Abstract: Based on core descriptions and logging data, the origins of Chang-6 reservoir of Yanchang Formation in Xinghe area of Ansai Oil Field are analyzed.The reservoir in this area represents a riverdominated delta progredation.Deposits consist of distributary channel, distributary mouthbar, front sant sheet and interdist ributary deposits.The distributary channel and distributary mouthbar sands constitute the dominated reservoirs.Grain-size distributions of the sandst ones are discussed in detail on the basis of cumulative curve, skewness-sdandard deviation, C-M pattern, and other distinction approaches.Earlier delta built out into the lacustrine shallow water from north of the area.Frequent division of distributaries produced many small distribut aries and a lobate delta.Later progradation produced cyclic alternations of distributary and interdistributary deposits from northeast of the area.The development of deltaic sedimentation produced overlapping sand bodies of a delta-complex.The sedimentary microfacies of individual sand bodies are described in detail based on depositional sequence and sand body geometry.