Problem 

 Identification of possible 

 producing zones 

 (sandstones and limestones) 



Identification of shales 



Identification of liquid- 

 bearing (porous) zones within 

 known producing formations 



Identification of potash 



beds, uranium or other highly 



radioactive zones 



Primary Curve 



ramma-ray 



Combination 



Neutron 



Identification of suitable 



horizons for correlation 



Identification of gas-filled 

 porous zones 



Typical Response 

 Low 



Gamma-ray — high 

 Neutron — minimum 



Medii 



to 1c 



Gamma-ray High or exception- 



ally high. 



Contrasting with 

 Either surrounding strata 



Gamma-ray — low 

 Combination Neutron — extremely 



high 

 By following these rules and referring to the typical responses shown in 

 Figure 15-3, or other locally established typical responses, one may use the 

 radioactivity log to identify a wide variety of formations. 



CORRELATION Radioactivity logs respond similarly from 



well to well and are, therefore, useful in 

 establishing subsurface maps and cross sections from which the depth and 

 thickness of various formations can be traced across a field, or, in many 

 instances, for hundreds of miles. By virtue of the considerable similarity of 

 appearance between radioactivity logs and electric logs, correlations between 

 these two types of logs are ordinarily established with ease. Similarly, the cor- 

 relation between radioactivity logs and core logs is usually quite apparent. 

 Thus the radioactivity log can be applied to detect and resolve depth discrepancies 

 which may be present in the other data. 



DETERMINING Questions regarding the condition or com- 



FORMATION position of the rocks; their porosity; their 



CHARACTERISTICS content of oil, water, and gas; and their shale 



or silt content cannot ordinarily be answered 

 directly from the radioactivity log alone. However, since the gamma-ray and 

 neutron curve responses frequently remain consistent over wide areas, it is 



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