Sedimentary microfacies of Chang-6 reservoir of Yanchang Formation in Xinghe area of Ansai Oil Field

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 river-dominated delta progredation. Deposits consist of distributary channel, distributary mouthbar, front santsheet and interdistributary deposits. The distributary channel and distributary mouthbar sands constitute the dominated reservoirs. Grain-size distributions of the sandstones 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 distributaries 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.