CONNECTION OF DISSOLVED INORGANIC CARBON TO BIOGENIC COALBED METHANE: ISOTOPE EVIDENCE FROM BLACK WARRIOR BASIN, ALABAMA (USA)

Dry gas from coal of the Black Warrior Basin (BWB) in Alabama, USA is thought to be at least partly biogenic. Questions remain about the specific pathways and controls of methanogenesis in this basin (e.g. thermal maturity, nutrient flux, and intermediate organic compounds). For example, the lowest δ¹³C‑CH₄ values are associated with the most saline formation water, one of the environmental factors that may control methanogenesis in the BWB. Although dry basinwide, the ¹³C-depleted CH₄ (lower δ¹³C) is clearly biogenic, whereas the ¹³C-enriched CH₄ may contain thermogenic and biogenic components (δ¹³C‑CH₄ range -60.4 to -42.3‰; Pashin et al., 2014, Int. J. Coal Geol.v126 p92).

Methanogenesis imparts a large isotopic fractionation on CH₄ and CO_2, so the presence of ¹³C-enriched CO₂ helps establish the biogenic nature of dry gas. Although methanogenic biodegradation yields both CH₄ and CO₂, CO₂ is anomalously low in BWB gas samples (median 0.1 mol %), implying that CO₂ dissolved into groundwater. To assess microbial CO₂, we have examined dissolved inorganic carbon (DIC) and present the first report of δ¹³C‑DIC values in produced waters from coal of the BWB. DIC concentrations correlate positively with pH (r = 0.77) and negatively with specific conductance (r = -0.74). Therefore, the highest DIC concentrations (8-15 mM) are typically associated with high‑pH (8.4-9.4) and brackish water, implying that a fresh, bicarbonate-dominated water is associated with methanogenesis. Positive values of δ¹³C‑DIC (median 25.6‰; range 7.4-36.3‰; n=19) confirm that microbial methanogenesis occurred throughout the basin (past and/or present) to yield such ¹³C enrichment. These δ¹³C‑DIC values are broadly consistent with ¹³C-enriched calcite in natural fractures in BWB coals, implying equilibration between the two during the evolution of formation fluids. Given that these δ¹³C‑DIC values occur across a spatial trend of coal thermal maturity, and thermally mature coal is thought to be less bioavailable, our results highlight the importance of identifying biodegradable compounds and active metabolic pathways in coal bed waters.