Abstract: The secondary porosity is the major porosity type of sandstones reservoirs in the first member of the Dainan Formation in the Qintong depression,northern Jiangsu.The distri-bution of the secondary porosity is closely related to oil and gas accumulation.The author inquires,on the basis of the studies of diagenesis,into the genetic mechanism of the sec-ondary porosity and owes its origin to the dissolution of organic acids.Some speculations upon the horizontal distribution of the secondary porosity were also made by the author in the light of organic maturity and compaction anomaly of the mudstones caused by clay mineral trans formation,thus resulting in the improvement of the knowledge of the distri-bution of the secondary porosity and oil and gas accumulation in different structural zones in the study area.