Abstract: Chang 8 oil reservoir in the Shui Mogou area of Fuxian county,Ordos basin is mainly composed of fine sandstone and powder-fine sandstone,which is medium in roundness and porosity,with medium structure maturity.The main sedimentary background of Chang 8 oil reservoir is lacustrine delta sedimentary system,which can be subdivided into subfacies of predelta and delta front.On this basis,three kinds of microfacies were mainly developed,namely underwater distributary channel,subaqueous diversion channel and the former deltaic mud.The main layer is 821,which is in the leading edge of the delta,and the large-scale development of underwater distributary channel sand body is the main oil and gas reservoir,and the reservoir is mainly controlled by sedimentary facies.