Based on well drilling and logging, as well as core observation, the reservoir rocks in the study area are mainly cryptite in open platform facies, followed by granular limestone. Combined with well logging interpretation and statistical data of fractures and cavity of core samples， it is concluded that the main reservoir space types are dissolved cavity and low angle dissolved fracture that is semi-filled and unfilled. The thick Upper Ordovician un-karstic strata made the Ordovician reservoir obstructed from the karst alteration. The paleokarst reservoir developed mainly below Caledonian parallel unconformity, and the reservoir quality is much poorer compared with the reservoirs in the main region of Tahe Oilfield where Upper Ordovician is absent. It is well developed when closer to the host of Tahe oilfield. The paleokarst reservoir developed at the top of the Ordovician Yijianfang Formation and the reservoir development decreased with the increase of burial depth. The reservoir development is controlled by sedimentary facies, Caledonian karstification, fault and fracture. It is concluded that Ordovician Yijianfang Formation may be the main reservoir space.