Diagenetic Path of Deeply Buried Clastic Rocks and Pore Evolution of Reservoirs in the Oligocene Huagang Formation of the Xihu Sag

To elucidate the development control factors, diagenetic evolution, and pore evolution of oil and gas reservoirs of the Huagang Formation in the East China Sea Shelf Basin Central Anticlinal Belt, this study involved geological analyses, including thin-section petrography, scanning electron microscopy (SEM), mineral analysis via TESCAN Integrated Mineral Analyzer (TIMA), X-ray diffraction (XRD), and petrophysical measurements. We investigated the reservoir characteristics and primary diagenetic processes of the Huagang Formation reservoirs using logging and nuclear magnetic resonance (NMR) data, identified provenance differences between the north-central (FN) and south-central (FS) areas, divided diagenetic environments, established distinct diagenetic sequences, and uncovered high-quality reservoir pore evolution patterns. The results showed that the provenance in the FN area of the Central Anticlinal Belt is primarily acidic igneous rocks, which exhibits low resistance to compaction but is susceptible to dissolution modification, and the “high-dissolution zone” developed at burial depths of 3600–3900 m constitutes the primary high-quality reservoir; the provenance in the FS area is a mixture of medium- and high-grade metamorphic rocks and acidic igneous rocks, which exhibits stronger resistance to compaction, but dissolution zones are poorly developed. The Huagang Formation has experienced multiple diagenetic processes, such as compaction, cementation, and dissolution. During destructive diagenesis, the average reduction in pore volume due to compaction accounts for 76% (FN area) and 81% (FS area), while cementation accounts for 18% (FN area) and 12% (FS area). Vertically, 3900 m and 4000 m are the boundaries between the acidic zone and acid-alkaline transition zone of the Huagang Formation in the FN and FS areas, respectively, and the whole Huagang Formation is considered within the meso-diagenetic A2 stage. The pore evolution is closely related to diagenesis. The porosity of the sandstones in the Upper Member of the Huagang Formation in the FN area changes from 37.5% to 10.62%, and the porosity of the sand-stones in the Lower Member of the Huagang Formation in the FS area changes from 36.5% to 8.90%. The results of this study provide a reference for the study of differential diagenetic evolution of sandstones in the Xihu Sag and the exploration of deep high-quality reservoirs.