Sequence stratigraphy and genesis of sand bodies of the Upper Triassic Yanchang Formation in the southwestern margin of Ordos Basin

Taking the Yanchang Formation in the southwestern margin of Ordos Basin as an example,by using the high resolution sequence stratigraphy theory,this paper studied the sedimentary characteristics of the Yanhe and other outcrops.It is concluded that unconformities,erosion surfaces,lithofacies contrast surfaces and flood surfaces mark the important sequence boundaries.The Yanchang Formation in the study area can be divided into one super long-term base-level cycle,five long-term base-level cycle and seventeen mid-term base-level cycles.It mainly developed braided fluvial and braided delta depositional system in the southwest steep slope,meandering river delta sedimentary system in northeast terrain-relief and turbidite fan deposits in the deep lake systems.The genesis of favorable sandstone reservoirs can be divided into submarine distributary channel,submarine distributary channel and composite river mouth bar,river mouth and slump turbidite fans.With changing lake level,the ratio of accommodation space(A) over sediment supply(S) affects the distribution of depositional systems from different source directions and the sedimentary evolution distribution patterns,as discussed in the paper.