The effect of grain anisotropy on the electrical properties of sedimentary rocks.

We have examined the effect of particulate anisotropy on the electrical properties of sedimentary rocks by generalizing the treatment of Sen et al (1981) to the case of ellipsoidal grains with a distribution of orientations and depolarizing factors. Two distributions in orientation have been treated in detail-randomly oriented grains in three dimensions and grains with aligned principal axes in two dimensions. In the former case the conductivity is a scalar satisfying Archie's law, sigma = sigma _wphi^m, with sigma _w the conductivity of the pore fluid and phi the porosity. The exponent m has a minimum of 1.5 for spherical grains. The presence of highly oblate (disk shaped) grains raises m significantly. As long as grains with extremely large eccentricities (= or >15) are not present, the exponent falls in the observed range 1.2= or <m= or <4. For aligned grains the conductivity is a tensor with principal values that satisfy a generalized Archie's law of the form sigma _j = a_j(phi)sigma _w phi^m, where alpha _j is the jth principal value of the conductivity and a_j(phi) can be expanded as a power series in phi with a constant leading term. For grain eccentricities in the range 0-0.95, the coefficients a_j fall in the range 0.1-4. The exponent m has a minimum value of 2 for two dimensions, independent of grain shape, if all grains have the same shape, and it is larger for any distribution of grain shapes.-Authors