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Clay has a significant influence on the relationship between resistivity index I and water saturation Sw (i.e, I-Sw relationship) of reservoir rocks because it complicates the current paths of these rocks.It is difficult to reveal the physical mechanisms of these clay effects on the conductivities of various rocks by physical laboratory measurements because the pore structure, micro distribution and content of clay inside a rock can not be observed and controlled during the experiments.We present a digital rock approach to study these clay effects on the electrical transport properties of reservoir rocks at pore scale using lattice gas automation (LGA) method.The digital rock samples are constructed with the information of grain size distribution from SEM images of reservoir rocks.The LGA is then applied on these digital rocks fully saturated with fluids to simulate the electrical transport properties for revealing the effects of volume and distribution patterns of clay on the non-Archie behaviors of the I-Sw relationship.The very good agreement between the simulated results and the laboratory measurements clearly demonstrates the validity of the LGA in numerical research of rock physics.Based on these studies, a new model has been developed for quantitatively describing the relationship between the saturation exponent and the volume of clay (V).This development may improve the evaluation for the fluid saturations in reservoir rocks.