1102 



EXPLORATION GEOPHYSICS 



proper choice of logging speed, the detector remains for a sufficient period in the 

 vicinity of each stratum to provide an accurately reproducible record of its radio- 

 activity. In areas where strong contrasts exist between various strata which it is 

 desired to differentiate, the logging speed may be made relatively rapid : in other areas 

 a slow speed may be required to obtain the accuracy necessary to differentiate between 

 strata whose radioactive properties are nearly alike. The time constant, or overall 

 speed of response of the system, is adjusted by the operator to suit the local conditions, 

 and the hoisting speed is then controlled accordingly. 



Gamma-Ray Curve Interpretation 



The radioactivity intensity ranges of the formations commonly associated with 

 oil-bearing structures are shown in Table 27 and Figure 688. The relative ranges 

 shown were determined by combining the results of measurements obtained in hun- 

 dreds of laboratory tests of the radioactivity of many rock samples. The relative 

 range of radioactivity intensity of each formation is indicated by the position and 

 length of the block in Figure 688. 



TABLE 27 



Average radio- 

 activity in 



Lithologic Type No. of radium equiva- 



Samples lents per gram 



Black and grayish-black shale 40 26.1 



Dark to black shales, neither calcareous nor sandy. ... 74 22.4 



Shales, including sandy shales 164 16.2 



Marls and limy shales, grayish-black and black 3 16.5 



Sand and shale 9 13.5 



Dark to black shales, not calcareous, but sandy 16 13.2 



Medium to light gray shales, neither sandy nor 



calcareous 17 11.3 



Siltstone 11 10.3 



Medium to light-gray shales, not calcareous, but sandy 18 9.0 



Marls and limy shales, dark 10 8.8 



Sandstones, silty but not shaly 26 7.3 



Shaly sandstones 40 7.0 



Marls and limy shales of light shades 16 6.8 



All sandstones, including shaly sandstones 131 5.3 



All sandstones, excluding shaly sandstones, but includ- 

 ing silty types 105 4.0 



Sandstones free from silt and shale 76 4.1 



Shale-free limestones and dolomites 64 4.1 



Microcrystalline to earthy limestones and dolomites of 



medium to light shade 28 4.0 



Medium to light shade, shale-free limestone 33 ^ 3.8 



Finely to coarsely crystalline limestones and dolomites 



of medium to light shade 24 3.1 



Medium to light shade, shale-free dolomite 21 3.1 



Effect of shade in shale-free limestone and dolomite: 



A. Light gray to white 30 3.1 



B. Medium shade 22 4.1 



C. Dark to black 10 6.1 



Estimated original permeability of sandstone before cementation : 



Very high 35 2.9 



High 37 5.1 



Low 40 6.6 



Very low 24 7.5 



(From "The Total Gamma-Ray Activity of Sedimentary Rocks as Indicated by 

 Geiger Counter Determinations," W. L. Russell, Geophysics, Vol. IX, No. 2, April, 

 1944.) 



