CHEMICAL AND STABLE ISOTOPIC COMPOSITION OF WATER AND GAS IN THE FORT UNION FORMATION OF THE POWDER RIVER BASIN, WYOMING: EVIDENCE FOR WATER/ROCK INTERACTION AND THE BIOGENIC ORIGIN OF COALBED NATURAL GAS

The chemical and isotopic compositions of groundwater samples from coal beds in the Paleocene Fort Union Formation of the Powder River Basin of Wyoming, where significant coalbed natural gas (CBNG) is currently being extracted, reflect the processes that generated the gas as well as the effects of water/rock interaction. Samples of water and gas were collected from 248 CBNG wells throughout the basin since 2000. Major cations and anions and δD and δ¹⁸O were measured on 163 samples of water and δD_CH4 was measured on 111 samples of gas. Water from Fort Union Formation coal beds is exclusively Na-HCO₃-type water with low dissolved sulfate content (<20 mg/L) and little or no dissolved oxygen (<1 mg/L), whereas shallow ground water (depth generally < 400 ft) in the formation is a mixed Ca-Mg-Na-SO₄-HCO₃ type. Water/rock interactions, such as cation exchange on clay minerals and precipitation/dissolution of calcium carbonate and sulfate minerals, can account for the accumulation of dissolved Na with the depletion of Ca and Mg. Bacterially-mediated oxidation-reduction reactions account for high HCO₃ (290-3,250 mg/L) and low SO₄ (<20 mg/L) values. The fractionation of δD_CH4 in a plot of δD_H2O (-121 to -167 per mil) and δD_CH4 (-294 to -327 per mil) indicates that the production of methane was primarily by biogenic CO₂ reduction. Values of δD (-121 to -167 per mil) and δ¹⁸O (-16 to -22 per mil) of water from Fort Union coal beds plot near the global, as well as the local, meteoric water lines and are consistent with a meteoric water origin. Information on processes that generate methane in the coal bed reservoirs is important for developing methods that will stimulate additional production.