Subsurface Logging Methods 



429 



of well-logging method is its ability to log through steel casing. This 

 permits the stratigraphic study of old wells drilled prior to the develop- 

 ment of geophysical well-logging methods now in common use. The loca- 

 tion of upper cased-off potential producing zones has presented a logical 

 field for this type of logging, particularly where no information was 

 available or where the available information was doubtful. 



Variations of this primary application are apparent. The location 



6200 



RUN He. ) UFORE ! 



Figure 197. Location of camotite (radioactive) cement with gamma-ray curve. 



(Lane-WeUs.) 



of the top of the producing zone for bottom-water shutoff, the correction 

 of some of the earlier drillers' logs, the location of upper potential fresh- 

 water sands for salt-water disposal, the supplying of additional infor- 

 mation where cores were not completely recovered or were lost, and the 

 location of the top and bottom of an oil-producing zone for gas-oil-ratio 

 control are all applications of this type. 



Many logs are run for measurement checks of various kinds. Drill- 

 pipe measure, casing measure, and electric-log measure often disagree. 

 On many of the older wells the original zero point has been lost. On 

 other wells, both old and new, a new bottom has been established. Many 

 of the earlier sample logs did not take into consideration the time lag 

 between the point of origin of the sample and the depth of the well at 



