Abstract: The study of diagenesis in the deep reservoir of Huagang Formation in the north area of Xihu Sag is the base for the prediction of deep favorable reservoir. Based on the analysis data of samples from four wells drilled at structure GZZ, the distribution characteristics of secondary dissolution zones in the deep reservoir were concluded. It shows that there are two controlling factors controlling the petrophysical properties of the deep reservoirs:(1) the unstable components of sandstone provided the dissolution space in later stage, and authigenic quartz enhanced the compaction resistance of sandstone to some extent; (2) the alternation of acidic and alkaline digenesis environment controlled the corrosion and cementation. In acidic environment, the dissolution of feldspar and debris and the mineral transition of mixed-layer illite/smectite significantly improved the petrophysical properties of the deep reservoirs; In alkaline environment, however, the recrystallization and interlocking cementation of carbonate, pore throat plugging caused by precipitation of authigenic clay (chlorite and illite) deteriorated the petrophysical properties of the deep reservoirs.