1108 



EXPLORATION GEOPHYSICS 



rience to date indicates the absence of any such relationship. The logs have proved to 

 be a very useful tool, however, and have been indirectly responsible for locating large 

 petroleum reserves. 



Prior to the development of radioactivity vi^ell-logging, it was necessary to obtain 



GAMMA RAY CURVE 



RADIOACTIVITY 



INCREASES 



ELECTRICAL LOO 

 SELF POTENTIAL RESISTIVITY 



NEUTRON CURVE 



RADIOACTIVITY 



INCREASES 



^^ZZZZZira^ 



NORTHWESTERN KANSAS 



Fig. 692. 

 (Courtesy of Lane-Wells Company.) 



all desired well information before casing was set. For this reason, completion practices 

 have included various types of logging and sample-taking in open holes to assure com- 

 plete information for subsequent operations. 



Determining the Tops and Bottoms of Formations 



An important part of the correct interpretation of any log is the accurate determi- 

 nation of the tops and bottoms of formations. This procedure requires particular 

 attention for radioactivity well logs because of the characteristically sloping transitions 



