SP. 



CONVENTIONAL RESISTIVITY LOG 



ohms-m 



INDUCTION LOG 



MICROLOG 



Figure 14-15. Conventional log, induction log and microlog in sand-shale series (Gulf Coast). 



The reservoirs in this type of formation are very often dirty sands, and 

 the contrast between the resistivities of water-bearing sands and oil-gas-bearing 

 sands is not very great; good producers can be obtained when true resistivities 

 are low. It is, therefore, important for a correct quantitative analysis that the 

 true resistivities can be determined with great accuracy. The above emphasizes 

 that when the beds are thin the necessary accuracy can be obtained with the 

 induction log only. 



Figure 14-16 shows the logs recorded opposite sands embedded with com- 

 paratively tight formations and shales (Lower Cretaceous-Mississippian). The 

 main difference with the preceding example is that most of the sands are more 

 consolidated and that the proportion of tight material is appreciably greater; 

 this fact is shown clearly by the comparison between the MicroLogs. 



Section B, for example, is a 7-foot sand located between appreciably 

 more resistive formations. The long normal accordingly reads 3.7 against 1.7 

 for the induction log. The lateral shows a steady decrease from the top to the 

 bottom of the bed, as is usual below a thick resistive formation, until a minimum 



304 



