414 Subsurface Geologic Methods 



proximate resistivities that would be measured on the microlog in the 

 different cases, and for the different electrodes combinations with the 

 assumption that Rto is about five times as high as the resistivity of the 

 mud cake, which again corresponds to a formation factor of about 10. 

 With them are also represented the formation resistivities. When the de- 

 partures between the microresistivities are small, nil, or slightly reversed 

 (within 20 percent), the interpretation can be aided by the fact that these 

 microresistivities are larger than the formation resistivity, contrary to 

 what would normally happen for an impervious bed of low resistivity, as 

 discussed in connection with figure 181 above. It is simpler, however, to 

 refer to the SP log to resolve the ambiguity in this case. 



In all the cases represented in figures 180, 181, 182, and 183, the 

 microresistivity corresponding to the Ixl-in. lateral could be computed 

 with reasonable accuracy. The microresistivities corresponding to the 1 

 and 2-in. normals are only approximate, because the effect of the limited 

 side of the pad cannot be accounted for accurately with these devices. 



It is interesting to notice that the microresistivity for the Ixl-in. 

 lateral is the same for the different cases represented on figures 180, 181, 

 182, and 183, which cases differ only by the value of true resistivity Rt 

 and the depth of invasion by the mud filtrate. This illustrates the fact that 

 the microresistivities, when measured with an electrode combination having 

 a very small depth of investigation, are essentially responsive to the 

 resistivity Rxg of the invaded zone and to the thickness and resistivity of 

 the mud cake. 



Summarized Rules of Interpretation 



From the above discussion, it is possible to derive a certain number 

 of simple rules of interpretation for the microresistivity logs of forma- 

 tions, whereby two electrode combinations of different depths of investi- 

 gation have been run simultaneously. These rules, which will apply in 

 the great majority of cases, are schematically represented on chart 1. 



Case I — The two microresistivities are higher than Rumt, that is, 

 higher than about 20 or 30 times the mud resistivity. The formation then 

 under study is a compact one, and should be interpreted as impervious, 

 regardless of the departure (Category li). 



Case II — ^The microresistivity determined by the shorter spacing, 

 generally Ixl-in. lateral, is smaller than the limit Rum- ^^ that case, the 

 sign and magnitude of departure should be examined, and, in case of 

 doubt, the ambiguity resolved by reference to the SP log. This gives the 

 following interpretation categories: 



(a) Large negative departure (more than 20 percent) — the forma- 

 tion is impervious (Category I2) . 



(b) The departure is not definite enough (less than 20 percent) — 

 the microlog alone cannot determine, in general, whether the formation 

 is permeable or not in this case, except that, when the resistivities of all 



