420 



Subsurface Geologic Methods 



active materials in one form or another are universally distributed. 

 Measurable quantities are found in all kinds of igneous, metamorphic, 

 and sedimentary rocks. From laboratory measurements and the experi- 

 ence gained in logging more than 15,000 wells, certain conclusions can 

 be drawn concerning the relative intensity of radioactivity in different 

 kinds of sedimentary rocks. Figure 190 is a chart that demonstrates the 

 relative radioactivity values of various formations encountered in well 

 logging. Anhydrite, salt, and coal are very low in radioactivity, while 



Figure 190. Drawing indicating radioactive values of various formations encountered 



in well logging. 



shale, bentonite, ash, and organic shale have the highest values of radio- 

 activity encountered. Producing formations such as sand, limestone, and 

 dolomite are relatively low in radioactivity. 



The neutron curve might well be referred to as a "fluid content or 

 hydrogen curve," since hydrogen is the controlling factor on the action 

 behavior of the curve in well logging. Neutron well logging is the 

 process of bombarding the strata with a strong source of fast-moving 

 neutrons and recording the secondary gamma rays that have been excited 

 by the neutron bombardment. Where hydrogen is present in the strata, 

 the neutrons are slowed down or stopped, and this in turn gives a low 

 value on the curve. Where there is no, or very little, hydrogen present, 

 the response is quite high in value and is shown as a throw to the right. 



