opposite very thin beds. The microcaliper is extremely helpful in general 

 since a decrease in hole diameter is likely to correspond to the presence of a 

 mud cake, and, hence, of a permeable bed. 



Hole Enlargements 



Caving is most often encountered at the level of shale beds but may 

 occur also opposite other types of formations. In this instance the pad 

 generally does not apply to the wall. When the caving is deep, the two 

 readings are equal to the mud resistivity. The fact that the MicroLog curves 

 read a resistivity that is equal, or close, to the mud resistivity is usually a 

 conspicuous indication of the presence of a cave. In many instances, however, 

 the interpretation is not so easy. If the cave is not too deep, the readings 

 may still be affected by the formations; and, since the mud is usually less 

 resistive than the formations, a positive separation is observed; and again a 

 confusion is possible with permeable streaks. As in the preceding example, 

 the ambiguity can be solved with the help of the SP curve and the microcaliper. 



Mud Log 



When the MicroLog pad is not extended, the curves are greatly influenced 

 by the mud. If parts of the hole are large enough, the microinverse will read 

 a value which equals the resistivity of the mud at that level in the hole. Often 

 this value is more accurate than that obtained from a surface sample of mud. 



Use of MicroLog in Quantitative Analysis 



In water-bearing formations the flushed zone is practically saturated with 

 mud nitrate. If the formation is clean, the formation factor can be taken 

 equal to K xo /K mf , R x0 being the resistivity of the flushed zone and R m/ the 

 resistivity of the mud nitrate. Porosity is deduced from the formation factor. 



The presence of the residual oil saturation in the flushed zone affects the 

 measurements, and a corresponding correction is necessary. To this end, 

 the proportion of residual oil has to be surmised. Taking a value of 20 percent 

 for the residual oil saturation would not usually entail too big errors, at 

 least in formations with granular porosity and in light oils. Greater residual 

 oil saturations are, however, frequent, chiefly in highly viscous oil. Gas- 

 bearing formations also seem to display high residual saturations in the 

 flushed zone because of the segregation effect if good permeability exists. 

 Because of these residual saturations, the MicroLog may show water contacts. 



The value of R^/ can be measured directly if samples of filtrate are 

 available. If this cannot be done, R m/ can be estimated from the value of R m , 

 the mud resistivity, according to average statistical data obtained in the lab- 

 oratory. The evaluation of the formation factor and, hence, of the porosity 

 finally depends on the determination of R^. 



297 



