Abstract: It's well known that lower segment of stream sandbody with positive sequence has better porosity and permeability than upper one of stream sandbody,but there is an exception in Chang-2 oil-bearing strata of Yanchang formation.Based on the study of petrology characteristics and types of pore,authors point out that bad porosity and permeability in the lower segment is the result of cementation in the diagenetic stage.We hold that compaction,cementation and secondary enlargement should be leading role.The best reservoir locates in the upper segment of stream sand stone in the area.