MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



cedures of soil analysis, one depending on a determination of the volatiliz- 

 able components — usually methane, ethane (propane, butane), and hydro- 

 gen — and the other involving the extraction of organic liquids and waxes 

 and inorganic constituents. The quantities investigated are exceedingly 

 small and, therefore, the accuracy requirements are high. The gaseous 

 paraffin hydrocarbons occur in quantities ranging from 10 to 1000 parts 

 per billion by weight. The solids (waxes) vary between several tens to 

 several thousand parts per million by weight, and the liquid organics from 

 1000 to 10,000 parts per million by weight. Table 82 shows the relation 

 of the more important gas and soil analysis methods, compared with the 

 older macroscopic procedures. 



B. Significant Hydrocarbons; Occurrence 



The question arises as to which of the hydrocarbons present in and 

 detachable from a subsurface oil concentration are the most significant, 

 the most unique, and lend themselves most readily to observation and 

 analysis. Crude oil is an exceedingly complex mixture of hj^drocarbons ; 

 the composition varies greatly with locality and therefore with the original 

 constitution of the organic source materials. It is assumed that they were 

 converted by anaerobic fermentation of cellulose and proteins to methane 

 and unsaturated fatty acids, probably in the presence of bacteria and salt 

 water; these presumably changed by polymerization, aided by pressure 

 and moderate temperatures, to the compounds of the naphthene series. 

 D. C. Barton has assumed that the originally naphthenic petroleums were 

 transformed to the more parafiinic types with time and depth. According 

 to Brooks,^* "no petroleum has ever been found which contains unsaturated 

 hydrocarbons of the olefine type, at least in the lighter fractions which can 

 be separated by distillations without decomposition." 



Table 83 illustrates schematically the relation and variety of the various 

 groups of the natural hydrocarbons. In crude oil the aliphatic group is 

 the most important; in coal and its derived products the aromatic com- 

 pounds predominate, alhough they are also found in certain types of 

 crudes. The saturated open-chain aliphatics (paraffins) are the chief con- 

 stituents of the paraffin-hsbse oils; the saturated closed-chain aliphatics 

 (naphthenes) occur chiefly in the asphalt-hsise crudes. The monolefin 

 and diolefin groups, both open- and closed-chain, are represented chiefly 

 in cracked oils and artificial products. Table 84 gives the composition of 

 some of the crude oils, (a) in the low-temperature range from 60°-95° C, 



'^ Problems of Petroleum Geology, p. 109 (1934). 



^* Dunstan, A. E., ei al. (eds.), Science of Petroleum, I, 48, Oxford (New York, 

 1938). 



