Taking Tahe oilfield as an example, the Ordovician carbonates experienced three periods of strong surface karst at the middle Caledonian period in the west and north of Tahe oilfield, early Hercynian in the main block, and later Hercynian in the northwest part, respectively. Since the extensive magmatic-volcanic activities at the later Hercynian period made the deep hydrothermal active, the hydrothermal alteration dissolved the carbonates remarkably. The hydrothermal-dissolved pores stacked on the previously existing reservoir space in the west and southwest of the Tahe oilfield. Therefore, the high quality of carbonate reservoir was formed rather by one single fluid than the composition of multiple dissolutions in a long-time dynamic process.