Abstract: The present paper gives a detailed description of lithology, porosity types and diagenesis on the basis of cores, thin sections and well logs of the reservoir sandstones from the third member of the Lower Shihezi Formation in the Tabamiao region, northern Ordos Basin. These reservoir sandstones have lower compositional and textural maturity. The porosity types include intergranular pores, intragranular solution openings, intercrystal solution openings, moldic pores and grain-margin fissures. The porosity between 0.7% and 18.69% with an average of 7.82% and the permeability from 0.008×10^-3 μm² to 23.01×10^-3 μm² with an average of 1.111×10^-3 μm² indicate the micropores-type and solution openings-type reservoir rocks. The channels may exercise a major control on the distribution and range of the excellent reservoir rocks. The constructive diagenetic processes such as rimmed chlorite cementation and dissolution during the early diagenesis are considered as the basic conditions for the generation of reservoir rocks. The diagenetic facies play an indispensable role in the further expansion of the reservoir area and improvement of the reservoir quality of reservoir rocks. The favourable blocks are delineated in terms of sedimentary microfacies of the reservoir sandstones, planar distribution of the physical properties and diagenetic facies of the reservoir sandstones. The results of research in this study may provide a geological basis for the further exploration in the study area.