764 Subsurface Geologic Methods 



It is also of theoretical interest to point out that the heat lost by the 1 

 mol of methane, which we assumed earlier to be migrating from a depth of 

 4500' to the surface, would account for a drop in temperature of the gas 

 of 101.8° F./mol as a result of its thermodynamic expansion through the 

 semi-permeable sediments. In addition, there is some heat lost by the 1 mol 

 of methane resulting from evaporation of the water vapor from the upper 

 portion of the sedimentary column and, at the same time, some heat ad- 

 sorped as a result of the water vapor condensed from the gas into the for- 

 mations. The net effect is a total heat loss producing a drop in temperature 

 of 102.3° F./mol. However, the actual cooling which takes place in going 

 from a reservoir temperature of 205° F. to 53° F. at the surface is 

 152.4° /mol. Therefore, the additional heat which is lost by the gas must 

 be dissipated to the rocks through which the gas passes and a slight rise in 

 temperature and a change in the local geothermal gradient is produced. 



The alterations in the chemical composition, the modifications of 

 certain physical processes, or the changes in micro-organic population 

 distribution caused by the migrating gas may be a second or third step 

 removed from the phenomenon regarding which information is sought. 

 The deviations of these properties from the normal may be very slight 

 or superimposed upon relatively large seasonal, diurnal, or nonpredict- 

 able fluctations. The problem of the geochemist is to select a property or 

 combination of properties which are influenced sufficiently by the econom- 

 ically important "unusual condition" so that by the use of analytical 

 techniques available to him it is possible to measure the selected property 

 with suitable precision. The fluctuations in the property from sample to 

 sample must be significantly greater than the statistical error of the analy- 

 tical process. 



Analytical Techniques 



The analytical procedure used in geochemical methods is dictated 

 by whether one is looking for the organic constituents or the inorganic 

 constituents, and whether one is taking samples of gas or soil or some 

 product of the soil. In this latter class is included such things as plants 

 which have an ability to concentrate a diagnostic substance in proportion 

 to its occurrence in the soil in which the plant grows, or the presence of 

 microorganisms, the growth of which is accelerated or depressed as a 

 result of hydrocarbons or inorganic ions in abnormal quantities. 



The Russian and German techniques consisted mostly of the analysis 

 of soil gases. Their scheme was to dig a hole with a hand auger and 

 extract from it, after a suitable interval of time, a sample of the collected 

 gases. The Russians developed a method for analysis in the field and a 

 second one for use in the laboratory. In the former case, the gas sample 

 was passed through a caustic solution to remove the carbon dioxide and 

 then into a combustion chamber. The resulting carbon dioxide was al- 

 lowed to bubble through a barium-hydroxide solution until the first tur- 



