Study On Biomethane And Microbial Community In Coal Reservoir Of The Southern Qinshui Basin

Coalbed methane?CBM?is an important unconventional resource in Qinshui Basin of China.Its commercial development has a long history.The geochemical characteristics of coal reservoir water are great significance to the exploration and development of CBM resources,and the indicating significance of biological metabolism activities to the reservoir environment cannot be underestimated.Therefore,the water from No.3 coal seam gas well in the Shizhuangnan block of Qinshui Basin was systematically collected,and geochemical test and 16S rRNA microbial sequencing were conducted to evaluate the water rock process and biogeochemical process in the coal reservoir,study the metabolic characteristics of methanogens and other microorganisms,and establish the response relationship between the coal reservoir environment and biological process.The main results are shown as follows:Coal reservoir water mainly comes from atmospheric precipitation and is controlled by water rock and biological activities.Sulfate reducing bacteria consume dissolved methane and organic matter by sulfate reduction.At the same time,the isotopic composition of water samples shows that they mainly come from the atmosphere and are affected by the activities of microorganisms.Influenced by the concentration of sulfate,sulfate reducing bacteria showed obvious difference in abundance at different water head height,and methanogens also had opposite regularity of abundance change.The spatial difference of dissolved inorganic carbon is produced by biogeochemistry in reservoir water resulting in typical carbonate minerals precipitation in some depth.Based on the investigation of the main isotopes and microorganisms of the coal reservoir water,the primary microbial community and the methane production pathway of the Shizhuangnan block were studied.The differences of carbon isotopes and hydrogen isotopes were analyzed respectively.It was found that there were two types of biogenic methane production,carbon dioxide reduction and acetic acid fermentation.The results of 16S rRNA sequencing showed that Methanobacterium and Methanosarcina were the main methanogens,which were affected by the geochemical factors of the coal reservoir environment.The methanogenesis of acetic acid fermentation type is more common than that of carbon dioxide type in the Jinhuo Fault of the study area.Several typical biogeochemical parameters are used to evaluate the redox environment of the corresponding coal reservoir and establish a relationship with gas production.It is found that high levels of Na⁺,HCO₃^-and D drift are often accompanied by high gas production,but these parameters are not reliable to some extent.Water dissolved inorganic carbon?DIC?isotope?¹³C_DICIC can be used to identify redox conditions.In the range of appropriately positive?¹³C_DICIC suggests that the reduction condition is suitable for methanogenic bacteria metabolism,with high gas production.The concentration of SO₄^2-and NO₃^-is affected by different bacteria,so it corresponds to different redox conditions,and can be used as an effective parameter to identify environmental conditions and gas production.The combination of?¹³C_DICIC and SO₄^2-can also be used to predict the potential favorable region.