Sedimentary Facies Characteristic and Sequence Stratigraphy Analysis of Yanchang Formation in Longdong Area,Ordos Basin

At Late Triassic,Ordos Basin is an asymmetry depressed-type lacustrine basin with gentle slope in the east and steep slope in the west.By the research of core,logging profiles and seismic data,four types of sedimentary facies have been recognized.Among these sedimentary facies,river facies and turbidity fans are found on the steep slope in the west of the depression,while braid river deltas are found on both the steep slope in the west and the gentle in the east of the depression.Based on the classic pattern of sequence stratigraphic framework,five three-order sequences have been identified in Yanchang formation,Longdong area.Five times of lake transgression can be distinguished.The sequence evolution is that they formed in early time,reached the maximum in middle time and declined in late time.Fischer's plots of Yanchang formation stratigraphy are scaled with retrieved stratigraphic thickness in Longdong area.There are well corresponding relations between the plots and five times of lake transgression.The classification of the sequence stratigraphy has been verified by using Fischer's plots,which may be a potential tool for sequence analysis.