Subsurface Logging Methods 



427 



than to show the presence or absence of hydrogen. However, the neutron 

 curve used in combination with the gamma-ray curve can be interpreted 

 to locate possible porous zones in producing strata very accurately. 



In figure 195 a portion of a combination radioactivity log is shown 

 with the gamma-ray curve to the left, indicating a productive limestone 

 of considerable extent. The tops and the bottoms of the formations have 



RADIOACTIVITY LOG 



GAMMA-RAY CURVE 

 RADIOACTIVITY INCREASES 



NEUTRON CURVE 

 RADIOACTIVITY INCREASES 



INDICATES POSSIBLE 

 POROUS ZONES. 



Figure 195. Combination radioactivity log indicating productive limestone of con- 

 siderable extent. 



been determined as previously explained by selecting the midpoint of 

 the transition. The neutron curve shown on the right is also interpreted 

 by selecting midpoints on the transitions. "Throws" to the right are con- 

 sidered barren or impervious strata, while "throws" to the left indicating 

 the presence of hydrogen are considered to have fluid in place and are 

 therefore interpreted as possible porous zones. 



Applications 



The radioactivity well log was originally developed with the thought 

 that some direct relationship might exist between natural radioactivity 

 and the presence of petroleum. Experience to date indicates the absence 

 of any such relationship. However, the indirect applications of the radio- 

 activity well log to the location of petroleum reserves are numerous and 

 varied. 



The first advantage of the radioactivity well log over any other type 



