CHEMICAL METHODS 961 



the producing zone is reached. It will be noted that the predominating 

 fraction is that of the pentane and heavier hydrocarbons. 



Figure 591 is a geochemical log of a dry hole located only 3,500 feet 

 from production in the Lovell Lake oil field. It is seen that the various 

 constituents are present in very small quantities, suggesting that the geo- 

 chemical effects produced by a petroleum accumulation extend laterally 

 for only short distances ; i.e., the migration of hydrocarbons is substantially 

 upward. 



It will be noted that other constituents, in addition to the hydrocarbons, 

 have been plotted on the logs. These inorganic constituents are of use for 

 correlation purposes only. 



Fluorologging. — A logging technique has been describedf which is 

 based on the principle that the rocks overlying an oil accumulation are 

 characterized by anomalously high fluorescent intensities. This technique, 

 known as Fluorologging, involves the preparation of logs by plotting the 

 fluorescent intensity of well cuttings against depth. A geologic section 

 overlying an oil accumulation will show a considerably higher average 

 fluorescent intensity than a section penetrated in barren territory. A porous 

 rock recovered from the subsurface which shows a value of 1.25 fluores- 

 cent units or more is considered to be from a commercial zone. 



Two logs are usually plotted for a well, showing : ( 1 ) the "fixed fluor- 

 escence" which represents the fluorescence of a formation after any oil or 

 gas present is flushed out; and (2) the "free fluorescence," which repre- 

 sents the fluorescent intensities of the free oil and gas in well samples. 

 From these data, predictions can be made as to whether the well will 

 encounter a petroleum accumulation. Also, commercial zones which have 

 been passed up during drilling can be located. 



Mud Analysis Logging. — The mud analysis logging system| con- 

 tinuously analyzes and records the oil and gas content of mud returns from 

 wells being drilled by the rotary method. Oil or gas detected in the return- 

 ing drilling fluid indicates oil or gas in the formation penetrated by the bit. 

 Another useful phase of the system plots, in detail, drilling speed as a 

 function of depth, see page 1093. 



For the purpose of detecting oil in the drilling mud, ultra-violet light 

 is used. The apparatus is capable of detecting quantities as small as one 

 part of oil in a hundred thousand parts of mud. The mud samples are 

 treated to remove any excessive viscosity and gel strength before they are 

 viewed in the detection apparatus. The oil in the samples appears as small 

 fluorescent spots under the ultra-violet light. By observing a fluorescent 

 spot through a magnifier, and then switching to visible light, it is possible 

 to distinguish crude oil from other oils or greases that may accidentally 

 get into the mud. Crude oil from a previous show that has already been 



t O. E. Campbell, "Principle Uses of Fluorologs," The Oil Weekly, December 30, 1946. 

 t Robert E. Souther, "Application of Mud Analysis Logging," Geophysics, Vol. 10, No. 1, pp. 

 76-90, January, 1945. 



