resistive because they do not contain electrical charge carriers. Indeed, hydrocarbons does not dissociate in ions because of the covalent nature of their chemical bonds. When a formation is porous and contains salty water, the overall resistivity will be low. When the formation contains hydrocarbon, or has a very low porosity, its resistivity will be high. High resistivity values may indicate a hydrocarbon bearing formation. 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 (, , and ) present in the interlayer space of clay minerals whose external electrical double layer is also much more developed than that of quartz. Usually while drilling, drilling fluids invade the formation, changes in the resistivity are measured by the tool in the invaded zone. For this reason, several resistivity tools with different investigation lengths are used to measure the formation resistivity. If water based mud is used and oil is displaced, "deeper" resistivity logs (or those of the "intact zone" sufficiently away from the borehole disturbed zone) will show lower conductivity than the invaded zone. If oil based mud is used and water is displaced, deeper logs will show higher conductivity than the invaded zone. This provides not only an indication of the fluids present, but also, at least qualitatively, whether the formation is permeable or not. See also Electric logs (in: Formation evaluation) References Further reading Apparao, A. (1997). Developments in geoelectrical methods. Taylor & Francis. Well logging