Objectives: This paper takes as target the carbonate reservoir in the Kh2 Member of Upper Cretaceous Khasib Formation in X oilfield, Iraq, studying the characteristics of the pore structure and their effect on oil movability. Methods: The methods of cast thin section, scanning electron microscope, high pressure mercury injection, constant- rate mercury injection, digital core and others are used to characterize qualitatively and quantitatively the micro- pore throats in Kh2 interval. Wettability test, low field NMR and two- phase relative permeability test are applied to evaluate oil movability. Results: A matching of the characteristics of pore structure and the movability of crude oil yields the following results. ①The principal pay zone in Kh2 interval is primarily composed of calcarenite and bioclastic—algalclastic micrite limestone. The pore distribution in calcarenite is characterized by strong heterogeneity, dominant channels, desirable permeability and "water channeling" during water flooding. The pore distribution in bioclastic—algalclastic micrite limestone is relatively uniform, but the connectivity between pores is poor, and the injected water is difficult to spread. This is an important micro geological factor that restricts oil recovery in this the oilfield at present. ②The wettability of rock samples shows that most of the samples from Kh2 interval are lipophilic or strongly lipophilic, resulting in capillary force to be displacement resistance, making water flood development difficult. ③Oil—water and oil—gas relative permeability tests of calcarenite and bioclastic—algalclastic micrite limestone reveal that residual oil saturation is high in the case of water flooding, whereas it can be significantly reduced in the case of nitrogen flooding. Conclusions:Therefore, for carbonate reservoirs characteristic of good permeability, strong heterogeneous pore structure and locally dominant channels, and those characteristic of relatively homogeneous pore structure and poor connectivity between pores, water flood development should be used with caution, for gas injection development is a better choice.