Subsurface Logging Methods 



411 



sistive as the mud, and that there is, between the pad and the formation, a 

 uniform mud film 1/64 inch thick. The order of magnitude of the different 

 microresistivities is about the same, but there can be a slight departure 

 either positive or negative, depending on the shape of the pad, the rugosit) 

 of the wall, the ratio of formation resistivity to mud resistivity, etc. 



Impervious Beds of Low Resistivity (Categories I2 and I3) 



Figure 181 represents an impervious bed whose resistivity has been 



R_=I8 at 80' 



MicroLog 



I'xl" LATERAL 



Figure 188. Example of microlog in shaly sandstone. 



assumed to be five times the resistivity Rm, of the mud. The resistivity 

 diagram on the right of the figure illustrates the fact that the resistivity 

 is uniform and does not vary with the distance from the pad, as would 

 occur for a permeable bed. This diagram also shows the resistivity scale 

 in relation to the resistivity Rm of the mud. 



The rectangles at the left of figure 181 represent, at the same scale, the 



