Comprehensive Assessment of Logging Geology in Self-generated and Self-storage Biological Gas Reservoir: A Case Study from Sebei Gas Field in Qaidam Basin

Based on the autogenic and self-storage characteristics of biological gas reservoirs, a quantitative identification model of formation stratum lithology is established, a characterisation method of mudstone thickness ratio is proposed, and a comprehensive evaluation index of potential reservoirs in terms of logging and geology is formed by combining the logging response of the sealing ability of caprocks and gas-bearing.The study shows that only small-scale water-bearing gas reservoirs can be formed under the condition of mudstone thickness ratio lower than 25%; the layers with mudstone thickness ratio above 40% are the advantageous hydrocarbon generating and reservoir-forming layers, and the potential layers dominated by sandy mudstone can be further explored; and in the strata with shallow capping layer with poor blocking ability, the layers with good hydrocarbon generating ability have the possibility of forming water-bearing gas reservoirs, which are of high exploitation value. The paper combines the needs of the middle and late stage of the development of biogenic gas reservoirs in the Qaidam Basin, re-evaluates the target layers of the four types, and tens of sets of low-permeability gas sand bodies obtain industrial production capacity, which makes an important breakthrough in the understanding.