Ch. 31] ELECTRIC LOGS 563 



they appear as "kicks" on the curve. In general, sandstones are nega- 

 tive with respect to shale. 



The causes of the potential differences are still somewhat obscure, 

 but they appear to represent the algebraic sum of several types of 

 electrical potential due to the physicochemical behavior of the drilling 

 fluid and the connate fluids. They probably are the result of the com- 

 bined effects of: (1) the potential that is developed at the interface 

 between a concentrated salt solution (the connate water in the rock) 

 and a dilute solution (the drilling fluid) ; (2) the potential that results 

 from "cataphoresis" when an electrolyte flows through a porous me- 

 dium as a result of a difference in pressure between the drilling mud 

 and that of the fluids in the formation; and (3) a potential difference 

 that develops across the interfaces when clay and sand, both saturated 

 with salt water, come into contact with fresh water. The relative im- 

 portance of the three effects is not known, and the physical chemistry 

 of the third source is not fully understood. In general, subsurface 

 sands have a low (high negative) potential with respect to shale, al- 

 though fresh-water sands, generally near the surface, have a positive 

 potential. The potential of limestones may be very low, intermediate, 

 or high. 



There is a partial relationship between the magnitude of the cata- 

 phoresis effect, or flowing potential, and the permeability. However, 

 the principal factor that affects the potential curve is the amount of 

 clay in the rock; the less the clay, the more negative the potential. 

 The amount of clay also affects the permeability; thus the cataphoresis 

 effect, amount of clay, and permeability are all interrelated factors. 



The resistivity log (or logs) is plotted on the right-hand side of the 

 strip with the amount of resistivity increasing to the right, as shown 

 in Fig. 2. In actual field practice a resistivity log is obtained by 

 lowering a system of electrodes into a well by means of an insulated 

 multiconductor cable. A voltage source (alternating current or com- 

 mutated direct current) is applied to one electrode at the surface. This 

 current passes down one of the conductor cables through another cur- 

 rent electrode traveling in the hole. A portion of the electrical field 

 established around the current electrode in the hole is picked up by 

 two other traveling electrodes spaced at successively higher intervals 

 up the hole. The supporting conductor cables, by which the two upper 

 electrodes are suspended, are connected at the surface to a potential 

 recorder (voltmeter) between these two electrodes. The difference in 

 potential between them is a measure of the apparent resistivity of the 

 rock formation in the neighborhood of the electrodes. Water in the 

 drilling mud nearly always invades the permeable sands, and, as this 



