Slow CO ₂ Leakage and Selective Mineralization Strengthening of Mudstone Seals: Implications for Geological CO ₂ Storage

A total of 111 mudstone samples from the southern Songliao Basin of China were analyzed to clarify how slow CO2 leakage influences mudstone seal capacity through diagenetic overprint diagenetic processes. Petrography and isotope data show that CO2-affected mudstones contain authigenic Dawsonite, ankerite, and calcite, whose carbon isotopic compositions consistently indicate a mantle-magmatic CO2 source. These carbonates precipitated later than Dawsonite in the underlying sandstones, demonstrating a progressive upward migration of CO2-charged fluids. CO2 leakage induced dissolution of feldspar, precipitation of authigenic carbonates, and formation of minor authigenic clays. Breakthrough pressure measurements reveal that ankerite has the strongest positive influence on mudstone seal capacity, whereas Dawsonite and calcite provide only limited improvements. The results indicate that CO2 leakage produces a vertical shift from carbonate dissolution zones to carbonate-cemented zones, enhancing long-term caprock integrity in the Songliao Basin. This study provides direct petrographic and isotopic evidence for the vertical migration of mantle-magmatic CO2 in mudstone caprocks, which is crucial for reconstructing the long-term CO2 leakage history of sedimentary basins and optimizing risk assessment frameworks of CO2 geological storage in similar continental petroliferous basins worldwide.