The Upper Paleozoic in the Ordos Basin is mainly made up of a terrigenous clastic sedimentary system. The fluvialdelta facies sandstone reservoirs are widespread and they constitute the main places for the Upper Paleozoic gas accumulation in this area.Owing to the terrigenous matrix filling, diagenetic compaction and cementation, however, most of the primary intergranular pores of the sandstones had been filled up. Therefore, most of the sandstones become too tight to make up effective reservoirs. Through the genetic analyses of the pores of the Upper Paleozoic sandstone reservoirs in the Ordos Basin, it can be suggested that the porous sandstones which are concentrated  in parts of the layers, are dominated by the secondary dissolution pores. Further studies show that the secondary dissolution pores were mainly caused by the genetic solution of the pyroclastic components, which had derived from the synsedimentary volcanism and were distributed in the sandstone formations. Therefore, the synsedimentary volcanism controlls the development and distribution of the main Upper Paleozoic sandstone reservoirs of this area. Owing to the easilydissolved component  supplied by the intermediate to basic volcanism, it controlls the distribution of porous sandstone reservoirs with higher permeability, while the acid volcanism mainly forms the low permeable sandstone reservoirs, which are dominated by the kaolinite intercrystal pores.