Ordovician carbonate fissuring/cavernous reservoirs are the important oil pay beds in Tahe Oilfield. Based on the K-Ar isotopic dating for illite from sandstone and mudrock filling in the karstic caverns of the reservoirs, the formation time of cave-fillings is during 369.33-451.90 Ma and thus that of the paleocaverns should be earlier than orequal to the 369.33-451.90Ma, which is corresponding to the late Caledonian-early Hercynian periods. There are three vertical cavern zones in Tahe area. Paleocavern development was controlled by paleo-landform. Karstic water input in different ways which resulted in the divisional distribution of caverns in plane in this area. In northern area, the karst water behaved asa planar way of input into the Ordovician Yingshan and Yijianfang strata while in the southern area, the water did asa lateral way of input into the same strata that are covered by impermeable upper Ordovician rocks. Exit of karstic water was located near the fault zone, The karstic water basically belongs toa unified system in the northern and the southern parts of the oilfield. A local model of meteoric karst reservoir is established.