426 



Subsurface Geologic Methods 



Neutron-curve interpretation is a matter entirely different from 

 gamma-ray interpretation. It has been stated previously that the response 

 of the instrument is controlled mostly by hydrogen. Response of the 

 instrument opposite hydrogen is very low. It matters not in what form 

 the hydrogen occurs, whether in gas, oil, water, or shale, the curve is 

 characterized by a low reading on the graph or to the left of the chart. 

 For this reason a neutron curve used alone cannot be interpreted further 



TABLE 22 

 The Relationships of Various Rock Types to Radioactive Values 



Lithologic type 



Average 

 radioactivity 



in radium 

 equivalents 



per gram 

 XlO-12 



1. 



2. 

 3. 

 4. 

 5. 

 6. 

 7. 



8. 



9. 

 10. 

 11. 

 12. 

 13. 

 14. 

 15. 



16. 

 17. 

 18. 



19. 

 20. 



21. 

 22. 



23. 



Black and grayish-black shale 



Dark to black shales, neither calcareous nor sandy 



Shales including sandy shales 



Marls and limy shales, grayish-black and black 



Sand and shale 



Dark to black shales, not calcareous, but sandy 



Medium- to light-gray shales, neither sandy nor 

 calcareous 



Siltstone 



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



Marls and limy shales, dark 



Sandstones, silty but not shaly 



Shaly sandstones 



Marls and limy shales of light shades 



All sandstones, including shaly sandstones 



All sandstones, excluding shaly sandstones, but in- 

 cluding silty types 



Sandstones free from silt and shale 



Shale-free limestones and dolomites 



Microcrystalline to earthy limestones and dolomites 

 of medium to light shade 



Medium- to light-shade, shale-free limestone 



Finely to coarsely crystalline limestones and dolo- 

 mites of medium to light shade 



Medium to light shade, shale-free dolomite 



Effect of shade in shale-free limestone and dolomite: 



A. Light gray to white 



B. Medium shade 



C. Dark to black 



Estimated original permeability of sandstone before 



cementation: 



Very high 



High 



Low 



Very low 



40 



74 



164 



3 



9 



16 



17 

 11 

 18 

 10 

 26 

 40 

 16 

 131 



105 

 76 

 64 



28 

 33 



24 

 21 



30 

 22 

 10 



35 



37 

 40 

 24 



26.1 

 22.4 

 16.2 

 16.5 

 13.5 

 13.2 



11.3 

 10.3 

 9.0 

 8.8 

 7.3 

 7.0 

 6.8 

 5.3 



4.0 

 4.1 

 4.1 



4.0 

 3.8 



3.1 

 3.1 



3.1 

 4.1 

 6.1 



2.9 

 5.1 

 6.6 

 7.5 



