376 Subsurface Geologic Methods 



water-base and oil-base mud, and the resistivity generally has a value of 

 from two to ten ohms m"/m. 



Effect of Variation in Hydrostatic Pressure 



It has been demonstrated many times that a change in the hydrostatic 

 pressure exerted by the mud column on a permeable formation will change 

 the magnitude of the SP. An increased pressure increases the SP while 

 a lowered pressure lowers the SP. Since the change generally occurs op- 

 posite only the permeable zones, these can sometimes be located and quali- 

 tatively compared by measuring the SP at different hydrostatic pressures. 

 Such a log is called an "SPD," spontaneous-potential-differential, log. 



Resistivity 



Rock formations, except for example, massive sulphide ore bodies 

 and graphitic beds, are capable of transmitting an electric current only by 

 means of the absorbed water which they contain. They would be non- 

 conductive if they were entirely dry. The absorbed water containing dis^ 

 solved salts constitutes an electrolyte able to conduct the current. The more 

 electrolyte contained in a formation and the richer this electrolyte in dis- 

 solved salts, the greater the conductivity and therefore the less the resis- 

 tivity of the formation. Fresh water, for example, has only a small amount 

 of dissolved salts and is, therefore, a poor conductor of an electric cur- 

 rent; salt water with a large amount of dissolved salt is a good conductor. 



Electric-logging practice is to measure, not the conductivity, but its 

 reciprocal, the electrical resistivity. This is the resistance of a volume of 

 rock having a unit of length and a unit of cross section. The resistivity of 

 rocks is expressed in ohm meter squared per meter (ohms m'-/m) or 

 ohmmeters. This has been found to be a convenient unit for practical pur- 

 poses, giving values between a fraction of an ohm and several thousand 

 ohms. 



The resistivity measured in a drill hole and recorded on the electric 

 log is called "apparent resistivity." This will vary from the "true forma- 

 tion resistivity" as a function of bed thickness, electrode spacing, diameter 

 of the bore hole, resistivity of the drilling mud, and, in the case of perme- 

 able formations, the nature of the invaded zone.^^ It is not feasible with 

 present measuring procedures to eliminate these effects; however, the in- 

 fluence of bed thickness on apparent resistivity is negligible if the forma- 

 tion is many times as thick as the AM spacing of the "normal" curve or 

 several times as thick as the AO spacing of the '"lateral." The other factors 

 noted above can be taken into account and true resistivity determined from 

 the apparent resistivity by the use of resistivity-departure curves.^^ 



The electric-log resistivity or apparent resistivity is satisfactory for 

 all problems except the determination of the fluid content of permeable 



*' Doll, H. C, Legrand, J. C, and Stratton, E. F., True Resistivity Determination from the Electric 

 Log — Its Application to Log Analysis: Am. Petroleum Inst. Drilling and Production Practice, p. 215, 1947. 

 ^* Resistivity Departure Curves, Schlumberger Well Surveying Corporation, Sept. 1947. 



