Abstract: The analysis of the pressure mercury data indicates that most of the parameters for the microporous structures are similar to those of the moderate-to high-permeable reservoir beds in the Yushulin Oil Field, Heilongjiang. The relative sorting coefficients and structural coefficients tend to be increased in response to the decrease of the permeability while the structural parameters and average pore throat radii generally decrease in response to the decrease of the permeability. The capillary pressure curves also show that the driving pressures are inversely proportional to pore throat radii. The low porosity and permeability of the reservoir beds result in the nonlinear flows of the oil and water in the pore medium. Furthermore, this kind of nonlinear flows becomes conspicuous when the pore throat radii and permeability decline. The coexistence of oil and water in the nonlinear flows lead to the higher bound water saturation, residual oil saturation and co-vadose point saturation. The smaller extend of the vadose areas and lower permeability of the water below residual oil tend to cause the lower oil-driving efficiency.