Abstract: The Middle-Lower Ordovician reservoir is considered the main developing interval in the 12th district of the Tahe Oil Field. Multi-phase orogeneses and karstifications are the main cause for the highly matured dissolved fissures and caves within the Middle-Lower Ordovician, which may also result in complex reservoir heterogeneity. This paper comprehensively utilizes data of seismic survey, coring, thin section, logging and laboratory test to clarify the fissure-cave reservoir characteristics within the Middle-Lower Ordovician in the 12th district of the Tahe Oil Field, and further discusses its dominating factor. The results indicate that, three reservoir types exist in the study area:cave type, fissure-cave type and fissure type. Primary pores take up only a tiny proportion in the accumulating spaces. Kart caves type occupies the majority, with the secondary fissure-cave type ranking behind. Dissolved fissure may undertake a favorable connecting conduit. Moreover, the reservoir dominating by fissure-fracture system, developing dissimilarly along the fissure-fracture system, may be controlled by multiple developing model along the paleochannel. Those findings may imply that underground-river conduit and monadnock existing in the lateral side of surface river are supreme zones for reservoir developing, while the dissolved fracture surface follows.