CHEMICAL METHODS 959 



tion. Thus from a study of these plant symptoms, information may be 

 obtained concerning the presence of certain metals in the area. 



Although plant chemistry studies show promise of increasing their 

 value in locating ore bodies, they have not progressed to the point attained 

 by soil and water studies. 



Geochemical Well Logging 



Geochemical well logging involves the analysis of cuttings or drilling 

 fluids, primarily for hydrocarbons. Various inorganic constituents, such as 

 chlorides, sulphates, and carbonates, may also be determined. The data 

 obtained by the various techniques used are very helpful in locating poten- 

 tial producing horizons, and in many instances to predict in advance of the 

 drill the probability of encountering a petroleum accumulation. 



Methods Using Well Cuttings 



Hydrocarbon Logging. — The techniquesf used for extraction and 

 analysis of hydrocarbons from well cuttings are the same as those described 

 in connection with geochemical exploration. 



The hydrocarbon values are plotted and, based on their order of magni- 

 tude and patterns they produce, interpretations are made. Logs of wells 

 located toward the center of an accumulation show low hydrocarbon con- 

 centrations in the upper part of the well, but at some distance above the 

 accumulation a definite increase is obtained. From this point on, the values 

 gradually increase until the deposit is reached, when maximum values are 

 obtained. After passing the first accumulation, the values rapidly decrease 

 to normal unless another deposit is being approached, in which case the 

 same pattern obtained above the first accumulation will be repeated. Logs 

 of wells located at the edge of an accumulation show higher values in the 

 upper part of the well than do centrally located wells. In such cases, the 

 values increase gradually until the accumulation is reached. Logs of non- 

 productive wells that are located at considerable distance, laterally, from a 

 petroleum accumulation show low hydrocarbon concentrations throughout. 



For the case of producing wells, the length of the section immediately 

 above the deposit over which significantly high values are obtained depends 

 upon the nature of the deposit. The higher the gravity of the accumula- 

 tion, the greater is the distance to the point above the deposit where the 

 first abrupt increase in concentration is observed. 



The relative quantities of the ethane-propane-butane, pentane, and heav- 

 ier hydrocarbons present serve as an index to the type of accumulation. 

 Approximately equal quantities of these constituents are found in the case 

 of gas-condensate accumulations. In the case of oil accumulations, the 

 pentane and heavier hydrocarbons predominate. 



t Leo Horvitz, "Well Logging Methods Conference," Bulletin No. 93 of the Agricultural and 

 Mechanical College of Texas, March 11-15, 1946. See also W. R. Ransone, "Geochemical Well Log- 

 ging," Geophysics, Vol. 6, No. 3, pp. 287-293, July, 1941. 



