Sedimentary microfacies and evolution in the Chang 6_1 member of Yanchang formation of Xinghe area in Ansai Oilfield

The delta front subfacies of the Chang 61 member of Yanchang formation,Xinghe area,Ansai Oilfield,Northern Shaanxi have been analyzed based on core descriptions,tests and logging data analysis.It is concluded that the meandering river dominated delta front subfacies in the lacustrine environment in this area can be divided into five sedimentary microfacies,i.e.subaqueous distributary channel,mouth bar,interdistributary bay,subaqueous natural levee and distal bar with the subaqueous distributary channel and mouth bar being the skeleton sand bodies and the distal bar being relatively underdeveloped.The study shows that the distributary channels have frequently relocated with the early distributary channels coming from the north,then from the northeast and the main flow path being in the northeast – southwest direction and the sediments also originated from the northeast direction.The paper has also illustrated the time-space distribution and evolution features of the five subunits of sedimentary microfacies in the Chang 61 sandstone unit,providing a basis for further development of the oilfield.