366 Subsurface Geologic Methods 



tion (2) of the drilling fluid into a permeable zone. The principle is a 

 recognized phenomenon of electrochemistry (streaming potential) and, if 

 effective in a well, may be expressed as: 



E=—^ (2) 



where £=electromotive force or spontaneous potential in millivolts. 

 /?2=resistivity of drilling fluid. 

 F=pressure differential (atmospheres) between drilling fluid and 



formation. 

 F=viscosity of filtering fluid. 



M=complex factor dependent upon the nature of the permeable 

 zone, the filtrate, and the filter (mud cake) . 



There may be other factors effective in generating bore-hole potentials, 

 but, at present, the phenomena just described appear to be those of major 

 importance. 



Whatever their origin may be (electrochemical or electrokinetic) , 

 the electromotive forces give rise to a current, which flows through the 

 permeable layers, then spreads into the adjacent impervious formations, 

 and returns through the mud filling the hole. The SP anomalies cor- 

 respond to the drop of potential created by the circulation of the current 

 in the hole, and thus measure only a part of the total electromotive forces. 

 Consequently, the characteristics of the SP log, and particularly the ampli- 

 tude of the anomalies, are a function of several factors, such as the salini- 

 ty of the mud and of the formation fluid, the resistivity of the surrounding 

 formations, bed thickness, hole diameter, amount of shaly material in the 

 permeable bed, and depth of mud invasion. 



Fresh-Water-Bearing Formations 



The SP developed by a fresh-water-bearing formation is usually very 

 small (fig. 153), frequently nonexistent (fig. 154), and sometimes reversed 

 (fig. 155) , as compared with the SP across a salt-water-bearing formation. 

 As most drilling fluids are comparatively fresh and as the electrochemical 

 effect has been recognized as being generally preponderant, it is obvious 

 from the formula (1) describing the action of the electrochemical cell 

 that, when the resistivity of the drilling fluid is appreciably higher than 

 that of the formation water, the SP is high and negative; when the two 

 are the same, the SP is zero; and when the drilling-fluid resistivity is 

 lower than the formation-water resistivity, the SP is positive. Therefore, 

 in the case of fresh-water sands and a fresh mud, SP's usually are small. 



Salt-Water-Bearing Formations 



The SP developed by a salt-water-bearing formation is generally 

 sharp, has an appreciable magnitude up to 100 or 200 millivolts, and 

 is negative with respect to the surrounding shale or nonpermeable forma- 



