Subsurface Logging Methods 437 



consideration must be given to the problems to be solved, the methods 

 available, and the adaptability of the methods and their cost. The method 

 selected should, therefore, have as many as possible of the following 

 properties: 



1. Significant relationship to the lithology of the rocks. 



2. Detection of oil, gas, or certain minerals penetrated by the bore. 



3. Many vertical variations, ability to differentiate strata. 



4. Persistence of vertical variations laterally, to provide good sub- 

 surface correlations. 



5. Ability to work under almost any bore-hole condition. 



(a) Cased holes, dry or filled with fluid 



(b) Open holes, dry 



(c) Open holes filled with mud or water 



(d) Open holes filled with oil or gas 



6. Location of porous strata. 



7. Economy of operation. 



8. Simplicity of interpretation. 



Radioactivity well logs fit nearly all of these requirements. Coring 

 and core analysis constitute the only positive means of lithologic identi- 

 fication, but owing to excessive cost the method is not very widely em- 

 ployed. The relationship of the radioactive content of the strata to lith- 

 ology has already been pointed out, and a suitable lithologic identification 

 can be executed by the gamma-ray curve. 



No positive means of the detection of either oil or gas has yet been 

 determined other than fluorescent and chemical examination of samples 

 and cores. At present the combination radioactivity log makes no claim 

 to the direct detection of oil or gas, but the indirect use of the log by 

 correlation or structural determination has been the means of locating 

 many new pay horizons. It is therefore stated that radioactivity logs are 

 as capable in the detection of oil or gas as other logging methods, with 

 the exception of visual examination of conventional cores. Research and 

 future development may well provide a sure means of oil detection, 

 either through the development of an additional curve or through the 

 better interpretation of analysis. The requirements 3 and 4 given above, 

 vertical variations for differentiating strata and vertical variations later- 

 ally of ample magnitude to provide subsurface correlations, are ade- 

 quately filled by the radioactivity well log. Abnormally high radioactive 

 shales in most areas serve as base markers for the long-range correlation 

 of gamma-ray curves. Within confined areas it has been found that the 

 neutron curve correlates readily, even to the extent that the evaluation of 

 potential reserves is possible. 



In considering requirement 5, the advantages and the flexibility of the 

 radioactivity log are most obvious. The radioactivity log can be obtained 

 in cased holes, dry or containing bore-hole fluid of any type, and in open 



