Evidence of Coal Biodegradation from Coalbed-Produced Water – A Case Study of Dafosi Gas Field, Ordos Basin, China

Bioconversion of coal to methane occurs in the coalbed aquifer environment. To investigate the evidence of coal biodegradation from coalbed-produced water, we collected six field water samples from the Dafosi gas field and prepared one laboratory-simulated water sample and one indoor anaerobic microbial degradation sample with the highest compound concentration as the two reference standards. Gas chromatography–mass spectrometry was used to detect the organic compound type, concentration, and differences in the biomarker compound sensitivity. Results indicate that extracted organic matter from coalbed-produced water samples can be evidence of biodegradation. Variations in range compounds (such as n-alkanes, tri- and pentacyclic terpenes, and steranes) and their sensitivity confirmed active microbial degradation in the studied area. A positive correlation between the n-alkanes content in the coalbed-produced water and the stable carbon isotope value of methane further verifies that the n-alkanes are primary substrates for maintaining microbial activity. Therefore, evidence including n-alkanes, tri- and pentacyclic terpenes, steranes, unresolved complex mixtures, and stable carbon isotope composition of methane contribute to biogenic methane generation in situ. Our limited data suggest that managing soluble organic matter in the coalbed-produced water may provide a viable route for coal biodegradation since most microorganisms survive within the coal seam water.