION OF WATER CO-PRODUCED WITH COALBED METHANE IN THE FORT UNION AND WASATCH FORMATIONS OF THE POWDER RIVER BASIN, WYOMING AND MONTANA

The U.S. Geological Survey, in cooperation with BLM and coalbed methane (CBM) operators, is conducting a study of water co-produced with methane in the Powder River Basin, Wyoming and Montana. CBM is produced primarily in the Paleocene Fort Union Formation although limited development is taking place in the overlying Eocene Wasatch Formation. Coal beds in these formations are aquifers, and the study objective is to determine basin-wide variations in the chemical and isotopic composition of water and relate those variations to characteristics of the coal beds and hydrology of the basin.

Total dissolved-solids (TDS) in water produced from Fort Union coal beds ranges from 270 to 2,400 mg/L. TDS increases from south of the Belle Fourche River to the north and west within the basin. Water produced from Fort Union coal beds is predominantly sodium-bicarbonate type with Na varying from 110 to 1,000 mg/L and HCO₃ ranging from 2,790 to 3,130 mg/L. Sulfate, except for two shallow (<500 ft) wells, is low with a median of 0.72 mg/L. TDS in Wasatch CBM wells ranges from 540 to 3,010 mg/L. Water type varies from sodium-bicarbonate to sodium-sulfate type with Na ranging from 210 to 730 mg/L, HCO₃ varying from 500 to 1,720 mg/L, and SO₄ ranging from 7 to 1,450 mg/L. Trace-metal concentrations in water produced from the reducing environment of the Fort Union and Wasatch coal beds are uniformly low and below the EPA’s maximum contaminant and secondary maximum contaminant levels for drinking water, except for iron (0.02 to 18 mg/L), manganese (1.4 to 350 µg/L), and barium (0.01 to 2.5 mg/L).

d²H values for water in the Fort Union and Wasatch Formations range from –130 to –167 per mil while d¹⁸O values range from –17.9 to –22.0 per mil. Values have not evolved away from the global meteoric water line and are consistent with a meteoric water origin. Isotopic values in the northwestern part of the basin are isotopically light relative to CBM water in other parts of the basin and this might be related to flow paths in the basin. Results of the study indicate that water composition in the coal beds of the Fort Union and Wasatch is apparently related to depth and is the product of geochemical reactions along horizontal and vertical flow paths. Basin-wide trends suggest that coal beds of the Fort Union function as a relatively continuous rather than compartmentalized reservoir.