Keywords: 
• Ordos Basin /
• Late Paleozoic Period /
• volcanic activity /
• volcanic material /
• sandstone reservoir

Abstract: Upper Paleozoic natural gas reservoir of Ordos Basin consists of lowpermeability sandstone which bears typical secondary porosity. After studying secondary porosity forming cause, it is turned out that there is some direct relation between formation of all kinds of dissolution porosity and dissolution and erosion of volcanic material. Based on the study of petrology, the observation of volcanic material composition, construction and structure, the state and secondary change, we can reach the conclusion that these volcanic materials has the characteristics of simultaneous sedimentation. Combing the formal achievements about simultaneous volcano eruptiverock on basin edge, there is causeeffect relation, which showing volcanic material of the sandstone comes from the simultaneous sedimentary volcanic eruption, which confirms that important geological events in Basinorogen coupling period have great influence on interbasin sedimentation. Not only the simultaneous volcanic material changes the composition of the sandstone, but also offers the dissolution matter for the formation of secondary porosity in these sandstones. The dissolution of volcanic material, with largely unsteady physical and chemical characteristics, led to the formation of dissolution porosity in crystalfragment, in rockfragment, and in ashfragment, as well as eroded kaolinite and chlorite intercrystal porosity in diagenetic period. Meanwhile, remnant, erosion of volcanic material and dissolution differences lead to the heterogeneous reservoir.