Abstract：The fissure-pore carbonate reservoirs constitute the dominant Ordovician reservoirs in No. 4 block of the Tahe Oilfield. The fissures make up the main connection channels and accumulating space. Based on core and thin section observations for more than 20 wells in combination with FMI and analysis of a variety of test data, this paper studied a certain characteristic parameters of reservoir fissures, such as the ranges, strikes, lengths and bulk densities of the fissures, and analyzed their filling situation. On such a basis，the controlling factors were analyzed, which included structure, lithology, former faulting, rock structure and the stages for the formation of the fissures. The fissures are assigned to four stage: I–Pre-Caledonian period, II–Caledonian period, III–Early Hercynian period, IV–Late Hercynian–Himalayan period. The genetic mechanism of the fissures are also discussed.