Abstract: The reservoir of Chang 4+5 Member of Yanchang Formation is characterized by ultra low permeability and complicated microscopicpore structure in Zhenbei area,Odors Basin.Conventional methods such as casting slice,scanning electron microscope(SEM)and physical properties can analyze petrological characteristics and pore structure types qualitatively,and can't analyze the quality of this reservoir reasonably.It should be combined with unconventional methods such as contant-speed mercury injection and nuclear magnetic resonance technique(NMR),in order to analyze the microscopicpore structure and movable fluid quantitatively of ultra low permeability reservoir.The results showed that the microscopicpore structure and physical properties of ultra low permeability reservoir were caused by impact of sedimentation and diagenesis.Ultra low permeability reservoir is characterized by the average throat radius,usually the bigger average throat radius,the bigger permeability,the more movable fluid saturation,and the better quality,especially if the average throat radius was smaller than 1.26 μm and the permeability smaller than 1.0×10^-3 μm².Comprehensive studies have shown that the microscopicpore structure and the development quality of ultra low permeability reservoir was mainly controlled by throats.