Abstract: The sand reservoir of the Shanxi Formation and lower Shihezi Formation of Upper Palaeozoic in the south of Ordos Basin belongs to tight reservoir.The calcite cementation is one of the key factors causing tight sand reservoir.The results of cement generation,inclusion enclave temperature measurement,O/C isotope analysis and electronic probe microelement analysis proved that most of the calcite cements were coming into being in the late diagenetic stage.After suffered compaction and early cementation,the porosity and permeability of the reservoir decreased greatly,and the opening of diagenetic environments became poor.With accompany of CO₂ formed by degradation of organics,the calcite cement was precipitated.The remnant intergranular pores were filled with this kind of late calcite cement,which leads to a great decrease of the effective porosity.Since the calcite cements were formed lately and the sand reservoir has not experienced any other constructive diagenesis essentially,the development of calcite cements had apparent controlling effects on reservoir characteristics.