Abstract:The deep Paleogene Huagang Formation and Pinghu Formation in the western subsag of Xihu sag are rich in oil and gas resources, making them a new field for exploration and development in the East China Sea shelf basin. However, the hydrocarbon accumulation mechanism is complicated due to the influence of differential densification of reservoirs. In order to clarify the hydrocarbon accumulation process in the deep reservoirs of the western subsag, we conducted systematic research on reservoir characteristics, hydrocarbon accumulation stages, and accumulation process. This was done through the experimental analysis of petrographic characteristics, microscopic temperature measurement, laser Raman spectroscopy, and geochemistry of the oil and gas contained in the fluid inclusions. This research was combined with an examination of structural evolution, stratigraphic burial history, and thermal history. The results of our study showed that the deep of Huagang Formation (>4000 m) and Pinghu Formation have both undergone densification, with dissolution pores being the main type of reservoir pores. Two stages of hydrocarbon inclusions were identified in both formations, with early stages dominated by oil- bearing inclusions and late stages dominated by natural gas inclusions. These findings indicate that the accumulation time corresponds to the Longjing movement and the Okinawa movement, respectively, with the late period being the most important. The geochemical characteristics of hydrocarbons contained in inclusions suggest that the hydrocarbon- generating material is mainly derived from higher plants and supplemented by lower organisms, which is consistent with the characteristics of coal- measure source rocks in the Pinghu Formation. The Okinawa movement, combined with pressurization caused by gas generation in the Pinghu Formation, served as the main driving force for gas accumulation in the late stage, particularly when the reservoirs in the H10 member and above were not densified. According to the relationship between the time sequence of reservoir densification and hydrocarbon accumulation, four types of reservoirs could be determined: conventional type, densification followed by accumulation, accumulation followed by densification and accumulation while densification. The research results provide a theoretical basis for the next step in the exploration and development of tight sandstone gas in the study area.