Abstract: The Ordovician oil reservoir in Tahe Oilfield,the northern Tarim Basin is so far the largest marine carbonate reservoir in China.Its main output layer is the karstic Lower-Middle Ordovician limestone.Based on the study of the Ordovician karstic reservoir rock,oil distribution,oil charge epoch,and considering the related study of structure and trap,it is concluded that the oil reservoir has been thrice charged,its evolution process can be summarized as "late Caledonian-early Hercynian oil accumulation and alteration-late Hercynian oil filling and adjustment-late Himalayan oil filling and readjustment".The large-scale palaeo-reservoir accumulated and altered in the late Caledonian-early Hercynian period is the basis of nowadays Ordovician heavy oil reservoir in the Tahe Oilfield.As a result of multiple oil charge and adjustment,the distribution of the Tahe Ordovician oils is complex enough.