Large differences in water flooding efficiency of patterns in different blocks of the Beibu Gulf oil field in China, were taken as an example of water flow problems leading to reduced recovery factor. Cores of formation W 3 IV in Well W and formation L 3 III in Well B were taken for study. By combining dynamic nuclear magnetic resonance, constantrate mercury injection and a visual micro plate model, the flow characteristics and factors influencing the pore scale flow of water were analyzed for these reservoirs. The pore and throat radii were small, but the pore throat ratio is large and its distribution range was wide. So, for example the pore volume of Well B samples was mainly controlled by smaller throats. It is easy for a dominant injection water flow path to form under such conditions and this adversely affects the volumetric sweep efficiency of the water flooding. The mechanism of this is explained. It results in oil recovery in medium and small pores of up to about 40%, while that in large pores is less than 5%. As a consequence the average oil displacement efficiency of Well B was only 44.7%, which is 22.3% lower than that of Well W.