424 



Subsurface Geologic Methods 



conditions. The fact that these factors do vary widely from well to well 

 and from field to field requires that theory be supplemented by wide 

 experience and empirical knowledge to derive the most reliable log inter- 

 pretation. The interpretation data that follow represent a summary of 

 extensive rJadioactivity-well-logging experience. They are based on the 



Figure 193. Interior of an instrument truck. 



behavior of gamma-ray and neutron curves under the geologic and bore- 

 hole conditions most commonly observed. An important part of the 

 correct interpretation of any log is the accurate determination of the tops 

 and bottoms of formations. This procedure requires particular attention 

 for radioactivity well logs because of the characteristically sloping transi- 

 tions, which represent formation contacts. 



Figure 194, which is a portion of a gamma-ray curve, represents the 

 correct point for determining the top and bottom of a zone. In all cases, 

 regardless of the magnitude of the break, the midpoint or center of a mini- 

 mum-maximum intensity value on the curve will most reliably indicate 

 the actual formation contact. Shown previously in figure 190 is the 

 relative radioactivity range of the rocks encountered in oil-well logging. 

 A working knowledge of the local stratigraphy is necessary for the cor- 

 rect geologic interpretation of the gamma-ray curve. 



