A re-appraisal of Waxman/Smits data for the electrical behaviour of shaly sands demonstrates that surface conductance (measured as Qv) is not the only, or even most important, phenomenon that occurs in shaly sands. Thus Qv measurements, either in the laboratory or from new logging tools cannot by themselves fully describe the electrical behaviour of shaly sands. A new model is derived which takes into account the change in porosity exponent with shaliness observed on Waxman/Smits data. This model demonstrates the response obtained from either aconductive or resistive addition of the sand and shale components, showing the influence of both circuit types on the data. The model provides an improved fit to Waxman/Smits data and can explain the non-linear resistivity index/water saturation data observed on microporous minerals. The model demonstrates the importance of the porosity exponent on the conductive behaviour of shaly sands and shows why neither the Waxman/Smits nor Dual Water models of the formation can predict the non-linear saturation exponents observed on microporous minerals.

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