Geological significance of karst section of Lower Triassic Feixianguan Formation in Puguang area, Northeast Sichuan

Reservoir strata with carbonate rocks of Feixianguan Formation are the main output horizons of natural gas in Puguang Gas Field, Northeast Sichuan. As to the division of four members of Feixianguan Formation, the controversy regarding whether the T1f1 member and T1f2 member should be classified in the same third-order sequence appears. Based on the core observation of the existing drilling in Puguang area, the karst breccia section is found atthe middle of the T1f1-T1f2 section, and there is a certain lithologic interface above and below it. The bottom of T1f1 is fine-grained dolomite. From the top of T1f1 to T1f2, oolites and mould casting holes begin to develop.The carbon and oxygen isotope study in Well PG104-1 shows that the fluid environment is atmospheric freshwater fluid, which proves the development of quasi-syngenetic karst, while the oxygen isotope value is more than -5‰, indicating that the relevant fluid is atmospheric freshwater fluid, which proves that the bottom of the karst breccia section experiences a certain period of exposure and that a sedimentary discontinuity appears.Considering the differences in carbon and oxygen isotopes between the upper and lower interface, the emergence and disappearance of die casting holes and the changes in logging curves, it is considered that the boundary between T1f1 and T1f2 can be distinguished and characterized by karst breccia sections. This study provides an important basis for the study of the storage and migration of oil and gas in Feixianguan Formation.