Subsurface Logging Methods 



373 



6. Quebracho. The eflfect on resistivity of quantities generally used 

 is negligible. 



7. Starch (i.e. "Impermex") . The resistivity is unaffected except as 

 the preservatives may change the resistivity. 



8. Caustic soda. The resistivity is decreased with the addition of 

 caustic soda. 



Figure 158. Comparison of two logs in the same well, one with a low resistive (salty) 

 mud, the other with a normal mud. 



Effect of Bed Thickness 



Research has indicated that, all other factors remaining the same, 

 when the bed is less than four times the hole diameter in thickness, the 

 SP will decrease with decreasing bed thickness. 



A section composed of interbedded thin shales and permeable sands 

 may have much less SP than the sand would have if the thin shale beds 

 were not present. This is an important factor in evaluating thin sand zones 

 and explains the good production obtained sometimes from sandy zones 

 with low SP.12 



Effect of Special Muds 



The normal constituents of drilling muds, natural clay, "aquagel," 

 "baroid," starch, and other additives have no effect on the SP log except 



•Doll, H. C, op. cit. 



