Carbon and oxygen isotopes and molecular fossil characteristics of unconventional gas reservoirs in Permian Maokou Formation, West Chongqing, Sichuan Basin

The well DB1 in West Chongqing area encountered unconventional gas in the first member of Maokou Formation (P 1 m 1 ); therefore, the P 1 m 1 core was systematically sampled and analyzed. Based on normal porosity–permeability, organic carbon, pyrolysis, and Vitrinite reflectance analysis, seven samples were selected for rock and ore identification, element analysis, carbon and oxygen isotope analysis of carbonate rocks, and GC–MS analysis of soluble organic matter. The results of organic geochemical analysis show that P 1 m 1 carbonate rocks have high organic matter abundance and are in an over mature stage, and a large amount of natural gas has been formed. The petrophysical property analysis results show that P 1 m 1 is an ultra-low porosity and permeability reservoir. In general, P 1 m 1 is a set of unconventional carbonate source rock gas reservoirs. The results of rock and ore identification and carbon and oxygen isotope analysis indicate that there are talc and fluorite minerals of hydrothermal origin in P 1 m 1 carbonate rocks, and the carbon and oxygen isotope composition of carbonate rocks is similar to that of hydrothermal dolomite in Chuanzhong area.. The GC–MS analysis shows that P 1 m 1 carbonate rock exhibits abnormal molecular fossil assemblages, such as N-alkanes with even carbon dominance, Isoalkanes with even carbon dominance iC 22+ , iPC 21+ long chain Isoprenoid alkanes, abnormally high abundance Hopane, relatively high abundance C 29 regular sterane, and high abundance sulfur compounds. These molecular fossil evidence indicates that the P 1 m 1 sedimentary environment has strong reducibility, and the water contains a large amount of H 2 S and sulfate. At the same time, a large number of chemoautotrophic bacteria and archea, heterotrophic bacteria and archea, and some hydrothermal animals jointly formed the hydrothermal ecosystem of P1m1, forming a large number of hydrocarbon generating parent materials.