Abstract: The diagenesis stages of the reservoir sandstones in the Upper Palaeozonic strata in Daniudi Gasfield,northern Ordos Basin,were studied.The results show that the vitrinite reflectance(R_o) of the Taiyuan Formation and the First Member of Shanxi Formation is between 1.4% and 1.5%,the inclusion homogenation temperature is 140℃ and the percentage of montmorillonite is usualy less than 30% in illite-montmorillonite mixed-layer mineral,indicating the stage B of metarmosis.The Lower Shihezi Formation(including P₁x¹,P₁x² and P₁x³ Members) is in the stage A of metarmosis with the R_o being 1.3%-1.4%,the inclusion homogenation temperature being 126℃ and the percentage of montmorillonite being 32%-36% in illite-montmorillonite mixed-layer mineral.The inclusion homogenation temperature of the Second Member of Shanxi Formation is complicated,but still can be considered as in the stage A of metarmosis.The main diagenesis processes that control the reservoir properties include compaction,cementation(silicide,carbonates,clay minerals),dissolution and fracturing.The types of pores are primary intergrain pore(residual),intercrystalline micro-pore and intergrain or intragrain solution pore(including moldic pore).The reservoir generally belongs to the type of low-porosity and low-permeability.The microfractures in formation greatly improve the reservoir properties and contribute to higher gas yield in P₁x³ and P₁x² than in P₁x¹ and other reservoirs.