In China， theories on secondary porosity of deep reservoirs need breakthrough urgently. The Qinan Sag， located in the Huanghua Depression， is an active hydrocarbon generation sag， and deve￣lops favorable reservoir sandbodies in the Paleogene Shahejie Formation， and has a significant exploration potential. Using thin sections，scanning electron microscope(SEM)，cathodoluminescence，Xdiffraction methods， integrated with measured reservoir properties and log interpretation, the authors summarized that distribution of secondary pores was different in different tectonic units. In the west slope， secondary pores were distributed in 3 belts: 1650～1800 m，2300～2600 m and 2880～2920 m. In the eastern Chengbei faulted belt， secondary pores were distributed in 2200～2800 m. In the central zone of the sag， secondary pores were distributed in 2900～3500 m. Lithic sandstone and arkosic arenite sandstone are the dominant rock types in the study area with low component and texture maturity which made the material basis for secondary pore development. Sedimentary environment also had some influence on reservoir quality. Lake shore zone and bar sandbodies possess thebest reservoir petrophysics. They are followed by main gravityflow channel sandbodies and channel of middle fan in subaqueous fan. The flankdistal sandbodies of gravityflow channel and middlebay in subaqueous fan constitute poor to fair reservoirs. The main mechanisms for secondary pore formation are as follows: ①The development of clay coating is favorable for development of secondary pore. ② The organic acid which formed during the process of organic material transforming into hydrocarbon and the acidic solution forned in resulting from the conversion of montmorillonite into illite could dissolve the feldspar and lithic grains. Asaresult, secondary pores could the develop. ③The activity of pore water entering into the sandbody reservoir influences the development of secondary pore. ④The dissolution dominated cementation during the hydrocarbon filling process and thus in the later period， secondary pores were developed. ⑤ Adjecent to fault and unconformity， a great deal of CO2 was dissolved in the atmospheric fresh water. Carbonate acids could be easily  to formed and they dissolved   feldspar grains when entering the reservoirs. As a result secondary pores were formed.