Subsurface Logging Methods 417 



A first exception, which is obvious, corresponds to cavings whose 

 diameters are larger than the maximum expansion of the spring system. 

 If, in such a case, the pad remains a few inches from the wall, the two 

 microresistivities will be equal to the resistivity R,n of the mud, irrespec- 

 tive of v/hat the formation is. If, on the contrary, the pad happens to be 

 near to the wall, the Ixl-in. lateral will measure approximately the resis- 

 tivity of the mud, whereas the 2-in. normal will already be substantially 

 affected by the formation resistivity. Inasmuch as the formation resis- 

 tivity is usually higher than that of the mud, even for a conductive shale, 

 there will be a positive departure between the micrologs which could be 

 falsely interpreted as indicating a permeable bed. These two cases can 

 generally be detected by observing the abnormally low value given by 

 the short spacing (Ixl-in. lateral) for which the microresistivity is fre- 

 quently close to Rm in that case, and they would not fail to be recognized 

 if a section-gauge log were run, as is highly recommended when large 

 cavings might exist. In those cases, the microlog indications should be 

 disregarded, and the interpretation based on the conventional log and, 

 in particular, on the SP log, as far as the determination of the permeable 

 beds is concerned. 



The interpretation rules could also be at fault if the mud cake, in- 

 stead of being built on the wall, and within the hole, were built within 

 the pores of the permeable bed. This case has not been encountered yet, 

 but it seems that it could occur in coarse-grain sands. If that did happen, 

 the part of the permeable bed where the pores are filled with the mud 

 deposit would be a little more resistive than the invaded zone behind, with 

 the result that the Ixl-in. lateral would measure an apparent resistivity 

 slightly higher than that measured by the 2-in. normal. This would result 

 in a negative departure, and the bed might, therefore, on the basis of the 

 microlog, be falsely classified as impervious. Here again, the SP log 

 would generally settle the question. 



Other exceptions, which have not yet been observed, may exist and 

 will reveal themselves when more examples are acquired. For that reason, 

 it will always be useful to confront the indications given by the microlog 

 with those obtained from the SP log, and also from the conventional 

 resistivity logs, supplemented by the section-gauge log when large cavings 

 are to be expected. 



Field Examples 



Figure 184 shows an example of a microlog recorded in a sequence of 

 shales and limestone. In this instance, most of the compact beds give 

 rise to microresistivities which are definitely higher than 30 7?,„, so that the 

 discrimination between permeable and impermeable formations is par- 

 ticularly easy. The permeability record, provided by core analysis, was 

 available in this hole and is reproduced in the figure as a check on the 

 indications of the electrical logs. 



