Miscellaneous Subsurface Methods 761 



It was further reasoned by American investigators that either some of 

 the liquid hydrocarbons might also have migrated to the surface or that 

 some of the gases had been converted by oxidation and by the influence 

 of sunlight into liquid and solid hydrocarbons through a series of condensa- 

 tion, polymerization, and oxidation reactions. Further thinking and field 

 experimentation along this line suggested that there might also be a vertical 

 circulation of formational fluids which would bring higher contents of 

 mineralized waters to the surface. 



The anomalous concentration distribution of certain trace-elements 

 and ions which have been observed over oil fields has also been explained 

 as a result of evaporation of the soil solution by passage of the gaseous 

 hydrocarbons. 



General Theory 



The underlying theory for all the geochemical methods and techniques 

 require that some of the hydrocarbon components in the oil and gas reser- 

 voir migrate toward the surface by a process of diffusion, effusion, and/or 

 permeation. The actual process of migration appears to be a rather com- 

 plex one which is not very clearly understood. The occurrence of oil or gas 

 at depth is recognized by the unusual concentration of these once-migrating 

 hydocarbon constituents or by disturbances in other chemical, physical, or 

 biological components at the surface brought about by the action of these 

 migrating hydrocarbons. 



The application of a modified D'arcy's equation to determine the rate 

 of flow of gas through the semi-permeable rocks of the sedimentary section 

 yields the interesting result that significant quantities of the gaseous hydro- 

 carbons are migrating toward the surface from a subsurface reservoir. The 

 equation is as follows: 



KA 



Pi ' — Pn 



1 + ' 



IX. [I + m) Lti 



where Q = flow of gas in cubic centimeters per second 

 K = permeability in D'arcy's. 



A = cross-sectional area of flow in square centimeters, 

 ju, — viscosity of gas in poises. 

 m = the thermodynamic character of expansion of the gas : for 



isothermal m = 1, for adiabatic m = Cy/Cp. 

 Ln — distance from reservoir to surface in centimeters. 

 pi = reservoir pressure in atmospheres. 

 Pn = partial pressures of gas at surface in atmospheres. 



If the permeability be taken equal to 10'^ D'arcy's, the value of rj for 

 methane at 20° C. equals 120 X lO*" poises; m equals approximately 0.86; 

 Ln, the depth of the reservoir, is assumed to equal 4500' at which the hydro- 

 static pressure will be equal to 138.5 atmospheres and p„ will be something 

 less than one atmosphere. Then, Q becomes 376 cubic feet of methane per 



