Diagenesis and Burial History Controls on Oligocene Huagang Sandstones, Southern Xihu Sag (East China Sea Basin): Implications for the Formation of Effective Reservoirs

Abstract

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, combined with burial history simulation and diagenetic analysis, to systematically analyze reservoir quality and diagenetic evolution, and establish reservoir space evolution models for effective reservoirs. Results indicate that E₃h sandstones underwent intensive diagenetic modifications including compaction, cementation, and dissolution. Diagenesis exerts critical controls on reservoir quality: compaction is the primary factor governing vertical variations in petrophysical properties (shallower intervals exhibit better quality than deeper ones), while cementation and dissolution act as secondary controls explaining property differences between Blocks A and B (Block A reservoirs are superior). A burial history-diagenesis coupling relationship was identified, involving three stages: (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 compaction resistance in quartz-feldspar-rich zones mitigates porosity loss. 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 burial history-diagenesis comprehensive analysis method presented herein offers a reference for sandstone diagenetic evolution studies and hydrocarbon exploration in analogous geological settings.

Keywords: Diagenetic evolution, Controlling factor, reservoir quality, Effective reservoir, Huagang formation, Xihu sag