Activity of bacteria and archaea in immature or lower-mature organic matter and new appraisal methods for biogas source rocks

The geochemical analyses of samples from both east Qaidam Basin and north Songliao Basin are implemented based on examining the influence of the activity of bacteria and archaea on the distribution of main biomarkers in saturated hydrocarbons and on the content of carbazole derivatives, with the aid of microbiological research results. Studies suggest that the regular sterane/17α (H) hopane value, C35αβ homohopane index, oleanane index, BNH/TNH ratio, and the abundance change of C28-C31 alkyl steranes can be used to appraise the activity of bacteria and archaea. In the strongly anaerobic environment from fresh water to saline water, the content of organic nitrogen in gas-generated materials is related to the nitrogen fixation of methanosarcina. Under the methane-producing condition, nitrogen-bearing heterocyclic compounds from protoplasm are degraded by methanogenesis partners. Thus, the contents of carbazole, methyl-carbazole and benzol-carbazole compounds are lower about 1 to 2 order of magnitude in excellent biogas source rocks than in other type samples.