The Jiyang sag consists of the Huimin depression, Dongyin depression, Zhanhua and Chezhen depression. Clastic rocks of Paleogene went through early and middle diagenetic stages and now the main reservoir is at a period of the middle stage. Primary and secondary pores were developed in sandstone, the secondary pores were generated from the dissolution of feldspar and calcite cement due to the organic acid from the maturated source rock, but the material dissolved was different in different depressions, for example, the secondary pores of the Dongyin depression were generated from the dissolution of calcite cement, the secondary pores in the Zhanhua and Huimin depression from the dissolution of feldspar, the secondary pores in the Chezhen depression from the dissolution of feldspar in upper section and the dissolution of calcite cement in lower section. The secondary pores were developed at two depths and the depth went down from the west to east, from the south to north in the Jiyang sag. The major controlling factors for secondary pore development were the maturity and location of source rock. Lastly, favorable reservoirs were evaluated according to reservoir depth, sedimentation and diagenesis. The reservoir with good quality was located in the northern and central parts in the Dongyin depression, there was some good reservoir in the Goudao, Goudong and Gounan areas in the Zhanhua depression, and favorable reservoirs were located in the northern steep slope and southern gentle slope of the Chezhen depression and central uplift, south gentle slope of the Huimin depression.