Abstract: The diagenesis of the Triassic Chang-8 oil reservoirs in the south-central part of the Ordos Basin includes compression, secondary overgrowth of quartz, secondary kaolinization of mica and matrix, overgrowth of authigenic chlorite membrane on the walls of primary pore spaces, replacement of intergranular matter and grain rims by intergrown calcite, nonselective dissolution of intergranular matter and grain rims and filling of asphalt into the secondary solution openings, and dissolution of feldspar. The present paper deals with the spatial distribution and origin of diagenetic products, and the relationship between the distribution of diagenetic products and existing total planar porosities on the basis of cast thin sections and carbon and oxygen isotopic data.The physical properties of the Triassic Chang-8 oil reservoirs are mainly controlled by secondary overgrowth of quartz, intergrown calcite replacement, feldspar solution openings, and distribution of residual primary porosities with authigenic chlorite membrane. The physical properties are generally poor in the areas where secondary overgrowth of quartz and intergrown calcite are developed. On the contrary, the physical properties are relatively good in the areas where the residual primary porosities with authigenic chlorite membrane and feldspar solution openings are developed.