432 



Subsurface Geologic Methods 



is lowered to a point where it is impossible to get a good electric log. 

 The high resistivity of fresh-water muds presents a similar problem in 

 attempts to log fresh-water sands electrically. Oil-base mud is used in 

 some areas to eliminate the infiltration where the productive zones are 

 partly depleted or where bottom-hole pressures are low. Since oil is 



GAMMA-RAY CURVE 



RADIOACTIVITY INCREASES 



NEUTRON CURVE 



RADIOACTIVITY 

 INCREASES ^ 



Figure 200. Characteristics of gamma and neutron curves in Cushing-Drumright field, 

 Oklalioma. This well is 20 years old. (Lane-Wells.) 



a dielectric substance, it shows a high electric resistance, which makes 

 it difficult to obtain satisfactory electric logs. Electric logs have been 

 run in oil-base mud by using electrodes that make a sliding contact to the 

 walls of the well. However, washed-out places and irregularities in the 

 well bore tend to defeat the purpose of the contact electrode and cause a 

 curve to be produced that is sometimes difficult to interpret. Since the 



