412 



Subsurface Geologic Methods 



respective values of the different microresistivities that would be obtained 

 in that case and the formation resistivity Rf. The apparent resistivities 

 measured on the different microresistivity logs are slightly lower than the 

 true resistivity Rt of the formation because of the limited dimensions of 

 the pad. 



When the resistivity of the impervious bed is not much different 



2.4 ot 86»F 



MicroLog 



3 4"x34" LATERAL 

 5 10 



Figure 189. Example of microlog in shaly sandstone. 



from that of the mud, there is practically no departure between the micro- 

 resistivity curves (Category I3), When, on the contrary, the resistivity 

 of the impervious bed is appreciably higher than that of the mud, there 

 is generally, at least with the pads presently used, a substantial negative 

 departure which is characteristic of an impervious bed (Category I2) . 



