Study on origin and accumulation mechanism of coalbed methane in Miquan area of southern margin of Zhunggar Basin

The southern margin of Zhunnggar Basin contains rich resources of medium-low rank coalbed methane(CBM).However,the exploration degree is relatively low,and lack of understanding of gas genesis and gas reservoir formation mechanism restricts the development to a certain extent.In order to explore the genesis and accumulation mechanism of medium-low rank CBM in Miquan Area on the southern margin of Zhunnggar Basin,based on gas samples and associated coal seam water samples from 18 CBM production test wells,this study systematically carried out laboratory experiments of stable isotopes,hydro-geochemistry and radioisotope dating of relevant samples,and comprehensive use of the classical natural gas identification chart and other means to analyze the data.The results show that most of the samples are located in the regions of “biogenic gas”“CO2 reduction” and “CO2 associated with microbial methanogenesis”;the water type is Na+·HCO-3·CI-,and the TDS of coal seam water is high; the radioisotopes of coal seam water are 3H<1.0 TU and 14C< 0.44 pmC.The data analysis shows that CBM is mainly biogenic in Miquan Area and carbon dioxide reduction is the main way of its formation,the abnormally high CO2 concentration in this area is closely related to the activity of microbial methane production.In addition,the hydrodynamic field in Miquan Area is relatively stagnant,the geological age of coal seam water is relatively old,no or little current surface water supply,and the current hydrogeological conditions are not conducive to the survival of methanogens and their methanogenic activities,most of the existing biogas resources should have been produced in earlier geological history.Finally,combining the regional tectonic setting,hydrogeological condition,Paleo-climate data and coal seam burial history,it is proposed that sealed secondary biogenic gas reservoirs are widely developed in Miquan Area,its formation has experienced four geological stages:burial and coal formation,uplift and denudation,surface water supply and biogas formation,methane genesis stopping and hydrodynamic storage.