Tracing Shale-Gas Migration in Complex Tectonic Settings: Evidence from Carbon-Isotope Fractionation Patterns

Late-stage tectonic deformation strongly influences shale-gas preservation and redistribution, but conventional methods rarely resolve gas-migration pathways and directions. Here, we test whether the direction of spatial carbon-isotope gradients can indicate the dominant direction of gas migration. We examine carbon isotopes of methane and ethane in the Jiaoshiba, Dongxi, and Pengshui blocks of the southeastern Sichuan Basin. These blocks have contrasting tectonic configurations but share the Wufeng–Longmaxi shale-gas system. We integrate the isotope data with multiscale pore-structure characterization and formation-pressure measurements. In the three selected shale samples, diffusion-dominated nanopores below 25 nm accounted for 7.8–13.2% of the measured pore volume and provided the physical basis for interpreting isotope fractionation. At Jiaoshiba, methane δ13C becomes heavier toward faults, consistent with preferential vertical escape of 12CH4. At Dongxi, ethane δ13C differs markedly between structural highs and deep sags, whereas methane δ13C remains comparatively uniform. This contrast is consistent with migration-related separation of oil-cracking and kerogen-cracking gas. At Pengshui, the well closer to the outcrop has lighter methane δ13C, defining a preliminary two-well pattern opposite to that at Jiaoshiba. Together, these observations support the use of spatial isotope patterns, interpreted in light of formation pressure and tectonic context, as potential indicators of gas migration pathways and preservation conditions.