217 / 2019-06-24 10:22:31

Paleogeographic framework and types of sand bodies during Late Triassic Chang 9 interval of the Yanchang Formation, Ordos Basin, China

paleogeographic framework; sedimentary pattern; sand body; Chang 9 interval; Yanchang Formation; Ordos Basin

The Chang 9 interval in the Upper Triassic Yanchang Formation is an important sequence in the oil exploration field in the Ordos Basin, north-central China. The analysis of heavy minerals, light minerals, and trace elements shows that the lake basin had been supplied with multiple drainage systems and provenances. Provenance studies demonstrated that the provenance systems can be divided into four main systems in the basin: northeast provenance derived from parent rocks dominantly composed of metamorphic rocks in the Yinshan Old Land, northwest provenance derived from parent rocks dominantly composed of metamorphic rocks in the Alxa Old Land, southwest provenance derived from parent rocks dominantly composed of metamorphic rocks in the Longxi Old Land, and south provenance derived from parent rocks dominantly composed of metamorphic and igneous rocks in the Qinling Old Land. Through comprehensive study of well logging, drilling, core, and well drilling, A shallow lake surrounded by multiple deltas was the main sedimentary pattern in the basin, representing three facies belts: western braided deltas, eastern meandering river deltas, and a southeastern shallow lake. Sand bodies mainly developed extensively along the braided river deltas and meandering river deltas, and can be divided into distributary channel-type sand bodies controlled by traction flow and bar-type sand bodies related to lake waves and lake flow, based on the genesis of the sand bodies. The sand body mainly develops 5 basic vertical structures, namely box shape, campaniform, infundibuliform, finger and dentoid. The vertical stacking patterns of multilayer sand body is complex, and the common shapes include box shape + box shape, campaniform + campaniform, campaniform + box shape, infundibuliform + infundibuliform, campaniform + infundibuliform, box shape + infundibuliform, etc. Based on analysis on major dominating factors of vertical structure of sand body, sedimentary environment, sedimentary facies and cycle of base level are identified as the major geological factors that control the vertical structure of single sand body as well as vertical stacking patterns and distribution of multistory sand bodies. There are four kinds of transverse structural features of sand body: "Isolated type", "Migrating stacked type ", "Contiguous type " and "Labyrinth type ". There are six kinds of planar structure of sand body: "Strip type ", "Branched type ", "Net type ", "Even chip type "," Flower-like type " and " Bird's foot type ". This work provides an ideal case study for provenance systems and sand bodies in a sedimentary basin, and also benefits petroleum exploration and development in sand reservoirs.