Subsurface Logging Methods 425 



Table 22 is a laboratory analysis of several hundred rock samples 

 showing the average radioactivity in radium equivalents per gram of 

 rock. This table is taken from "The Total Gamma Ray Activity of Sedi- 

 mentary Rocks as Indicated by Geiger Counter Determinations" by Rus- 

 sell,^^ who has pointed out that an increase in radioactivity is directly 

 proportional to an increase in shale or silt in the strata. Furthermore, 

 the shade or color of the rock has a relationship to the amount of radio- 



Figure 194. Showing method of determining true thickness of sandstone or limestone 

 using midpoint of transition for top and bottom of zone. 



activity. The darker the shade or color, the higher the radioactivity, ex- 

 cept for rocks stained by oil or asphalt. One notable exception is coal, 

 which is very low in radioactivity. For general interpretation purposes, 

 we can conclude that formations encountered during a radioactivity survey 

 will be of the value shown in table 22.^^ Two things must be kept in mind, 

 however. The first is that a knowledge of the local stratigraphy is im- 

 perative for the correct interpretation because sandstone, limestone, and 

 dolomite have so nearly the same value that they cannot be accurately 

 differentiated by gamma-ray evaluation alone. The second is that in some 

 areas of complex geology some sandstone and/or limestone have been 

 encountered that are as high in radioactivity material as the usual shale. 

 This phenomenon has occurred only in some Gulf Coast and California 

 areas; none has been logged in Midcontinent fields. 



-^ Russell, W. L., The Total Gamma Ray Activity of Sedimentary Rocks as Indicated by Geiger Counter 

 Determinations: Geophysics, vol. 9, no. 2, Apr. 1944. 

 ^ Russell, W. L., op. cit. 



