The types of substances in coal rock used by microbes,the specific ways in which microbes produce secondary biogenic gas(SBG)and whether substances exist in the coal seam for the formation of a large amount of SBG are important basic scientific issues.This paper conducts a systematic and comprehensive research study on the above issues using methods such as the isotopic tracing of gas,the analysis of coal organic geochemistry,and gas-producing simulation experiments of coal.Results show that the formation of SBG is by the microbial reduction of CO2and the SBG-producing coal seam undergoes microbial degradation.The thermogenic heavy gaseous hydrocarbons have also been degraded by microorganisms and possibly transformed into microbialoriginated CO2.A large amount of CO2,a relatively large amount of H2and a certain amount of heavy gaseous hydrocarbons may form in the thermal evolution of coal.These substances and the microbial-originated CO2and coal seam water can finally become parent materials of SBG.These components are rich in coal seams of medium–low thermal evolution,which should be the main coal seams for SBG formation and exploration. The types of substances in coal rock used by microbes, the specific ways in which microbes produce secondary biogenic gas (SBG) and whether substances exist in the coal seam for the formation of a large amount of SBG are important basic scientific issues. This paper conducts a systematic and comprehensive research study on the above issues using methods such as the isotopic tracing of gas, the analysis of coal organic geochemistry, and gas-producing simulation experiments of coal. Results show that the formation of SBG is by the microbial reduction of CO2and the SBG-producing coal seam undergoes microbial degradation. The thermogenic heavy-hydrocarbon hydrocarbons have also been degraded by microorganisms and possibly transformed into microbial origin. CO2. A large amount of CO2, a relatively large amount of H2 and a certain amount of heavy-hydrocarbon hydrocarbons may form in the thermal evolution of coal.These substances and the microbial-originated CO2 and coal seam water can finally become parent ma terials of SBG.These components are rich in coal seams of medium-low thermal evolution, which should be the main coal seams for SBG formation and exploration.