Difference characteristics in gas-bearing between multiple coal seams and coalbed methane accumulation models in Liulin Area

Based on the difference characteristics in gas-bearing between Shanxi Formation and Taiyuan Formation in Liulin Area,with an emphasis on preservation Co nditions,the controlling factors of the difference characteristics are analyzed with a view to coal roof lithology and hydrogeological conditions,and accumulation models of th e coalbed methane( CBM) in two formation are summarized.The study shows that the coal roof lithology and hydrodynamic conditions control the difference in gas-bearing between coal seams in two formations comprehensively.The gas content and methane( CH4) concentration in Shanxi formation coal seam are higher than Taiyuan formatio n coal seam generally.The carbon dioxide( CO2) concentration of Taiyuan formation coal seam is higher,and the CH4 carbon isotope is lighter.The coal seam roof in Shanxi formation is mudstone and sandstone,which is a weak aquifer and is favorable to the accumulation of CBM.However,the coal seam roof in Taiyuan formation is limestone,w hich is a strong aquifer and is not conductive to the accumulation of CBM.With the active hydrodynamic conditions, CH4 carbon isotope fractionation effect and carbon isoto. pic exchange between CH4 and CO2are the primary causes of higher CO2 concentration and lighter CH4 carbon isotope in Taiyuan formation coal seam.The CBM reservoi r in Shanxi formation belongs to pressure closed gas reservoir,and the CBM reservoir in Taiyuan formation belongs to hydraulic closed gas reservoir.