Study On The Stratigraphic Classification And The Evolution Model Of River During Fuxian Formation And Yan8Reservoirs Of Yan’an Formation In Wuqi Area，Ordos Basin

Wuqi area is located in the west of Shanbei slope of Ordos Basin. FuxianFormation and Yan’an Formation of early and middle Jurassic are dominated byfluvial deposits. In this paper, classification and correlation of the stratum, theidentification of sedimentary facies are studied and depositional model are established,by using the knowledge of sedimentology and well logging of Fuxian Formation toYan8Reservoir, Ordos Basin.Based on the study on early and middle Jurassic in Ordos Basin, the results andnew understanding are obtained as follows:1. The K9tuff layer is preserved in Fuxian Formation to Yan8Reservoir in Wuqiarea. The sedimentary cycle is obvious and change regularity of electricalcharacteristic is strong, which are the signs of the stratigraphic classification andcorrelation.2. Based on the analysis of drilling cores, logging and thin sections, sedimentaryfacies recognition marks of Fuxian Formation to Yan8Reservoir of early and middleJurassic are determined. There are more than one river types, and the river types indifferent periods during the evolution process of from braided river to meanderingriver. Gravelly braided river is developed in Fuxian Formation, sandy braided river isdeveloped in Yan10Reservoir, while low sinuosity meandering river and highsinuosity meandering river are developed from Yan9Reservoir to Yan8Reservoir.3. There are differences in the characteristics of sedimentary microfaciescombination of different types of braided river and meandering river. Channel bar,channel lag deposit and flood plain are developed in gravelly and sandy braided riversand point bar, channel lag deposit, natural levee, flood fan and flood plain aredeveloped in low sinuosity meandering river, but compared with the high sinuositymeandering river, the natural levee and flood fan are not developed. 4. Through the sedimentary models of different types of rivers established andthe sand body thickness maps of different periods, the results are discovered thatbraided river has the characteristics that large-thickness, multiphase channel stack,well-continuity and channel relatively single, while the meandering river has thecharacteristics small-thickness, little-Sand ratio, weak-continuity and multiple channelin the plane.