RELATIONSHIP BETWEEN FORMATION WATER SALINITY AND BIOGENIC COALBED METHANE: BLACK WARRIOR BASIN, ALABAMA (USA)

Several factors are thought to influence biogenic coalbed methane generation, including the carbon source (thermal maturity, bioavailability), microbial community, and conditions affecting microbial growth and competition (e.g. nutrients, substrate availability, competing electron acceptors, and salinity). Large variations in coal thermal maturity and formation water salinity occur within the Black Warrior Basin (BWB) coal beds in Alabama. The basin hosts a dry gas (median C₁/(C₂+C₃) 2470) that is thought to have significant biogenic content (Pashin et al., 2014, Int. J. Coal Geol.v126 p92). Therefore, the BWB is a natural site for evaluating the relative importance of C source effects vs. environmental conditions (salinity) for methanogenic biodegradation. In this study we examined formation water across a range of salinity in relationship to isotopic signatures of gas origins.

Wells sampled in 2015 generally exhibited low water production typical of mature CBM wells. pH ranged from 7.4-9.4 (median 8.0). Chloride concentrations were 7-664 mM (median 237 mM) and sulfate was uniformly <1 mM. This is within the salinity range of methanogenesis, but rates and/or metabolic pathways might vary across this salinity range. Dissolved inorganic carbon (DIC) was 1.6-15.0 mM (median 5.7 mM). Chloride concentrations were negatively correlated with pH, alkalinity and DIC concentrations. All waters are dominated by Na (89-98% of cations). Overall, groundwater resembles a mix between methanogenic waters (low Cl^-, high alkalinity, slightly basic pH, and very low Ca/Na ratios) and saline waters with few signs of methanogenesis (high Cl^-, low alkalinity, near-neutral pH, and slightly higher Ca/Na ratios). Waters are supersaturated with respect to calcite (median saturation index 0.68), consistent with undated, ¹³C-enriched secondary CaCO₃ shown in previous studies of BWB coals.

δ¹³C‑DIC was uniformly positive, consistent with methanogenesis (7.4-36.3‰, median 25.9‰), and we see no strong relationship between salinity and δ¹³C‑DIC. Overall, these results imply that salinity is not the primary control on gas generation in BWB, as methanogenesis seems to occur across the entire salinity range. Salinity may play a role along with other limiting factors such as C_org sources (e.g. thermal maturity).