416 Subsurface Geologic Methods 



formations are known to be much higher than that of the mud, the fact 

 that the microresistivities are comparatively low gives a good probability 

 that this is due to mud infiltration in a permeable bed. The ambiguity, if 

 any, can be resolved by noting the trend of the SP curve. If that trend 

 is positive, i.e. if the convexity is toward the positive side, or if the SP 

 is on the positive limit (shale line), then the bed must be interpreted as 

 impervious and the microlog gives its exact boundaries (Category I3). If, 

 on the contrary, the trend of the SP log is negative, the bed is permeable, 

 and again the microlog gives the accurate boundaries (Category Pj). 



(c) Large positive departure (more than 20 percent) — the forma- 

 tion must be permeable (Category Po). 



Remarks 



(1) It is indeed wise to check the trend of the SP log in any case; 

 unless this log is completely flat, it should always be possible to determine 

 the trend of the SP. 



When the mud is saturated with salt, the SP curve is completely 

 flat. In that case, however, there is generally little doubt about the in- 

 terpretation of the microlog, because the permeable beds, which are in- 

 vaded by the saturated mud filtrate, are the only ones to give microresis- 

 tivities lower than the limit Rum, which itself is lower than the true 

 resistivity of all impervious beds. The interpretation can then be based 

 on the observation of the lows in the microlog, while a good positive 

 departure, if it exists, brings a useful confirmation. 



(2) The interpretation is also facilitated by the consideration of the 

 conventional resistivity log, insofar as this log makes it possible to evalu- 

 ate the true formation resistivity Rf. 



When the value of the microresistivity Rmicro is less than Rum, it 

 can reasonably be assumed that a bed cannot be impervious unless Rt is 

 comprised between values respectively equal to about Rmicro and 2 Rmicro- 

 When Rt is between the two values, the SP log will generally give a 

 definite anomaly that will make the interpretation safe. When Rt is not 

 between the above limits, there is a strong probability that the bed is 

 permeable. 



(3) In the particular case of a highly resistive permeable bed, 

 with a comparatively conductive invaded zone, and interbedded with 

 shales and highly resistive compact formations, the SP log may show 

 no appreciable deflection with respect to the shale line. But, in that case,, 

 a good correct departure should be obtained from the microlog. 



Exceptions to the rules 



There are few cases which do not fall within the simple set of rules 

 discussed in the previous section. Fortunately, these cases are rare, and 

 the ambiguity can generally be resolved if the remarks made in the prev- 

 ious section are taken into account. 



