BORE-HOLE INVESTIGATIONS 



1053 



SELF 

 POTENTIAL 



RESISTIVITY 

 Short Normol 10" 



+ 



Limetton* 32" 



sands containing a great amount of colloidal material, whether stratified or not, give 

 rise to low resistivities, whatever the fluid content may be. 



In all these difficult cases, the use of instruments for the recovery of samples from 

 the interesting layers is particularly helpful, as will be explained later. 



Hard Formations. — In hard formations the interesting zones, from the standpoint 

 of the oil production, are those which show some porosity and permeability. These 

 zones generally contain highly 

 mineralized connate waters, and, 

 even if oil-bearing, they are more 

 conductive than the surrounding, 

 more compact rocks. But porous 

 and impervious layers (shales) 

 may also exist inside these forma- 

 tions, and are also conductive. The 

 problem is therefore: (1), to de- 

 termine the location of the con- 

 ductive zones in highly resistive 

 formations, and (2), to find out 

 which of these zones are perme- 

 able and which are impervious. 



Conventional resistivity logs 

 generally give a rather confusing 

 representation of the boundaries 

 of the layers of different resistivi- 

 ties in hard formations. The 

 special device called the limestone 

 sonde is better adapted for this 

 purpose, and the combination of 

 the limestone device and the S.P. 

 log may provide, in favorable 

 cases, a fair solution to the prob- 

 lem. Figure 649 is an example of 

 a log recorded opposite a thick 

 limestone bed containing several 

 conductive streaks. The limestone 

 sonde indicates these by a series 

 of well-marked depressions. Op- 

 posite depressions 1, 3, and 6, the 

 S.P. log shows slight inflections, 

 with a convexity towards the 

 negative side, which is represen- 

 tative of permeable zones. In front 

 of depressions 2, 4, and 5, the S.P. 

 log shows inflections, with con- 

 vexity towards the positive side, 

 characteristic of impervious beds. 

 But it is clear that the indications 

 of the logs, and particularly of the 

 S.P. logs, are far from being as 

 definite as in the case of soft for- 

 mations ; the approximate location 

 of the interesting beds is barely 

 suggested, and sometimes, as in 

 the case of 7, the S.P. curve shows 

 no response at all. No indication 



Fig. 649. — S.P. and resistivity curves opposite a lime- 

 stone formation containing thin conductive streaks. 



