Diagenesis and burial history controls on Oligocene Huagang sandstones, southern Xihu Sag (East China Sea Basin): Implications for the formation of effective reservoirs

This study aims to reveal the diagenetic evolution sequence and controlling factors of Oligocene Huagang Formation (E₃h) sandstone reservoirs in the HY area, and predict high-quality reservoir distribution to guide hydrocarbon exploration. A comprehensive approach integrating core observation, thin-section petrography, geochemical analysis, and seismic-log interpretation was employed. By combining burial history simulation and diagenetic analysis, we systematically analyzed reservoir quality and diagenetic evolution and established evolution models for effective reservoir pore spaces. Results indicate that E₃h sandstones underwent intense diagenetic processes including compaction, cementation, and dissolution. Diagenesis plays a critical role in controlling reservoir quality: compaction is the primary factor that governs vertical variations in petrophysical properties, where shallower intervals typically exhibit better quality than deeper ones. In contrast, cementation and dissolution serve as secondary controls, explaining property differences between Blocks A and B, with Block A reservoirs being superior. A three-stage relationship has been identified between burial history and diagenesis: (1) early gradual subsidence stage (GSS) with weak feldspar dissolution, early carbonate cementation, chlorite coating, and mechanical compaction; (2) intermediate tectonic activity stage (TAS) with massive feldspar dissolution, quartz overgrowth precipitation, and ferruginous cement formation; (3) late stable burial stage (SBS) featuring intensified compaction and clay mineral transformations. The spatial configuration of fault systems and sandstones, combined with compositional attributes, strongly controls diagenetic evolution. Favorable zones for dissolution development occur at intersections of fault zones and acidic fluid migration pathways, while quartz-feldspar-rich zones mitigate porosity loss via compaction resistance. Three effective reservoir development models were proposed: high dissolution-low cementation-low compaction, moderate dissolution-low cementation-low compaction, and low compaction-low cementation-low dissolution. This research provides a critical basis for predicting E₃h high-quality reservoirs in the study area. Furthermore, the comprehensive burial history-diagenesis analysis method presented here offers a valuable reference for studies on sandstone diagenetic evolution and hydrocarbon exploration in similar geological settings.