ADVANCES IN OIL PROSPECTING 99 
effect on a concentrated solution, a pressure corresponding with a 
depth of burial of nearly 1o kilometers would bring about a change 
in resistivity of only 1o per cent. 
More details on the effect of water on rock resistivities may be 
obtained from K. Sundberg’s papers (ref. list I9,10). 
II. DETERMINATION OF RESISTIVITIES 
The experimental determination of the resistivity of rocks or for- 
mations is by no means a very easy matter. This is chiefly due to the 
RESISTANCE RATIO R/R, 
8 

500 000 1500 2000 2500 3000 3500 2000 
PRESSURE IN BARS 
Fic. 2.—Relation between pressure and resistivity of electrolytes (after L. H. 
Adams and R. E. Hall). 
fact that a sample can not well be kept in the same condition after 
being removed from its location as it has been in before it was dis- 
turbed, chiefly in reference to its mechanical constitution and its 
moisture content. Therefore, experiments to test the resistivity of 
rocks are usually made, whenever possible, on outcrops, or by means 
of special equipment, in wells (Schlumberger method). If there is no 
possibility of avoiding the testing of samples, the tests should be 
made in a portable apparatus immediately after the sample has been 
taken, or else, the resistivity of a formation may finally be computed 
from the pore volume of the constituent rock and the conductivity 
of the impregnating water (Sundberg method). 
429 
