Investigating the Genesis and Migration Mechanisms of Subsea Shallow Gas Using Carbon Isotopic and Lithological Constraints: A Case Study from Hangzhou Bay, China

This study addresses the challenge of data scarcity in research on the migration patterns of shallow gas in submarine sediments. Taking the northern Hangzhou Bay area of the East China Sea as an example, we integrate borehole core geophysical surveys and geochemical data to elucidate the migration and fractionation mechanisms of shallow biogenic gas. A three-zone conceptual model—“disturbed zone–active zone–residual zone”—dominated by lithology-controlled migration is established, revealing the dominant roles of gas escape, mixing-homogenization, and adsorption fractionation in heterogeneous sedimentary systems. The results show that high-permeability sand layers can act as adsorption-fractionation windows, causing significant enrichment in δ13C-CH4 (–57.4‰). We propose an analytical framework of “zonal verification–mechanism tracing”, which overcomes the limitations of traditional Rayleigh fractionation models and enables accurate interpretation of gas migration patterns in heterogeneous systems using limited data such as δ13C-CH4 and CH4 concentration. This provides a new paradigm for engineering surveys and risk assessment in low-data-density contexts. The findings indicate that the shallow seepage zone poses low engineering risks, while the residual zone serves as an indicator of depleted gas reservoirs. The proposed analytical approach can be extended to preliminary submarine engineering surveys and hazard assessments in other regions.