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.

The Cuyu gold deposit in central Jilin Province in Northeast China is located in the eastern segment of the northern margin of the North China Craton (NCC), as well as the eastern segment of the Xing’an–Mongolian Orogenic Belt (XMOB). Gold ore-bodies are controlled by NW-trending faults and mainly occur in late Hercynian granodiorite. The mineralization process in the Cuyu deposit can be divided into three stages: quartz + coarse grained arsenopyrite + pyrite (stage I), quartz + sericite + pyrite + arsenopyrite + electrum + chalcopyrite + sphalerite (stage II), and quartz + calcite ± pyrite (stage III). Stage II is the most important for gold mineralization. We conducted analyses including petrography, microthermometry, laser Raman spectroscopy of fluid inclusions, and H–O–S–Pb isotopic analysis to elucidate the mineralization processes in the Cuyu deposit. Five types of primary fluid inclusions (FIs) are present in the hydrothermal quartz and calcite grains of the ore: liquid-rich two-phase aqueous fluid inclusions (L-type), vapor-rich two-phase aqueous fluid inclusions (V-type), CO₂-bearing two- or three-phase inclusions (C1-type), CO₂-rich two- or three-phase inclusions (C2-type), and pure CO₂ mono-phase inclusions (C3-type). From stages I to III, the fluid inclusion assemblages changed from L-, C2-, and C3-types to L-, V-, C1-, C2-, and C3-types and, finally, to L-types only. The corresponding homogenization temperatures for stages I to III were 242–326 °C, 202–298 °C, and 106–188 °C, and the salinities were 4.69–9.73, 1.63–7.30, and 1.39–3.53 wt.% NaCl equiv., respectively. The ore-forming fluid system evolved from a NaCl-H₂O-CO₂ ± CH₄ ± H₂S fluid system in stage I and II with immiscible characteristics to a homogeneous NaC-H₂O fluid system in stage III. Microthermometric data for stages I to III show a decreasing trend in homogenization temperatures and salinities. The mineral assemblages, fluid inclusions, and H–O–S–Pb isotopes indicate that the initial ore-forming fluids of stage I were exsolved from diorite porphyrite and characterized by a high temperature and low salinity. The addition of meteoric water in large quantities led to decreases in temperature and pressure, resulting in a NaCl-H₂O-CO₂ ± CH₄ ± H₂S fluid system with significant immiscibility in stage II, facilitating the deposition of gold and associated polymetallic sulfides. The Cuyu gold deposit has a similar ore genesis to those of gold deposits in the Jiapigou–Haigou gold belt (JHGB) of southeastern Jilin Province indicating potential for gold prospecting in the northwest-trending seam of the JHGB.

In recent decades, an increasing number of non-ferrous and precious metal deposits have been discovered all over the world