Discovery and generation mechanisms of epigenetic biogenic H₂S from coal seams in the Fukang block, southern Junggar Basin, China

In the Fukang block located in the eastern part of the southern Junggar Basin, coalbed methane (CBM) from CBM wells exhibits a gradually increasing H₂S concentration in the process of CBM production via water drainage, severely threatening production safety. Based on the CBM exploration and extraction data, as well as the anaerobic fermentation experiments, this study preliminarily investigated the causes of abnormal H₂S concentrations during the CBM production in the Fukang block. As indicated by the gas content test in the CBM exploration stage, the original CBM showed a low H₂S concentration of only up to 2.152×10⁻⁶. Abnormal H₂S concentrations did not occur at the beginning of CBM production. However, some wells exhibited abnormal H₂S concentrations as CBM production proceeded. For example, the No.13 CBM well showed an abnormal increase in H₂S concentration after seven years of gas production, with the H₂S concentration reaching 700×10⁻⁶. The grey relational analysis reveals that the H₂S concentration is closely related to the yield and quality of water in CBM wells. H₂S generation can be promoted under a sufficient supply of nutrients from groundwater for microbial metabolism. As shown by the anaerobic fermentation system constructed with the coal and water produced from the Fukang block as the anaerobic broth, the H₂S production was inversely and positively proportional to the SO₄ ²⁻ and HCO₃ ⁻ contents in the fermentation broth, respectively. In this system, CH₄ showed a lagging gas production peak and significantly lower cumulative gas production compared to H₂S. However, the anaerobic fermentation system constructed with coal from the block and distilled water as the fermentation broth primarily produced CH₄, with only a small amount of H₂S. These findings indicate that H₂S was generated from the reduction of SO₄ ²⁻ by sulfate-reducing bacteria (SRB) using CH₄ as electron donors. The gradually decreasing content of low-molecular-weight organic acids in the fermentation broth indicates that SRB also reduced SO₄ ²⁻ using organic acids as electron donors. Therefore, the field production data and the anaerobic fermentation experimental results indicate that H₂S in the Fukang block was generated from the reduction of SO₄ ²⁻ in the coal seam water by SRB. The biogenic gas generated in the CBM production stage, which is different from primary and secondary biogenic gases, is referred to as the epigenetic biogenic gas, in which the H₂S is called epigenetic biogenic H₂S. The generation of epigenetic biogenic gas during the CBM production further corroborates the feasibility of implementing CBM bioengineering under human intervention.