Sedimentary and evolution characters of the delta in the Lower Permian Shanxi Formation in the northeast of Ordos Basin,China

On the basis of the research on field outcrops,well cores and well logging data,this paper discusses the sedimentary and evolution characters of the main sedimentary facies(delta facies) of the Lower Permian Shanxi Formation in the northeast of Ordos Basin.It identifies two delta systems lying in the east part and west part of the research area,respectively.The delta plain can be divided into distributary channel,natural levee,splay,plain swamp and flood plain microfacies.The delta front includes underwater distributary channel,estuary bar,tide sandbar,distal sandbar,inter-distributary bay microfacies.The distributary channels were braided channels in P1s2(Member 2 of the Shanxi Formation) stage,and they changed into meander channels afterward.In P1s2-3 and P1s2-2(Member 2-3 and Member 2-2 of the Shanxi Formation) stages,river-tide controlled delta developed in the background of tide flat,and river controlled-lake delta grew with sea water withdrawing from the southeast and lake environment emerging.Different types of facies,sub-facies and microfacies will control the lithofacies change character, sandbody distribution character and reservoir performance of the sandbody.Because the underwater distributary channels,tide sandbars and estuary bars in the delta front and the distributary channels in the delta plain of P1s2-3 sub-segment have more excellent quality in lithofacies,sandbody thickness and reservoir performance of the sandbody,they are key targets of exploration and exploitation.