906 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



it is likely that some of them have been carried in solution by circulat- 

 ing waters along fissures, and the like, or up the dip in formations. 

 Modifications may occur along the path because of adsorption on colloidal 

 matter; they do occur near the surface in the aerated zone because of oxida- 

 tion, polymerization, and actinic action. Besides, changes must be ex- 

 pected from one formation to another when there is a difference in diffusion 

 constants. 



If the process of diffusion of gases through formations without lateral 

 variations were alone responsible for the distribution of hydrocarbons and 

 related organics at the surface, their maximum concentration should in 

 all cases occur directly above the source, since the migration may be as- 

 sumed to take place from it in essentially a vertical direction. As a matter 

 of fact, this has been found to be true in many cases, particularly above 

 faults and fracture zones. It has also been claimed, particularly by the 

 Russian investigators, that maximum concentrations of methane should 

 be formed above gas fields, and those of the heavy hydrocarbons above 

 oil fields. Further, it has been stated that in a given field where both oil 

 and gas are found, the maximum methane concentration should occur over 

 the highest point of a dome or monocUnal trap where the gas occurs, and 

 the heavy hydrocarbons should occur above the oil accumulations at the 

 flanks. The Russians' own data do not bear out this conclusion in all 

 cases. 



To begin with, natural gas is not pure methane but is frequently asso- 

 ciated with ethane and other heavy hydrocarbons (Table 85). Con- 

 versely, methane is often absorbed in and liberated by oil deposits. Hence, 

 a clean-cut separation of oil and gas occurring in the same structural or 

 stratigraphic trap appears hardly possible by surface measurements. 

 Furthermore, the process of diffusion is not so simple as theoretically 

 indicated, and the surface expression of subsurface hydrocarbon distribu- 

 tion is modified considerably by lateral variations in the permeability and 

 diffusion characteristics of the overlying formations. For this reason, 

 the simple explanation given by Pirson,*^ that heavy hydrocarbon halos 

 are due to the marginal arrangement of oil below the gas on the flanks of 

 an anticlinal or domal trap does not appear tenable, besides being in dis- 

 cord with the observation that in many cases production is found under 

 the hare spot in the center of a halo and not under the halo itself. 



It is evident, therefore, that other causes besides uniform diffusion must 

 be responsible for the halo formation or must at least interfere with the 

 process of normal diffusion. The solution of the problem is rendered 

 difficult because the published data frequently lack information about 



«2 Ibid. 



