326 Subsurface Geologic Methods 



follows, therefore, that when a well is drilled in any particular field and sam- 

 ples from each of the various sands penetrated are tested and classified accord- 

 ing to their respective fluorescent qualities, the record thus obtained becomes 

 an index for that field. When subsequent wells are drilled the new samples 

 may be compared with the index and correlated accordingly. It is necessary, 

 of course, that the samples to be tested consist of oil or of sands containing oil. 



The observations of Melhase, while classic, have given way to more 

 accurate and standardized, though often empirical, laboratory and field 

 methods. The correlations are not limited to shales, but also include any 

 earth-bearing oil, either of surface or of subsurface origin. 



The method, it must be emphasized, is neither foolproof nor per- 

 fectly reliable. As many of the techniques are empirical, developed in 

 the laboratory according to the bents of the technician, there may be con- 

 siderable latitude in the extent of dependability of results. The inspection 

 of untreated earths with the unaided eye is open to much more error than 

 the study of these materials either with instruments or by extraction and 

 subsequent objective measurement. Trace amounts of highly fluorescent 

 minerals may negate results for an inexperienced worker. Likewise, em- 

 phasis should be given the possible untoward effects of air on the sample, 

 since some cores, after aging and exposure to subtropical climate, may 

 lose their fluorescence. The role of hydrocarbonoxidizing bacteria in the 

 destruction of oil traces in rock or earth should not be discounted. 



It has been found that well cores and cuttings, even when small, 

 broken, or partly contaminated, fluoresce if they contain liquid hydro- 

 carbons.^- In general it is found that a producible sand will give uniform 

 fluorescence throughout, but that a sand carrying salt water in addition 

 to oil will show a mottled fluorescence. In local areas increases in oil 

 saturation and productivity appear to increase fluorescence intensity. When 

 properly employed, fluorescence discloses the presence of petroleum un- 

 detectable by odor, taste, stain, saturation, analysis, or electric log. 



Fluorologging 



Well-log curves prepared from fluorochemical data provide extensive 

 and reliable information.^^ The curves are called "fluorologs" (fig. 134), 

 and the data are obtained from cuttings and core samples as customarily 

 taken for paleontologic analyses. Core samples are not necessary in the 

 procedure which has been developed by Ferguson, but it is desirable to 

 have samples of all cores taken. The cuttings samples are usually taken 

 at an interval of not more than thirty feet, but samples taken at shorter 

 intervals provide more accurate results. Sampling is initiated at the 

 surface and continued at regular intervals to the total depth of the hole. 



The assay is made by the photographic method, and fluorescence in- 

 tensities are plotted against depth. It is not necessary to rely upon the 

 photographic procedure, but it is useful for very weak fluorescences. A 



^- De Ment, Jack, op. cit., 1947. 



^Ferguson, W. B., and Campbell, 0. E., The Fluorographic Method of Petioleum Exploration: 

 10 pp., Houston, Texas, Fluorographic Exploration Co., Apr. 24, 1945. 



