Formation mechanisms of H2S in coal seam of southern Junggar Basin in Xijiang, China

Although the coal seams in the Fukang block of southern Junggar Basin is characterized by low sulfur content, abnormally high concentrations of hydrogen sulfide (H₂S) have been detected during the middle to late stages of coalbed methane (CBM) drainage. This unexpected H₂S enrichment cannot be adequately explained by conventional thermogenic or biogenic formation mechanisms. To investigate the origin of this anomaly, we conducted integrated field monitoring at CBM well CSD-05 and designed a laboratory-scale dynamic anaerobic digestion system to simulate varying rates of H₂S recharge. The results indicate that, during the CBM drainage process, groundwater continuously transports nutrients and sulfate ions into the coal seams. Under dynamic anaerobic conditions, sulfate-reducing bacteria (Desulfobulbus, Desulfomicrobium) reduce the sulfate in groundwater to form H₂S by utilizing acetic acid provided by acid-producing bacteria (Pseudomonas, Romboutsia) or methane generated by methanogens as electron donors. Peak H₂S generation occurs when three critical factors are optimally aligned: groundwater recharge rate, CBM drainage rate, and the metabolic cycle of sulfate-reducing bacteria. This newly identified mechanism of H₂S generation is termed as epigenetic biogenic H₂S. Field monitoring and laboratory simulations confirm the synergistic influence of these factors on H₂S production: Specifically, when the field daily water production remains around 6 m³/d, the adequate supply of nutrients and sulfate lead to increased sulfate-reducing bacterial abundance, resulting in H₂S concentrations as high as 641 ppm. In laboratory simulations conducted at a flow rate of coal seam water 0.05 mL/min, the cumulative H₂S production reaches 12.24 mL/g. In contrast, when the nutrient and sulfate supply is insufficient or interrupted, the H₂S concentrations drop to 25 ppm or 0 ppm, respectively. When the supply is too rapid, the external recharge rate surpasses the growth and metabolic rate of bacterial community, causing a decline in cumulative H₂S production to 5.9 mL/g. The results indicate that the maximum production of epigenetic biogenic H₂S is achieved when the liquid recharge and drainage rates of the fermentation system are adjusted every two weeks, precisely matching the metabolic cycle of sulfate-reducing bacteria.