Sedimentary microfacies of the Chang-6 reservoir of Yanchang Formation in Wangyao district, Ansai oilfield

The origins of Chang-6 reservoir horizons in Wangyao district of Ansai oilfield are interpreted to be a river-dominated delta deposited in lacustrine environment. The main sedimentary microfacies of Chang-6 reservoir horizons in Wangyao district include distributary channel, distributary mouthbar, front sand sheet and interdistributary deposits. The distributary channel fills to show a finning upward sequence. The distributary mouthbar deposits consist of overall coarsing upward sequence and other homogeneous sequences. The distributary channel and distributary mouthbar sands coalesce to constitute the dominated petroleum reservoirs. Progradation of two distributaries constructed at first only a delta skeleton in the Chang-6~2_1 sandlayer. Rapid sedimentation during the period of Chang-6~1_1 lead to rapid progradation basinward of multiple distributaries. The deltaic microfacies are interpreted and delineated based on depositional sequence and sandbody geometry. Sandbody microfacies are the main factors affecting movement of oil and water. Comprehensive descriptions of sedimentary microfacies are good methods to predict reservoir performance.