can, therefore, give an accurate and detailed definition of the boundaries for 

 very thin beds. 



Tight Formations 



Since these formations usually do not cave much, the pad is seated against 

 them; and very high readings are obtained. 



Shales 



When there is no caving, the pad is applied to the formations. Because 

 of the focusing system, a thin mud film between the pad and the wall has no 

 influence on the measurement which, therefore, is equal to the resistivity of 

 the shale. 



In the MicroLog, caving may give similar features on the curves as porous 

 beds. The discrimination of these features is again possible with the help of 

 the SP and the microcaliper curves. 



Porous and Permeable Formations 



The MicroLaterolog is essentially a tool for the determination of R xo . 

 It seems superfluous to elaborate here on the significance and the use of this 

 factor in the quantitative analysis of the electric logs for the evaluation of 

 porosity and saturation. 



Mud-Cake Thickness Less Than % Inch 



The resistivities recorded with the MicroLaterolog give directly the values 

 of K x0 without need for correction. 



Thin mud cakes are observed in wells drilled with high salinity muds. 

 The MicroLaterolog, accordingly, is one of the components of the so-called 

 salt-mud survey technique, which is used more and more extensively in hard- 

 rock territories where high-salinity muds are used. 



Mud-Cake Thickness Greater Than 3 /s Inch 



The effect of the mud cake is no longer negligible, and the resistivities 

 recorded with the present type A are generally lower than R x0 . Interpretation 

 charts have been established in the laboratory, which make possible the deter- 

 mination of R x0 when the mud cake thickness is known. 



The equipment for the simultaneous recording of the MicroLaterolog and 

 the MicroLog is still under development. Pending its introduction, the value 

 of the mud-cake thickness as given by a MicroLog run in the same well can be 

 used to correct the single curve MicroLaterolog, at least to a first approximation, 

 by using the interpretation charts mentioned above. 



It is sometimes possible to use the indications of the microcaliper for the 

 determination of mud-cake thickness. But such a determination should be made 



315 



