ADVANCES IN OIL PROSPECTING 105 
d. DETERMINATIQN OF RESISTIVITIES FROM WATER ANALYSES 
AND POROSITY DETERMINATIONS 
If it is not possible to determine underground resistivities by the 
Schlumberger method, that is, if there is casing in the well at the 
depth to be investigated, or if the wells are dry, sufficiently accurate 
estimates of resistivities of formations may often be obtained by a 
method suggested by Lundberg (reference list No. I,10), provided 
water analyses are available for the formations under investigation, 
which is urten the case. If the water analyses can, in addition, be 
supplemented by a determination of the pore volume of the forma- 

Fic. 6.—Schlumberger’s method of measuring resistivities in wells (after C. and 
M. Schlumberger). 
tion from an investigation of drill cores, for instance, a fairly accurate 
determination of the resistivity of the formation can be made. 
If the porosity of a sample is known, the resistance factor P may 
be found from the diagram in Figure 1 a, if it is assumed that all the 
pores are filled with water; this is ordinarily true, as already stated, 
below the ground-water level. If the chemical composition of the 
waters at a certain depth is known, and if it is assumed, with suf- 
ficient accuracy, that only sodium chloride is in solution, the diagrams 
in Figure 1 6 may be used to determine the resistivity of the water; 
to convert the results into actual resistivities at a certain depth, the 
temperature correction illustrated in Figure 1 ¢ may be applied. 
The resistivity factor P is of the order of 50 to 100 for dense lime- 
435 
