Provenance systems and types of sand bodies during Upper Triassic Chang 8 time of the Yanchang Formation, Ordos Basin, China

Abstract

The Chang 8 interval in the Upper Triassic Yanchang Formation is an important sequence for strategy replacement in the oil exploration field in Ordos Basin, north- central China. Provenance studies demonstrated that the provenance systems can be divided into mainly four systems in the basin: the northeast, northwest, southwest, and south provenance systems. It is inferred that terrigenous clastic sediments mainly originate from the oldlands around the basin. Parent rocks in these provenances are composed of a variety of rocks dominated by metamorphic and igneous rocks. A shallow lake surrounded by multiple deltas was the main sedimentary patterns in the basin, representing three sedimentary facies belts: west braided deltas, east meandering river deltas, and southeast shallow lake. Sand bodies mainly developed extensively along the braided river deltas and the meandering river deltas, which can be divided into the distributary channel type of sand bodies controlled by the traction flow and the bar type of sand bodies related to the lake wave and lake flow reconstruction according to the genesis of sand bodies. Moreover, the structural characteristics of the different types of sand body in vertical, horizontal, and plane views were classified. Vertically, there were seven types of single sand bodies, which were related to braided subaerial distributary channels, braided subaqueous distributary channels, distal bars, subaerial distributary channels, subaqueous distributary channels, mouth bars, and natural levees, respectively. Horizontally, sand bodies have sheet shape structure, migration and stack structure, isolated structure, and composite structure. There were five types of plane shape of sand bodies developed in the basin, including widely strip shape, mesh shape, cramped and strip shape, “bird foot” shape, and branched shape.