Abstract: The diagenetic effect, porosity and its influence on physical properties of clastic reservoirs in the Jiufotang Formation in the Zhangwu Depression were analyzed by means of core observation, ordinary and casting thin section microscopic identification, SEM and X-ray diffraction. Compaction, cementation and erosion effects are common in the Jiufotang Formation, and are mainly in stage A of the middle diagenetic phase. The diagenetic process of clay minerals was described as clay rim, compaction, early dissolution, chlorite, illite, secondary quartz overgrowth, micro quartz, sodium feldspar, calcite and late dissolution. Both primary and secondary pores were found. The secondary pores include intergranular dissolution pores, giant pores, intragranular dissolution pores, mold pores and fracture pores. A set of fan delta front sediments provided reservoirs. Rock type, clay content, diagenetic effect and sedimentary micro-facies influenced the distribution of high-quality reservoirs.