Abstract: Based on data of casting sheet images,SEM,physical property,mercury injection curves,oil saturation and formation water salinity,forming mechanism of Chang 2 low resistivity oil layer of Yanchang Formation in Jiyuan region of the Ordos Basin has been analyzed.There are mainly 3 reasons for the formation of the low resistivity oil layer,including high irreducible water saturation,low oil saturation and high formation water salinity.Complex porosity structure,development of micro-pores among kaolinite crystal and fine grained rock framework form thin throat and small pore throat radius,which cause high irreducible water saturation.Low oil saturation is mainly caused by low amplitude tectonic setting and low oil column height.High formation water salinity reduces resistivity contrast between oil layer and water layer.According to statistics of geochemical pyrolysis parameters of Change 2 oil layer,combined with data of oil testing and well logging porosity,the cross plot of S1 and Φ has been built,by which the Chang 2 low resistivity oil layer of Yanchang Formation in Jiyuan region can be recognized.