370 Subsurface Geologic Methods 



tions (fig. 156). Such a behavior is entirely in agreement with results to 

 be expected from a consideration of the formulae as described above when 

 the drilling fluid has a higher resistivity (lower salinity) than the forma- 

 tion water. The presence of oil in the shaly sand will often lower the SP 

 (fig. 157). 



Effect of Porosity and Permeability 



As noted beforehand, there is no quantitative relationship between 

 SP and porosity or permeability. However, in a particular section, marked 

 variations in the magnitude of the spontaneous potential generally are 

 associated with changes in the physical properties of the formation, 

 although such changes can be identified only in a qualitative manner. 

 For example, in a sand with only 50 millivolts, SP in a section where 

 the sands average, say, 100 millivolts, the lower SP may be the resultant 

 of a bed-thickness effect, of shaliness, or of an increased resistivity. 

 Shaliness probably would mean less permeability, whereas increased re- 

 sistivity probably would indicate less porosity or less water saturation. -^^ 



Effect of Drilling Mud 



It has been seen that the SP is directly affected by the resistivity and, 

 therefore, by the salinity of the drilling fluid. Considering a salt-water- 

 bearing formation, the SP will decrease with decreasing resistivity or in- 

 creasing salinity of the drilling mud. Not only is there a decrease in the 

 magnitude of the SP, but the anomalies lose definition and the log becomes 

 featureless. Figure 158 is a typical comparison of two logs in the same 

 well, one with a low-resistivity (salty) mud, the other with a normal mud. 



Mud-resistivity measurements are given on the log headings as well as 

 the temperatures at which the measurements were made. Since drilling- 

 fluid resistivities vary inversely with temperature and since well-bore tem- 

 peratures usually are different from surface temperatures, corrections have 

 to be made for mud resistivities measured at the surface in order to 

 evaluate their effect in the hole. For example, a mud with a resistivity of 

 2.0 ohms at 64° F. will have a resistivity of only 0.70 ohms at 200° F. 



Figure 159 shows the variation of resistivity of a sodium-chloride 

 (salt) solution with temperature and with changes in salinity expressed in 

 parts per million. 



Factors Influencing Resistivity of Drilling Fluids 



The factors that influence the resistivity of drilling fluids are the fol- 

 lowing: 



1. Temperature. Resistivity decreases with increasing temperature 

 (fig. 159). 



2. Sodium-chloride salinity. Resistivity decreases with increasing 

 sodium-chloride salinity (fig. 159) . 



"Doll, H. G., The S. P. Log: Theoretical Analysis and Principles of Interpretation: Am. Inst. Min. 

 Met. Eng. Petroleum Technology, Sept. 1948. 



