(fyafiten, 2% 



GEOCHEMICAL 

 PROSPECTING 



Harold Bloom 



The geochemical method of prospecting for petroleum has been based 

 primarily on the premise that buried oil and gas accumulations emanate gaseous 

 hydrocarbons which migrate to the earth's surface. In the early days of explor- 

 ation, visible oil and gas seeps were often the only clues that led to the drilling 

 and discovery of oil and gas fields. In the Gulf Coast salt-dome province, for 

 example, 35 of 141 salt domes discovered before 1936 were detected visually 

 (Sawtelle, 1936). As visible clues became less conspicuous, it became necessary 

 to develop techniques sufficiently sensitive to detect the ever-diminishing volume 

 of gas or oil leakage. In a general way, it seems apparent that the amount of gas 

 reaching the surface would depend upon the depth of the reservoir rock, the 

 volume of gas present, the reservoir pressure, and the accessibility to the surface. 

 Today, methods capable of measuring 1-part-per-billion of methane are used 

 to identify surface hydrocarbons. 



The mechanism of gas migration to the surface is not fully understood 

 because many factors influence the flow of molecular gas: porosity and perme- 

 ability of the strata, pressure gradient, temperature, and viscosity of the hydro- 

 carbon are but a few of these factors. The subsurface fractures, faults, folds, and 

 ground-water movements further complicate any analysis of gas movement. 



Gaseous hydrocarbons conceivably could rise vertically from a pool to 

 the surface if the subsurface pressure gradient is upward and if there were no 



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