Subsurface Laboratory Methods 325 



lowed by elution with different solvents.-^ Qualitative differentiation of 

 the crude oils is made by observing the fluorescence of the different frac- 

 tions in chloroform solution and the differences in color between the main 

 bulk of the solutions. The capillary-strip method is also employed to 

 distinguish between crude oils of different origins, as crude and refined 

 oils from Digboi and Assam, some artificial crudes, and a chloroform 

 extract of a resin. 



Fluorescence Microscopy 



The fluorescence microscope is an instrument which permits ex- 

 amination of an object at high or low magnifications under filtered ultra- 

 violet light. Fluorescence microscopy may be of the reflected variety of 

 the transmitted variety. Actually there is little difference between the 

 two, but for opaque specimens the reflected method is generally the best. 



Since petrographic methods play an important role in geochemical 

 and geophysical prospecting, fluorescence microscopy would appear to 

 have a promising future in this field as an auxiliary tool. The features 

 of petrographic sections that may not be revealing by bright-field micro- 

 scopy may well be so in ultraviolet light. The fluorescence microscope 

 detects oil traces in drill cores, earths, muds, cuttings, water residues, 

 and the like.^^ 



Quenching Analysis 



The fluorescence of petroleum is quenched or reduced in color and 

 brightness by certain substances. The prudent use of such substances 

 makes possible measurement of fluorescence intensity and in certain in- 

 stances identification. Thus, nitrobenzene has been used to measure the 

 fluorescence, and, by additions of this substance to a sample, the change 

 in response appears to have some value for distinguishing oils.^*^ The pres- 

 ent writer has found that in complex organic substances, such as essential 

 oils, the application of a given quencher to oil samples of different origin 

 causes the brightness and color of fluorescence to vary greatly among the 

 samples. In addition to nitrobenzene as a quencher, other substances of 

 this character include trinitrotoluene, picric acid, and chlorinated hydro- 

 carbons. 



Fluorescence Exploration 



The values of fluorescence and ultraviolet light as aids in correlating 

 oil sands were first pointed out in 1936 by Melhase.^^ He stated: 



During the past two years the writer [Melhase] has tested oils from many 

 of the California fields and from different sands that may occur in these fields 

 and it was found that no two samples of oil exhibited the same quality and 

 degree of fluorescence unless they were obtained from identical sands. ... It 



^ Mukherjee, N., and Indra, M., Nature (London), vol. 154, pp. 134-145, 1944; Inst. Petroleum 

 Jour., vol. 31, pp. 173-178, 1945. 



^ De Ment, Jack, Oil Weekly, vol. 103, pp. 17-19, 1941. 



'" Mukherjee, N., and Indra, M., idem. 



^ Melhase, J., Mineralogist, vol. 4, p. 9. 1936. 



