Abstract: In Yanchang formation period, the environment is favorable to the development of the lake delta.The delta front mainly developed in Chang 6, which includes microfacies, such as distributary channel sandbodies, river mouth bars, distal bars and so on.The pore spaces are completely dominated by primary porosity and pores, vogs and fractures with secondary origination.Study indicates that the formation and evolution of the porosity are controlled by diagenesis.The diageneses mainly comprise cementation, compaction and dissolition.Cementation, compaction are unfavorable for the evolution of the porosity.Dissolition is favorable for secondary porosity, thus forming available reservoirs in some sections of low porous and low permeable clastic rocks.According to synthesize evaluate, the distributary channel sandbodies and river mouth bars are best reservoir.