An analytical approach previously proposed to model the electrical properties of shaly sands has been adapted to obtain the cementation exponent (m), the water saturation (S w ), and the shaliness distribution from electromagnetic and porosity log measurements in an oil well. Shaliness is described by the clay volume fraction (p) and by the clay type parameter (beta 1 /a) in the sand. For a shale-coating structure these parameters can be related to the exchange cation density (Q v ). Based on an equivalent flow regime inside this granular model, a Kozeny-Carman type equation has been derived, expressing the intrinsic permeability (k) of the medium in terms of a porosity-tortuosity factor phi (super (m-0.5)) and of the parameter Q v . The power-law derived expression shows that k decreases with the amount of clay, not only because a high Q v implies a narrowing of the pore channels, but also because it modifies the hydraulic tortuosity of the medium. This new equation has been statistically tested with extensive petrophysical laboratory data for different types of sandstones and satisfactorily applied to core and log data of an oil well from Potiguar Basin, Rio Grande do Norte, Brazil.