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

K. S. Mendelson, Morrel H. Cohen

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Abstract

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 wphim, 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 = aj(phi)sigma w phim, where alpha j is the jth principal value of the conductivity and aj(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 aj 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

Original languageEnglish (US)
Pages (from-to)257-263
Number of pages7
JournalGeophysics
Volume47
Issue number2
DOIs
StatePublished - Jan 1 1982
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology

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