CHEMICAL METHODS 939 



In 1936, Americans became interested in this new approach to petroleum 

 prospecting. Instead of collecting samples of the soil air in the field, 

 however, a technique was developed whereby samples of the soil itself 

 were collected and analyzed for hydrocarbons. Saturated hydrocarbons 

 even heavier than those reported by the Russians were extracted from the 

 soil samples. An important advantage of this technique is the fact that 

 samples may be collected under practically all conditions, while with the 

 techniques which require pumping the gas from the bore hole itself, it is 

 impossible to collect samples in water-covered or in arid areas. 



During the migration of hydrocarbon gases from a petroleum deposit, 

 the formations penetrated are believed to be modified chemically. Further, 

 at the surface of the earth, where the hydrocarbons come in contact with 

 oxygen and sunlight, it is believed that the gases are converted to heavier 

 materials such as waxes and liquids. Thus a geochemical exploration tech- 

 nique has been devised whereby samples are collected from the top few 

 inches of soil and analyzed for their wax-like materials. Concentration dis- 

 tribution patterns are often obtained which agree with those resulting from 

 the analysis of the deeper samples for saturated hydrocarbons. 



Another method is based on the fact that there are present in shallow 

 soils certain inorganic materials which fluoresce under ultra-violet light. 

 Proponents of this method believe that these materials are associated with 

 petroleum accumulation at depth. Still another method is based on the 

 observation that the growth of certain bacteria is accelerated in the presence 

 of hydrocarbons. Therefore, this method is applied by collecting shallow 

 samples at depths of approximately six inches and making bacterial counts 

 on these samples. 



In addition to the foregoing methods, determinations are made for 

 various minerals present in soils. These are also believed to be indicators of 

 petroleum at depth. 



Physical Principles. — All geochemical methods are based on the 

 assumption that the lighter components of an oil or gas deposit flow 

 continuously, although at a very slow rate, from the reservoir, through 

 the various overlying strata, into the atmosphere. The exact mechanism 

 of migration is unknown. Undoubtedly, diffusion plays the most important 

 role, with effusion and permeation being involved to a lesser extent. What- 

 ever the mechanism, the phenomenon becomes readily conceivable when it 

 is considered that diffusion can take place through metals,f fused silica,| 

 various glasses, § and other solids normally regarded as impervious. 



t W. R. Ham, "Diffusion of Hydrogen Through Platinum and Nickel and Through Double 

 Layers of These Metals," Jour. Chem. Physics, yo\. 1, pp. 476-481, 1933. 



% E. O. Braaten and G. F. Clark, "The Diffusion of Helium Through Fused Silica," Jour. Am. 

 Chem. Soc, Vol. 57, pp. 2714-2717, 1935. ^ 



§ William D. Urry, "The Permeability of Various Glasses to Helium," Jour. Am. Chem. Soc. 

 Vol. 54, pp. 3887-3901, 1932. 



