Abstract: The Lower Huagang Formation in Xihu Sag is rich in tight gas resources, which is a hot spot for oil and gas exploration and development in this area. Based on observation of rock slices, scanning electron microscopy, physical property testing of the core and X-ray diffraction, the basic reservoir characteristics, the diagenesis and the pore evolution of the reservoir are studied. The results show the reservior porosity is mainly between 7% and 10%, the permeability mainly between 0.10 to 0.50 mD, and the fracture undeveloped, so the reservoir belongs to a kind of tight sandstone reservoir. The reservoir has experienced the early diagenesis stage A and B and the middle diagenetic stage A, and is currently in the middle diagenesis stage B, and the reservoir are greatly influenced by compaction, dissolution and cementation. The results of pore quantitative evolution show that the compaction reduces porosity by 29.67%, which is the main reason for the undeveloped primary pores; the dissolution increases porosity by 15.14%, which is the key factor for the development of deep reservoirs; with the influence of cementation, the reservoir porosity reduces by 13.55%, which is an important reason for the deterioration of reservoir pore structure. Based on the relationship between pore evolution results and oil-gas charging time, the Lower Huagang Formation in the south-central of the Xihu Sag is a kind of tight reservoir that denses before the accumulation of oil and gas.

Key words: Xihu Sag, Lower Huagang Formation, tight reservoir, diagenetics, pore quantitative evolution