902 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



E. Interpretation and Results of Gas and Soil Analysis 



Gas and soil analysis in its present form is of comparative!}' recent 

 development. There is still some dispute over its merits. Divergent 

 opinions have been expressed regarding the type of organic compounds to 

 be considered significant and the relation of their surface distribution to 

 subsurface accumulations of oil and gas. More information is needed, 

 particularly about the variation of oi-ganic and inorganic constituents with 

 depth in wells, and on known fields. In any event, whatever interpreta- 

 tion procedure is developed will not be the equivalent in physical and 

 mathematical rigorousness to procedures currently employed in other geo- 

 physical methods. The fundamental problem is, in principle, no different 

 from that which faced the surface geologist a number of years ago, namel}', 

 that of locating subsurface oil and gas accumulations from the surface 

 distribution of seeps. Factors which have made possible an advance in 

 his interpretation methods are: (1) the\grcatcr areal completeness of 

 sampling points and greater independence of random indications; (2) a 

 segregation of the surface materials by physico-chemical analysis, in regard 

 to geologic significance; (3) information on their variation with depth, b>' 

 the analysis of well samples; and (4) data on geologic structure, by simulta- 

 neous application of other geophysical methods to a given problem. 



Any attempt to deduce from the surface oi- near-surface accumulations 

 of organic products the existence, location, or depth of a subsurface oil 

 deposit must, of necessity, involve some definite assumption i-egarding the 

 mechanism of their migration. Various ideas have been advanced on the 

 basis of observed horizontal and vertical distribution and theoretical 

 possibilities. 



What little data have been released on the variation in organic and inor- 

 ganic constituents at the surface and in wells suggests a picture indicated 

 schematically in Fig. 12-14. Above an oil deposit, the heavier gaseous 

 hydrocarbons of the paraffin series show the most significant indications; 

 they appear to be the most reliable indicator of subsurface oil deposits, 

 since they are not known to be associated with near-surface decomposition 

 of organic materials. The heavy hydrocarbon content (ethane, propane, 

 and the like) of rocks overlying an oil deposit decreases toward the sur- 

 face, although that decrease is far less regular than the scheme suggests 

 and varies with the permeability and related characteristics of the indi- 

 vidual formations. Methane appears to be more irregular in its vertical 

 distribution, since decomposition processes within formations tend to be 

 superimposed on the regular decrease away from the deposit. This is 

 particularly true near the surface where anaerobic decomposition resulting 

 from fermentation of vegetable matter (as in marshes, peat, lignite) may 



