RESEARCH OF THE EVALUATION INDICATOR SYSTEM FOR BIOGENIC GAS SOURCE ROCKS

Biogenetic gas has advantages of huge resource potential, wide distribution, shallow buried depth and low cost for exploration. It is an important natural gas resource as well as the main source of natural gas hydrate. The generation mechanism of biogenetic gas is obviously different from that of the hydrocarbon and pyrolysis gas. The formation condition also has some particularities. So many indicators usually used for evaluation of oil source rocks and overmature gas are unserviceable for the evaluation of gas generation capability of biogenic gas source rocks. The paper establishes the evaluation indicator system and evaluation criteria of gas generation capability of biogenic gas source rocks through the analysis of formation and evolution characteristics of biogenic gas and controlling factors. The main control factors of biogas formation include the material conditions that can be used by microbial population like methanogenic bacteria and the environmental conditions that are suitable for growth and activity of microbe. Organic carbon, chloroform "A", organic acid, organic matter types and vitrinite reflectance can be the evaluation indicators of material conditions of biogenic gas source rocks. The sedimentation rate, formation temperature, Pr/Ph and the content of Cl^- are the evaluation indicators to reflect the geologic environment. The content of Sapropelinite, PME, carbazole compounds, the ratio of sterane and hopane and the ratio of nC₂₁^- and nC₂₂⁺ are the indicators reflect the activity and intensity of microbe and can be used as the evaluation parameters for biogenic gas source rocks.