Abstract: Based on chemical data analysis, such as cast ing thin slice, scanning electron microscope, sizing test,pressure mercury, we studied Chang 4+5 reservoirs charac teristics and low permeability origin in Ordos basin Heshuiarea. The result indicated that the regional rock type is nature arkosic sandstone primarily, the ingredient maturity andthe structure maturity is low; The reservoir porosity typetake the remaining intergranular hole primarily, next as agrain in dissolution pore and the intergranular dissolutionpore; The throat type is the laminated shape, the curved lamellar throat primarily, the throat is the tiny throat, theseparation and the connectivity is bad; primary origin of lowpermeability is sedimentary and diagenesis function.