Localized CO2-Induced Alteration at a Sandstone–Mudstone Interface and Its Implications for a Potential Self-Sealing Mechanism in the Lishui Sag, East China Sea Basin

Mudstone caprocks are important sealing formations for geological CO2 storage, but the extent and mineralogical response of CO2-induced alteration within mudstone caprocks remain poorly constrained. This study examines a Paleocene mudstone caprock from the Mingyuefeng Formation in the Lishui Sag, East China Sea Basin, where natural CO2 charging has occurred. Mineralogical and geochemical analyses, including X-ray diffraction (XRD), rare earth elements (REE), and carbon–oxygen isotopes, were used to characterize CO2–water–rock interaction at the reservoir–caprock transition. The studied caprock is dominated by quartz and clay minerals, with mineralogical characteristics favorable for caprock sealing. REE signatures indicate a felsic source and limited vertical REE redistribution within the studied interval. Carbonate minerals display distinct vertical variations across the sandstone–mudstone interface, with localized carbonate redistribution near the transition and carbonate depletion accompanied by clay-mineral alteration above it. Most samples retain δ13C values close to background levels, whereas a decimeter-scale interval adjacent to the interface exhibits depleted δ13C values accompanied by localized δ18O variations, consistent with the localized influence of CO2-bearing fluids. The altered zone is restricted to a decimeter-scale interval above the reservoir–caprock transition, suggesting that CO2-related alteration was spatially restricted within the studied interval. Carbonate redistribution and clay-mineral transformation may have contributed to geochemical buffering during CO2–water–rock interaction, although their quantitative effects on pore structure and sealing performance require further petrophysical investigation. These results provide a natural analogue for understanding localized CO2-induced alteration and potential geochemical self-sealing processes at reservoir–caprock interfaces.