762 Subsurface Geologic Methods 



square yard per year at the mean temperature of 53° F. It should be 

 pointed out that the other gaseous hydrocarbons would show a faster rate 

 of flow because of their lower viscosity. It is rather paradoxical that meth- 

 ane has the highest viscosity of all the gaseous hydrocarbons and, conse- 

 quently, will migrate slower under the assumed conditions. It is interesting 

 to observe that, perhaps, this paradoxical condition concerning the vis- 

 cosity of the gaseous hydrocarbons has had something to do with determin- 

 ing the high methane content of most natural gases. Since methane is much 

 slower to migrate under the same conditions than the other gases, it would 

 be retained longer and, consequently, would form the major portion of the 

 residual gas in any given reservoir. 



A thermodynamic consideration of this migrating gas leads one to 

 some interesting conclusions, as follows. If one assumes that one cubic foot 

 of methane in a reservoir 4500' below the surface at a temperature of 205 ° 

 F. and a pressure of 138.5 atmosphere migrates toward the surface along 

 a pressure gradient of 0.0306 atmospheres per foot and a temperature grad- 

 ient of 0.0337° F. where the mean temperature is 53° F. and the mean 

 pressure 0.8 atmospheres, then the condition of state at any depth may be 

 represented by the following equation: 



PV = nzRT 

 where P = pressure 



V = volume of gas 

 n = mol fraction 

 z = compressibility factor 

 R = universal gas constant 

 T = absolute temperature 



In figure 408 is plotted the water-vapor content of that same volume 

 of methane, assuming that it is saturated at all depths along its migratory 

 path. 



Attention is drawn to the fact that water vapor is being condensed 

 from the gas during the lower three thousand feet. More than 80 percent 

 of the total water which is evaporated from the formations comes from the 

 twenty-five feet immediately below the surface. It is possible that evapor- 

 ation of these waters might cause the concentration of dissolved mineral 

 matter to such an extent that substances with a very small solubility product 

 constant which occur in the soil solution at saturation levels might be pre- 

 cipitated in the interstices of the rock and soil. This is one reason for de- 

 termining the variation in concentration of certain minerals and substances 

 of low solubility and more or less universal occurrence in the soil solution. 

 It has been reasoned that substances of this sort may be precipitated and 

 thereby stabilized or fixed in the path of the migrating gases; and, if suffi- 

 ciently sensitive techniques are devised for their detection, they become 

 indices of subsurface conditions. In some cases, it is probable that they 

 influence the subsequent developments of micro-flora and fauna which may, 

 in turn, become the observable index. 



