Evolution of Deep Diagenetic Fluid Environments in the Xihu Sag of the East China Sea Shelf Basin and Their Impact on Reservoir Quality

ABSTRACT Systematic investigations into the deep diagenetic fluid environment, diagenetic evolution and their effects on reservoir properties in the deep layers of the Pinghu Formation within the N structural belt of the Xihu Sag in the East China Sea Shelf Basin remain scarce. Drawing upon petrographic thin sections, scanning electron microscopy (SEM), whole‐rock and clay mineral X‐ray diffraction (XRD), fluid inclusion analyses, well logging and other analytical data—integrated with burial and thermal history modelling—this study elucidates the alternating acidic and alkaline fluid conditions, their diagenetic evolutionary pathways and their consequent influence on reservoir performance. The results indicate that the deep Pinghu Formation reservoirs experienced multiple episodes of acid‐alkaline diagenetic environments during their geological evolution. Presently, they are at the mid‐diagenetic stage A 2 , characterised by a transition from weakly alkaline to acidic and then to alkaline conditions. Acidic environments are primarily marked by the dissolution of feldspar and lithic fragments, precipitation of authigenic kaolinite, development of secondary quartz overgrowths and minor calcite dissolution. Alkaline environments are recognised by quartz dissolution, illite and chlorite cementation, carbonate precipitation and replacement, as well as albite precipitation. We establish a pore evolution model featuring ‘early compaction, intermediate dissolution and late cementation’. Four fluid environment types can be identified: weakly alkaline–weakly acidic, weakly acidic–moderately to strongly acidic, acidic–weakly alkaline and weakly alkaline–alkaline. Acidic fluids derive mainly from organic acids, complemented by humic acids, whereas alkaline conditions arise when acidic fluids are extensively consumed and metal cation enrichment elevates the pH of formation waters. Repeated cycles of acidic and alkaline diagenesis foster multi‐phase carbonate cementation and clay mineral transformations. Overall, acidic fluids enhance reservoir quality, while alkaline fluids exert both constructive and destructive influences. Compaction‐related porosity loss is largely depth‐dependent, cementation‐related porosity loss correlates with alkaline diagenesis and dissolution‐related porosity gain hinges upon the intensity of acidic fluid supply.