372 



Subsurface Geologic Methods 



3. Barite and limestone. Resistivity tends to increase slightly with 

 the addition of these common weighting materials.^^ 



4. Cement and sodium bicarbonate. The addition of either or both 

 of these materials tends to reduce resistivity. 



5. Sodium pyrophosphate. Resistivity decreases but not uniformly as 

 it does with increasing temperature or sodium chloride salinity. 



Figure 157. Log showing less SP (self potential) across an oil-bearing formation as 

 compared to SP of a salt-water bearing formation. Note also decrease in re- 

 sistivity opposite water-bearing interval. 



'^ Sherborne, J. E., and Newton, F. M., Factors Influencing Electrical Resistivity of Drilling Fluids: 

 Am. Inst. Min. Met. Eng. Tech. Pub. 1466, Mar. 1942. 



