904 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



merized paraffin hydrocarbons Most of these are assumed to be reaction 

 products of organic materials and inorganic agents. In the case of the 

 liquid and visco-solid pseudohydrocarbons, the organic materials are prob- 

 ably furnished by surface vegetation. The solutions carrying the sub- 

 stances (chlorides?) with which they enter into (catalytic?) reactions are 

 assumed to come from below. Since the significant inorganics have their 

 greatest concentration - usually in halo fashion, the pseudohydrocarbon 

 reaction products are likewise arranged in the form of halos. 



The depth at which the maximum amounts of these reaction products 

 occur varies greatly. The maximum of the "liquid" hydrocarbons may be 

 found several inches to several feet deep. One kind of "wax," assumed 

 to be caused largely by bacterial action, occurs in greatest concentration 

 about six inches from the surface,*^ whereas another, resulting from oxida- 

 tion and actinic effects, has its maximum at the immediate surface. The 

 chemical nature of these pseudohydrocarbons has not been definitely deter- 

 mined; they are assumed to be fatty acids. In the second case conditions 

 are reversed. The organic materials are assumed to come from below 

 and are changed near the surface not so much by catalytic reactions as by 

 oxidation and polymerization. The product so formed is a heavy paraffin 

 hydrocarbon^^ and occurs in much smaller quantities than do the pseudo- 

 hydrocarbons. 



This very generaUzed scheme of the distribution of hydrocarbons and 

 pseudohydrocarbons is subject to revision as more field data are accumu- 

 lated and the chemical nature of the substances mapped becomes better 

 known. It is but partially in accord with what should be expected 

 theoretically from the laws controlling the migration of gases and liquids 

 through homogeneous media. According to Pirson,^" there are three ways 

 in which gases emanating from a subsurface source may reach the surface : 

 (1) by permeation, (2) by effusion, and (3) by diffusion. 



Permeation takes place by virtue of the porosity, or better, the per- 

 meability of rocks; it is governed by d'Arcy's law which states that the 

 velocity of gas flow is proportional to the pressure gradient, multiplied by 

 the ratio of permeability times density, divided by the viscosity of the 

 gas. Limes, slates, and moist strata have litte permeability; therefore, 

 migration by permeation is hardly to be expected across the bedding 

 planes of stratified formations (except sands and sandstones) and is con- 

 fined to loose overburden, faults, and fissures. Assuming, therefore, that 

 a fracture zone of high permeability crosses a series of impermeable forma- 



*^ Rosaire, op. cit. p. 8. 



68 Ibid. 



6^ "Pseudohexane," Horvitz, loc. cit. 



6" Oil Weekly, Oct. 10, 1938. 



