The Xihu depression is an offshore sag located on the East China Sea Shelf Basin, which is currently one of the major oil and gas basins along the coast of China. In this study, an integrated approach using thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), high-resolution 3D CT core scanning and stable isotope analysis was applied to examine the diagenetic evolution and investigate the microscopic characteristics and formation mechanisms associated with effective reservoirs. Four types were distinguished: upper conventional reservoirs (UC reservoirs), lower conventional reservoirs (LC reservoirs), “bottom calcium” low-permeability reservoirs (“bottom calcium” reservoirs) and “MI clay” low-permeability reservoirs (“MI clay” reservoirs). Poikilotopic calcite cements play an important role in the diagenetic alterations and reservoir quality evolution, precipitating during early eogenesis, provided a framework that retards the adverse impacts of UC reservoirs by compaction. Conversely, in LC reservoirs, with limited poikilotopic calcite, secondary porosity is mostly due to the dissolution of feldspar or unstable rock fragments. UC reservoirs normally develop in the middle of tidal channels and in subaqueous distributary channels, with the base of the sand-body being extensively cemented by carbonate cements, such as late calcite, Fe-calcite and dolomite, which formed the “bottom calcium” reservoir. Combined evidence from petrographic and geochemical analyses suggests that calcite precipitates from diagenetic fluids of mixed marine and meteoric waters, with additional external sources from calcareous siltstones and bioclasts. The carbon sources of calcite mostly originate from the dissolution of carbonates clacts or bioclasts within sandstone beds or adjacent silty mudstones, while dolomite cements have an isotopic composition that is more comparable to the generation of biogenic methane. This study demonstrates how poikilotopic calcite, developed in tide-dominated delta systems, affects the vertical heterogeneity. The results can be used to improve the reservoir evolution model of tide-dominated delta systems and provide a basic understanding for researchers conducting reservoir studies of similar sedimentary systems. Our results can act as a geological basis for further oil and gas exploration.

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.