BORE-HOLE INVESTIGATIONS 



1037 



Study of True Resistivities 



The Resistivity Departure Curves. — It has already been pointed out that the 

 apparent resistivity is dependent on the resistivities of all the different media in the 

 vicinity of the electrodes and may thus differ considerably from the true resistivity of 

 the formation being logged. 



The evaluation of the fluid content of permeable beds from electrical log data 

 requires an accurate knowledge of the true resistivity of the beds; hence it becomes 



e + L 



Fig. 642. — Resistivity log opposite a thick, highly resistive layer, 

 using electrode arrangement of Figure 641. 



important to have a means of detennining the true resistivity of a formation. A method 

 of arriving at the desired values, at least in favorable cases, is furnished by the resistiv- 

 ity departure curves.* These curves can also be used to evaluate the effect on the 

 apparent resistivity of various factors, such as, for instance, the size of the hole or 

 the extent of invasion. 



Potential Measurements 



The S.P. log, or spontaneous potential log, is a record of the naturally- 

 occurring potentials measured in the mud at different depths in a drill hole. 

 Usually, the S.P. log indicates an increase in potential by excursions or 



* The first issue of the Resistivity Departure Curves was published by the Schlumberger Well 

 Surveying Corporation in 1947. A second issue, which includes a number a.i additional curves and 

 which also contains departure curves for beds of finite thickness, is being published in 1949. 

 Detailed instructions for the use of the departure curves are given in these publications. 



H. G. Doll, J. C. Legrand, E. F. Stratton, "True Resistivity Determination from the Electric 

 Log — Its Application to Log Analysis" (A.P.L meeting, Los Angeles, 1947). 



