Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

► Molecular and isotopic compositions of natural gases from the Songliao Basin indicate varying genetic gas types existed in shallow reservoirs. ► Various geochemical parameters can be used discriminate secondary microbial gas from primary microbial gas. ► Biodegradation and carbon dioxide conversion produced gases with widely varying chemical and isotopic compositions. ► Rayleigh kinetic models that describe closed systems are not applicable to the Songliao Basin.