Relationship between sandstone reservoirs densification and hydrocarbon charging in the Paleogene Huagang Formation of Xihu Depression, East China Sea Basin

The tight sandstone gas resources in Xihu Depression of the East China Sea Basin are abundant. In recent years, the Paleogene Huagang Formation is a hot spot for exploration. However, the unclear densification of the reservoir and the matching relationship with the oil and gas charge restrict the effective exploration and economic development of the tight sandstone gas of the Huagang Formation. This study reveals the reservoir densification process and its relationship with hydrocarbon charging based on the analysis of sandstone reservoir characteristics, the classification and evolution of diagenesis type and evolution sequence.The study shows that the reservoir sandstone mainly undergoes three diagenesis processes: compaction, cementation and dissolution. Compaction is the root cause of sandstone pore loss and densification in the Huagang Formation reservoir during burial diagenesis. The formation experienced a total of three stages and five periods of diagenetic evolution, including the syngenetic stage, the early diagenetic stage (A, B period) and the middle diagenetic stage (A, B period). At the A₂ period of the middle diagenetic stage, the diagenetic environment began to change from acidic to alkaline, accompanied by cementation of late carbonate and siliceous. When the strength of mechanical compaction continued to increase, the reservoir gradually became denser. Combined with the coupling relationship between oil and gas accumulation period (7-0 Ma), the Huagang Formation reservoirs in the study area are divided into three types: accumulated reservoir, synchronous dense reservoir, accumulated dense reservoir. The study combs the reservoir characteristics, diagenetic evolution sequence, reservoir densification and coupling relationship with hydrocarbon charging in the Huagang Formation of Xihu Depression, providing theoretical support for the exploration of tight sandstone gas in the next study area.