Subsurface Logging Methods 



375 



as they change the resistivity of the mud. On the other hand, two muds 

 used occasionally, silicate and oil-base, affect the SP in a manner unre- 

 lated to resistivity. 



A positive SP generally is recorded in a silicate mud opposite a 

 permeable sand, although the mud resistivity is higher than the resistivity 



Figure 160. Example of a log made in sodium-silicate mud showing reversal of 



SP curve. 



of the formation water (fig. 160) . The anomalies are small but sharp 

 and are easily seen when the proper SP scale is used. 



SP logs in true oil-base muds, those with a water content of less than 

 five percent and a resistivity of several thousand ohms, usually are not 

 satisfactory for distinguishing between permeable and nonpermeable for- 

 mations or for locating formation boundaries. On the other hand, normal 

 SP logs are obtained in oil-emulsion muds. Such muds are a mixture of 



