Subsurface Logging Methods 413 



Permeable beds (Categories Pi and P2) 



At the level of a permeable formation, the rubber pad slides over 

 the mud cake against which it is applied, and the mud cake itself is separ- 

 ated from the uncontaminated part of the formation by the invaded zone, 

 wherein the original fluid has progressively been replaced by mud filtrate. 



Two cases must here be considered, depending on whether the invaded 

 zone is less or more resistive than the uncontaminated zone. 



The first case, which is the simplest as far as interpretation of the 

 micrologs is concerned, is represented in figure 182, which is similar to 

 figure 181 already discussed, except that now the resistivity varies in the 

 formation with the distance from the pad. Here again, the abscissae on 

 the diagram represent the depths from the pad, while the ordinates show 

 the resistivity of the ground at the corresponding depths. 



The resistivity Rmc of the mud cake has been assumed to be twice 

 the resistivity R^ of the mud. The resistivity Rx^ immediately behind the 

 mud cake, where the permeable bed should be practically saturated by 

 mud filtrate, has been taken equal to 10 times Rm', this would correspond 

 to a value of 10 for the formation factor, if the saturation by mud filtrate 

 is complete, and if the pores are reasonably free of conductive solids. ^^ 



On the left of figure 182 are represented, at the same scale, the ap- 

 proximate values of the microresistivities that would be obtained in that 

 particular case from the microlog. As can be seen, the departure is positive 

 and quite substantial. This result is general when Rt is larger than R^g. 

 A large positive departure between microcurves is characteristic of a 

 permeable bed, provided, however, that the resistivity measured by the 

 Ixl-in. lateral, or by the 1-in. normal, be lower than approximately 30 

 times that of the mud (Category P2) . 



The interpretation is less definite for beds, such as salt-water-bearing 

 beds, where the resistivity of the invaded zone is larger than that of the 

 uncontaminated zone. This is particularly true when the mud is of the 

 low water-loss type, with the consequence that the mud cake is thin and 

 the formation is invaded by the mud filtrate to only a short distance from 

 the wall. 



When the mud cake has an appreciable thickness, and when, simul- 

 taneously, the depth of invasion is large enough, the microresistivities 

 measured with the different electrode combinations show good positive 

 departures (Category P2). This effect is represented on figure 183a. For 

 a smaller penetration of the mud filtrate into the permeable bed, the 

 departure would disappear (Category Pi), as represented in figure 183b. 

 For still less invasion, the departure might even be slightly negative, as 

 represented in figure 183c, but this negative departure never exceeds 20 

 percent (Category Pi) . 



On the left-hand side of figures 183a, b, c are represented the ap- 



" Panode, H. W., and Wyllie, M. R. J., The Presence of Conductive Solid in Reservoir Rocks as a 

 Factor in Electric Log Interpretation: AIEE Meeting, San Antonio, Oct. 5-7, 194£>. 



