The log must run in holes containing electrically conductive mud or water, i.e., with enough ions present in the drilling fluid.
However, in most common cases (oil and gas drilling, water-well drilling), the solid mineral phases do not contribute to the electrical conductivity: electricity is carried by ions in solution in the pore water or in the water filling the cracks of hard rocks.
In contrast to aqueous solutions containing conducting ions, hydrocarbon fluids are almost infinitely resistive because they do not contain electrical charge carriers.
In geological exploration and water-well drilling, resistivity measurements also allows to distinguish the contrast between clay aquitard and sandy aquifer because of their difference in porosity, pore water conductivity and of the cations (Na+, K+, Ca2+ and Mg2+) present in the interlayer space of clay minerals whose external electrical double layer is also much more developed than that of quartz.
If oil based mud is used and water is displaced, deeper logs will show higher conductivity than the invaded zone.