Subsurface Logging Methods 



405 



which will be illustrated by schematic drawings (figs. 180, 181, 182, 183) . 

 Each category will be given a reference letter, namely, Ix, h, h (Imperv- 

 ious beds), Pj, Pg (Permeable beds). These letters will be placed on the 

 field examples exhibited (figs. 184 through 189) at the levels of each sec- 

 tion of bore hole which can be considered as belonging to the correspond- 

 ing category. 



i 



Apparent 

 Resistivities 



MicroLog 



R^ Specific resistivity ot the 

 ground at distance "x° 



~ R j~ Uncontominated zone 



ini 



Resistivity through 

 invaded zone 



Mud cake 



I xl 



6 1 2" 



Distances from pad 



Figure 182. Permeable bed (oil-bearing) invaded by mud filtrate (category P2). 



Rt < Rx„ 



Highly Resistive Impervious Formations (Category Ii) 



In impervious formations of high resistivity — i.e. whose resistivity is, 

 for example, more than 50 times that of the mud — all the microresistivities 

 are high. Because the wall of the drilled hole is generally somewhat 

 rugose, the rubber pad cannot fit perfectly against it, and a sort of mud 

 film may remain between the two. For that reason, and al^o because of 

 the limited dimension of the rubber pad, the apparent resistivity recorded 

 on the micrologs can be substantially lower than the true resistivity for 

 that type of formation. Wlaen the formation resistivity is very high, the 

 rugosity of the wall, which causes a mud film to remain under the pad, 



