Abstract: The Upper Paleozoic in Ordos includes four gas formations such as Tai2 , han2 , He8 and He7. Studies on them show the reservoirs are defined as the poreOtype and fractureOpore type sandstone with low porosity and low permeability , as well as markable heterogeneity. The type of reservoir’s pore space is mainly pore , which can be subdivided into four sorts , i. e. , intergranular pore , residual intergranular , intragranular dissolved pore as well as Kaolin intercrystal pore. The widespreading micro - fracture has minor effect on changing the reservoir’s porespace , but it can increase the reservoir’s permeability. The main factors that control reservoirs include deposition , diagenesis and tectogenesis. The reservoir with coarser grain size and higher maturity has better porosity and permeability. It’s compaction , silicon cement and Kaolin cement that account for the
tight reservoir. Dissolution and tectonic microOfracture are favorable to improve the reservoir.

Key words: Ordos basin, Upper Paleozoic, reservoir, clastic rock, diagenesis