Subsurface Logging Methods 



433 



radioactivity well log is a measurement of radioactive emanations, it is 

 not affected by salt-water mud, fresh-water mud, oil-base mud, or other 

 contaminated fluids. 



Radioactivity well logs are easily correlated with other types of 

 geophysical well logs and such information obtained from well bores 

 as sample logs and core analysis. Radioactivity logs and the types of 

 electric surveys in common use today are easily correlated. Although 

 there is no relationship between the two types of surveys and they are 



Well A 



V/ell B 



Well C 



Gamma Ray 



Gamma Ray 



Figure 201. Correlation of radioactivity logs made in cased wells along a two-mile 

 strike section, Long Beach field, California. (Lane-WeUs.) 



often surveyed under different well conditions or years apart, there still 

 exists a similarity in response that permits accurate correlation. This is 

 most fortunate, for it is simple logic to comprehend that a multiconductor 

 cable such as is employed in electric logging used under varying fluid 

 and mud conditions cannot be expected to measure with complete accuracy. 

 Figure 203 illustrates the typical response of radioactivity and elec- 

 tric logs to the usual formations encountered in oil-well drilling. These 

 generalized curves are shown to illustrate the most typical response of 

 radioactivity and electric logs to the various types of formations. These 

 typical curves should not be used as a criterion for analysis of any par- 

 ticular log since in practice a wide range of response may exist. In making 

 correlations between radioactivity and electric logs, the gamma-ray curve 

 is correlated with the self-potential curve, while the neutron curve is 

 compared with the shallow-resistivity curve. 



