Abstract: Tahe oilfield is a carbonate oilfield with the largest reserves in China, characterized by large buried depth of karst reservoir and complicated reserving space. Through geological investigations on present and ancient karsts, this study identifies the paleogeomorphic, karst zonings and fracture-cave system in Ordovician karst sequence of the North Tarim uplift. Through an analysis on the geneses of fracture-caves in different karst zonings, it is proposed that the surface karst zoning consists of surface rivers, dolines and other fracture-caves; the fracture-caves in porous flow karst zones are composed of water storage cave and porous flow well, and those in karst runoff zones include hall-like cave, mainstream cave, tributary cave and peripheral cave. The filling characteristics of karst fracture-caves and the formation of fracture-cave units are clarified though analyzing the surface river and subsurface river sedimentation in the karst stage as well as the collapse, travertine and the roles of hydrothermal chemical fillings in the burial stage. The research results indicate that the fracture-cave system is consisted of carbonate rocks dissolved by the running water from the dolines at karst highland or on a karst slope to the outfall of subsurface river at karst depression; if the fracture-cave system partially undergoes subterranean river sedimentation, collapse and chemical fillings, one or more fracture-cave units can be formed. Based on analyzing the fracture-cave structure and filling characteristics, it is believed that the formation of fracture-cave units is controlled by the paleo-geomorphology and phreatic fluctuation in the karst stage as well as the fault activity in the burial stage. In this study, seismic, logging and geological data are comprehensively applied in creating the methods for identification of fracture-cave and filling types, so as to unify the terms describing the fracture-caves with different geneses, thus providing a detailed basis for the establishment of geological model for oilfield development.