Densification process of Huagang Formation in northern and central Xihu Sag of East China Sea Shelf Basin

Technical methods such as cast thin section identification, scanning electron microscopy, constant rate mercury intrusion, X-ray diffraction, fluid inclusion homogenization temperature measurement, laser Raman component analysis and illite isotope dating were used to analyze the densification process of Huagang Formation in the northern and central Xihu Sag of the East China Sea Shelf Basin. The controlling factors of reservoir densification were discussed. The reservoir heterogeneity of the Huagang Formation is controlled by the pore throat structure, and the reservoir with a throat radius less than 1 μm is regarded as a tight reservoir. Burial compaction is the main reason for the generally low permeability and tightness of reservoirs, and differential diagenesis aggravates the differential evolution of reservoirs. When the formation temperature exceeds 160℃, the inhibition of cementation by the ring chlorite is significantly weakened. The alkaline closed environment of the middle diagenesis B stage makes a large amount of iron-containing carbonate, pom-shaped chlorite and bridging illite enriched and directly blocks the throats, resulting in large-scale tight reservoirs in the Huagang Formation. Recovering the key diagenetic evolution process of the Huagang Formation reservoirs, it is speculated that the H4 and H5 sand formations of the Huagang Formation entered large-scale compaction during 9-7 Ma, and the strong lateral compression of the Longjing Movement accelerated the densification process.