Abstract: The deep-buried sandstone reservoirs of the Huagang Formation in the Xihu Sag, which usually occur 3 500 m below sea bottom, are quite dense and low in gas saturation. The process of reservoir densification and its bearing on hydrocarbon accumulation are discussed in this paper based on the data from cast sections, SEM, C-O isotopes, laser roman spectrum and fluid inclusions. The results indicate that strong compaction is the main reason for primary porosity loss and cementations by authigenic quartz, carbonates and illites are the key factors to reservoir densification. According to the time scale of cementation, reservoir densification occurred mainly in the stage A of the middle phase of digenesis in Middle and Late Miocene about 17~13 Ma. Study of timing of reservoir densification and hydrocarbon accumulation suggests that primary hydrocarbon accumulation mainly occurred before the early stage A of the middle digenesis phase, when the reservoir has not yet densified. Owing to the low hydrocarbon accumulation rate and short accumulation time; the reservoir could not fully infilled. The majority of hydrocarbon accumulation occurred in the late stage A of the middle digenesis phase up to the stage B of the middle digenesis phase, about 15~5 Ma before present in late Miocene, when reservoir became densified. As the result, the reservoir of Huagang Formation is low in gas saturation since hydrocarbon is difficult to charge the reservoir completely. Reservoir densification before hydrocarbon accumulation is the main reason for low gas saturation.