Enrichment mechanism and evaluation indicators of normal pressure shale gas in the complex structural area of southeastern Chongqing

Normal pressure shale gas is widely distributed in the complex structural area of southeastern Chongqing, with good prospects for resource exploration. Normal pressure shale gas has gradually become a hot topic in shale gas exploration and development. Due to the influence of multiple stages of structural evolution, there are significant differences in the preservation conditions of shale gas in the complex structural area of southeastern Chongqing. Shale reservoirs and gas geochemical characteristics are distributed differently under different pressure systems. The overall trend of alkane isotopes in shale gas in the study area shows a gradual increase in weight from the basin margin toward the basin exterior, while the dryness coefficient gradually decreases, indicating differences in gas preservation and dissipation in different structural zones. Lighter alkane carbon isotopic values and a higher dryness coefficient indicate better shale gas preservation conditions. A comprehensive comparison of shale gas evaluation indicators from the Lower Silurian Longmaxi Formation in the complex structural area of southeastern Chongqing reveals that static geological parameters including porosity, face porosity, deformation index, and gas content exhibit strong heterogeneity within synclinal structures, with significant variations among shale samples from individual wells. Meanwhile, dynamic geological parameters such as alkane carbon isotopes, dryness coefficient, and gas production rate show marked differences across various structural zones. An integrated evaluation of shale reservoir properties and gas geochemical characteristics demonstrates that the Pingqiao and Dongsheng anticline zones exhibited favorable gas preservation conditions, representing optimal shale reservoirs and sweet spots for shale gas enrichment.