OVERVIEW

Sedimentary formations are capable of transmitting an electric current only by means of the interstitial and adsorbed water they contain. They would be non-conductive if they were entirely dry. The interstitial or connate water containing dissolved salts constitutes an electrolyte capable of conducting current, as these salts dissociate into positively charged cations, such as Na ⁺ and Ca ²⁺, and negatively charged anions, suchas Cl ^? and SO ^? ₄. These ions move under the influence of anelectrical field and carry an electrical current through the solution. The greater the salt concentration, the greater the conductivity of connate water. Freshwater, for example, has only a small amount of dissolved salts and is, therefore, a poor conductor of an electric current. Oil and gas are non-conductors.

The electrical resistivity (reciprocal of conductivity) of a fluid-saturated rock is its ability to impede the flow of electric current through that rock. Dry rocks exhibit infinite resistivity. In electrical logging practice, resistivity is expressed in ohm-m ²/m or simply ohm-m. The resistivity of most sedimentary formation ranges from 0.2 to 2000 ohm-m. The resistivity of poorly consolidated sand ranges from 0.20 ohm-m for sands containing primarily saltwater, to several ohm-m for oil-bearing sands. For well-consolidated sandstones, the resistivity ranges from 1 to 1,000 ohm-m or more depending on the amount of shale inter-bedding. In non-porous carbonate rocks, resistivity may be as high as a few million ohm-m. The resistivity of reservoir rocks is a function of salinity of...