StTBSURFACE LOGGING METHODS 



499 



under the fluoroscope every two feet for any oil content. As many minerals 

 will give off fluorescence of various colors, it is necessary to distinguish be- 

 tween mineral and oil fluorescence. This is done by putting a portion of 

 the fluorescent material into a color-reaction plate and applying a petro- 

 leum solvent to it. If oil is present, a "cut" will be obtained; that is, some 

 of the oil will be dissolved out and will come to the surface of the cutting 

 agent, causing the fluid to "bloom" or fluoresce. If no cut is obtained, it 

 can be assumed that the fluorescence is from a mineral source. Examina- 

 tion under the microscope will probably verify the presence of the mineral. 

 Refined rig greases and oils that may contaminate the cuttings may easily 



Figure 235. Examining cuttings under ultraviolet light for oil content. 



be detected by experimentation. The color of the fluorescence obtained 

 from crude petroleum may range from brown to yellow and through var- 

 ious tints of green and blue to near-white, dependent upon the gravity or 

 impurities. The color may be important in distinguishing separate zones 

 in the same well where cavings from above may be present, as well as in 

 correlating the same zone from well to well. 



A microscopic examination of all cuttings is made by the geologist 

 assigned to the laboratory as the well progresses. A detailed interpreta- 

 tive lithologic log is made from this examination. An attempt is made 

 before and during the logging of a well in an unfamiliar area to obtain 

 as much available geologic knowledge as possible. These studies, together 



