Study On C-N-S Microbial Geochemical Cycle And CBM Production Potential In Coal Reservoir Of Shizhuangnan Block Qinshui Basin

Methane carbon isotope indicates that CBM generated in high rank coal of Southern Qinshui Basin is thermogenic.But 16S r DNA sequencing of CBM wells’produced water in Southern Qinshui Basin detected Methanogenic Achaea,sulfate reducing bacteria,nitrate reducing bacteria and denitrifying bacteria,suggesting that the secondary biogenic methane should exist in the study area,which contradicts with the understanding of CBM origin in the high rank coal.By systematically collecting produced water samples of CBM wells in Shizhuangnan Block,Southern Qinshui Basin,water ions,hydrogen and oxygen isotope,REE,dissolved inorganic carbon isotope and microbial diversity were tested in this paper to research the C-N-S biogeochemical characteristics closely related to the microbial metabolic activity.Research shows that Shizhuangnan Block is a monoclinal structure that distributes in nearly SN direction and elevation of coal seam gradually decreases from the east to the west.The rainfall runoffs from east to west,gradually transforming from oxidation runoff environment to the reduction stagnant environment as the Sitou Fault in the west has water blocking effect.Water types of the water samples in the study area are mainly Na-HCO₃-Cl and Na-HCO₃type.The concentration of KDS,Na⁺,K⁺,Cl^-,Ca²⁺,Mg²⁺and HCO₃^-which increases gradually from the eastern runoff area（oxidation environment）to the western residence area（reduction environment）is controlled by mineral dissolution reaction,which indicates the change characteristics of redox conditions in the coal seam.Through the analysis of the 16S r DNA sequence of microorganisms in the collected water samples,Methanogenic Achaea,sulfate reducing bacteria and nitrifying bacteria were detected.Methanogenic Achaea inlucdes Methanobacteriales,Methanomicrobiales and Methanosarcinales;sulfate reducing bacteria includes Desulfarculales,Desulfovibrionales and Desulfuromonadales and nitrifying bacteria includes Nitrososphaerales.The model of C-N-S microbial geochemistry in the study area is established,and the C-N-S microbial zones are divided into:methanogenic zone,sulfate methane anaerobic oxidation zone（SMTZ）and denitrification zone.The effect of methane producing Achaea in methanogenic zone is efficient,which is the high CBM bearing zone in the study area.The transition zone above is the SMTZ,due to the high sulfate concentration,most of the methane is consumed by the anaerobic oxidation of sulfate reducing bacteria in the transition zone.Above the SMTZ is the zone of denitrification,which corresponds to the increase of N₂ and the decrease of nitrate content.The methanogenic zone’s production potential is the highest,the SMTZ zone is the second,and the denitrification zone is the lowest.