Abstract: Ultra-low permeability reservoirs have the characteristics of tremendous development potential and complex pore structure. In order to make definite the microscopic geologic origin of Yanchang Formation in Ordos basin, the authors integratedly use a lot of advanced technology test methods, such as convention thin section, cast thin section, X-ray, high pressure mercury penetration, constant rate mercury penetration, NMR, EMS techniques. The results show that the key elements of cause physical property variation are high cement content, high chlorite content and complex form, halite content, high ferrocalcite content, illite cement upgrowth, lomontite cement corrosion bad. All those elements cause ultralow permeability reservoir showed the characteristics of poor scope-pore coefficient, high displacement pressure, fine pore-throat upgrouth, low movable liquid saturation.

Key words: microscopic origin, ultra-low permeability, Yanchang Formation, reservoirs, Ordos basin