Quantitative characterization and distribution of petrologically superior sandbodies in a shallow-water delta: Chang 81 sub-member, Triassic Yanchang Formation, Longdong area, Ordos Basin, China

Due to its huge production potential, tight sandstone oil reservoir is the most promising exploration target in the Upper Triassic Yanchang Formation, Ordos Basin. Although tight sandstone reservoirs have received much attention in recent years, their strong internal heterogeneity makes the identification of superior sandbodies a great challenge in the industrial community. In this paper, we describe the detailed sedimentary structural characteristics of the Chang 81 sub-member sampled by 52 coring wells in the Longdong area at the Ordos Basin, present well logging data to establish the standards for the identification of superior sandbodies, aiming to investigate the planar distribution regularities and the deposit mode of shallow-water delta superior sandbodies. Five typical sandbody types were identified using samples description: homogeneously and heterogeneously bedded distributary channels, mouth bars, respectively, and levee sandbodies. Among in them, homogeneously bedded distributary channels and mouth bars are of a superior type, developed mainly in the middle of the respective microfacies; other sandbody types are mainly located at the edge of them. The use of sedimentary structures to assess the quality of sandbody reservoirs provides a new method for the identification of superior sandbodies in tight sandstone. The workflow proposed in this paper, based on a step-by-step analysis of beddings and microfacies, can be applied in the Ordos Basin and other similar shallow-water delta systems to improve the accuracy and reduce the economic cost of hydrocarbon exploration.