Characteristics and evolution of the sedimentary system of Upper Triassic Yanchang Formation in Ordos Basin,China

Based on observing outcrops, describing cores and analyzing the data of well logging and tests,as well as labor measurements,this paper analyzes the sedimentary system of the Upper Triassic Yanchang Formation in Basin involving the classification and characteristics of the system,and detailed discusses the rule of vertical evolution of the sedimentary system.The result shows that the depositional system of the Yanchang Formation in Ordos Basin composes of alluvial fans,fluvial deposits,fan deltas, braid deltas,meandering river deltas,and lakes.During No.8 period of Yanchang Formation,the lake basin formed.In the study area,there developed the Shenmu-Wushenqi delta,Ansai delta and Zhengning-Heshui delta,and so on.In No.7 period of Yanchang Formation,the lake basin further developed,and the delta began narrowing.The lake water large-scale held back in Member 6 of Yanchang Formation.The delta fronts connect each other and possess the largest area.The delta fronts arrive at Wuqi-Ganquan in the northeast and Huachi-Qingyang in the southwest.In No.4+5 period of Yanchang Formation,the shallow lake was the main subfacies.During No.3 and No.2 periods of Yanchang Formation,the delta plains developed dominantly.In No.1 period of Yanchang Formation,the lake basin continued shrinking until it died out,and swamp environments appeared partly.The evolution of the basin sedimentary system in the basin has formed five sets of reservoirs and cap rock assemblages.