HYDROGEOCHEMICAL CONTROLS ON MICROBIAL COALBED METHANE ACCUMULATIONS IN THE WILLISTON BASIN, NORTH DAKOTA

Microbial methane (CH₄) in organic rich strata contributes ~20% of all natural gas resources worldwide; furthermore, coal biodegradation may be stimulated to produce new fuel products and/or to convert anthropogenic carbon dioxide (CO₂) into CH₄. This study aims to determine hydrogeochemical controls on coalbed methane distribution in the Williston Basin and relationships between microbial methane accumulations, extent of coal seams, water type, and redox chemistry. Groundwater samples from 23 wells completed in Fort Union formation coalbeds were analyzed for dissolved gas composition, gas isotopes (δ¹³C_CO2, δ¹³C_CH4, & δD_CH4), water isotopes (δD_H2O, δ¹⁸O_H2O, δ¹³C_DIC, & δ³⁴S_SO4), and major ion chemistry.

Results from groundwaters with adequate dissolved CH₄ indicate that CH₄ is of microbial origin and is comprised of three distinct isotopic populations. One population of gas samples has isotopic values indicative of CH₄ produced through CO₂ reduction with δ¹³C_CO2, δ¹³C_CH4, and δD_CH4 values between -12.3‰ to -17.0‰, -79.1‰ to -88.1‰, and -284‰ to -319‰, respectively. One ¹³C-enriched sample has a δ¹³C value of -18.6‰ and -58.3‰ for CO₂ and CH₄, respectively, and appears to have been produced through acetate fermentation, but may be an artifact of methane oxidation. Another anomalous sample has extremely low δ¹³C_CH4 and δD_CH4 values (-105.3‰ and -392‰) _­, respectively, and is likely representative of very early stage methanogenesis.

Significant amounts (>50 mole %) of dissolved microbial CH₄ commonly occur in wells screened beneath numerous coalbeds and near areas of groundwater discharge. In the majority of these wells, Na-HCO₃ dominated groundwaters have sulfate (SO₄) concentrations below detection limits (<1 mg/L), high alkalinities (>25 meq/kg), and relatively positive δ¹³C_DIC values (-7.5‰ to -0.6‰), indicative of methanogenic activity. Wells with little or no CH₄ contain nitrate (NO₃) below detection limits (<1 mg/L), excess N_{2 (g)} (>78 mole %), and variable concentrations of SO₄ (18 to 1623 mg/L). Relationships suggest that microbial methanogenesis is predicted to occur in areas where groundwater has traveled through multiple coal seams, allowing redox reactions to evolve until conditions allow methanogens to outcompete SO­₄ and NO₃ reducing bacteria for critical substrates.