Based on the analysis of core observation, rock thin sections, logging data and a variety of testing data of the Middle Ordovician Yijianfang Formation on the northern slope of middle Tarim Basin, reservoirs are mainly distributed in grain limestone of intra-platform shoals. The main reservoir types in this area are fractured reservoirs, fractured-vuggy reservoirs and pore-cave reservoirs. The controlling factors of reservoir genesis are sedimentary microfacies, faults and dissolution. The carbonate shoals provide the early material basis for the later selective dissolution. Meteoric-water dissolution and burial dissolution improved the properties of reservoir to some extent. The multi-period activities of the NE-trend faults played a key role in controlling the high quality reservoir.