An empirical equation has been developed to correct resistivity data from core plugs and logs. A combination of rock conductivity dependent on water salinity, conductivity as a result of the interaction between cations in brine and shale, and conductivity of the rock itself may occur in reservoir sediments. The commonly used Waxman and Smits equation incorporates a rock conductivity which is result of the cation exchange between the brine and the rock, i. e. the clay portion of the rock matrix. A new model has been established by introducing the possibility of the rock itself to conduct electrical current, combined with the Waxman and Smits model. For reservoir rocks with high matrix conductivity, it is shown that the conventional Waxman and Smits correction indicates abnormally high water saturation in oil zones. The introduction of this new concept has proved its usefulness in a case study. The study presents resistivity data of core plugs from porous plate drainage including multiple salinity measurements (C_OC_W). The saturation n-exponents have been corrected to values applicable for log interpretation. Secondary log evaluation, using the new method for resistivity correction, show a increase in net-pay for the reservoir.
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