Abstract: Yangchang Series in Ansai area is characterized by low Porosity and low Permeability.This comprehensive study is based on the data obtained from the capillary pressure curvs, casting slices, Petrophysical properties, SEM and static conditions.By using the method of moment and Q-type group analysis, the reservoirs are classified, and the characteristics of the pore distribution are studied.The diagenesis is the predominant factor of forming the low permeability sandbodies and making the secondary porosity deevelop.It lays a googd foundation for increasing the economic effectiveness of the exploration and for the reserve calculation.