2009 Volume 62 Issue 4 Pages 385-396
Archie's equation is often used for the interpretation of resistivity structures obtained from electrical and electromagnetic methods. The equation assumes that the contribution of electric conduction of the electric double layer, which is called surface conduction, is negligible. Therefore, the resistivity of clay-bearing rocks becomes generally lower than the resistivity expected from the Archie's equation. Some empirical formulas which modified the Archie's equation based on the equivalent parallel circuit model were proposed by experimental works.
In this research, the empirical formulas were reviewed to consider the influence of clay minerals on resistivity. The effectiveness of the empirical formulas was examined using the experimental resistivity measurements of clay-bearing artificial samples. The influence of clay minerals on resistivity is greatly dependent on the type of clay minerals and the salinity of pore-water. Moreover, some experimental works were reviewed to investigate the temperature dependency of the resistivity of pore-water and ionic mobility within the electric double layer. I also carried out the laboratory experiment in which the temperature dependency of resistivity of clay-bearing artificial samples was measured. The results indicate that the temperature dependency of the resistivity of clay minerals is larger than that of pore-water. When applying the empirical formulas to clay-bearing rocks, it is necessary to compare the conductivity of pore-water with the excess conductivity of electric double layer generated by the clay minerals, and to take the effectiveness and the limitation of the formulas into consideration.
