CHAPTER VIII 

 CHEMICAL METHODS 



Geochemical Prospecting for Petroleum 



Geochemical prospecting methods are based on the premise that the 

 lighter and smaller molecule hydrocarbons which make up gas and oil 

 accumulations slowly migrate upward from the deposits to the surface 

 of the earth. In practice, samples of soil or soil air are collected at rela- 

 tively shallow depths in a systematic manner over an area to be investigated. 

 The samples are analyzed for hydrocarbons and from the distribution 

 patterns obtained, significant conclusions concerning the possible presence 

 of petroleum accumulation may be drawn. This method, then, unlike geo- 

 physical methods which locate structure with which oil and gas may be 

 associated, attempts to localize the oil or gas accumulation itself. 



In addition to the hydrocarbons themselves, other constituents, both 

 organic and inorganic, have been found to be present in the soil over 

 petroleum accumulations and these materials are believed to result directly 

 from hydrocarbon migration. 



The first method of searching for oil and gas fields involved the search 

 for visible gas and oil seepages. The success of this early method is proven 

 by the fact that of 141 salt domes discovered prior to 1936, 35 owe their 

 discovery to the detection of these gas and oil seepages.f 



In view of the fact that oil and gas, in sufficient quantities to enable 

 detection by the naked eye, can migrate to the surface of the earth, it was 

 logical to assume that microscopic seeps over gas and oil fields would be 

 much more numerous. The first systematic attempt to search for microscopic 

 seepages as indicators of gas and oil deposits was made by G. Laubmeyer in 

 1929. This investigator described methods for collecting samples of soil 

 air from systematically located bore holes and analyzing these samples for 

 traces of methane. The results of his work indicated that the soil air over a 

 producing area is richer in methane than is the soil air over a barren area. 



In 1932, a group of Russian investigators, headed by Sokoloff, became 

 engaged in similar work. SokolofT and co-workers improved Laubmeyer's 

 sampling technique and found heavier gaseous constituents as well as 

 methane. 



The conclusions of the Russian investigators substantiated those of 

 Laubmeyer in that they, too, reported that maximum hydrocarbon concen- 

 trations occur in the center of the region overlying the subsurface accumu- 

 lation, with minimum values over the surrounding barren area. 



t George Sawtelle, "Salt Dome Statistics," Gulf Coast Oil Fields, A.A.P.G. Publication, pp. 

 109-118, 1936. 



