Conditions for Appearance of Life on Earth 71 



The second trend is represented by the hypothesis held by V. A. Sokolov, 

 V. B. Porfiriev, J. Roberts and others. These authors clearly demonstrate the 

 complete unsoundness of the hypothesis of 'least severe' conditions for the 

 formation of oil as far as hydrocarbon chemistry is concerned. Instead, they 

 offer a scheme for the formation of oil and gas as a result of changes in biogenic 

 organic matter under 'the most severe' conditions, i.e., at temperatures of 150- 

 500° and at a considerable depth [42, 43]. But the fact that a number of rich 

 oil deposits have been found in the sedimentary cover of the shields lying on 

 the pre-Cambrian crystalline basement, has proved fatal for all these variations. 

 The thickness of that covering strata often does not exceed 1-5 km, while the 

 temperature at its base, corresponding to the geothermal gradient of the tectonic 

 platforms, could not rise above 40-60°, i.e. certainly could not reach the limits 

 necessary for the formation of petroleum according to these schemes (the Ural- 

 Volga region, Kansas and Oklahoma, Egypt). The pre-Cambrian basement 

 itself, with its insignificant graphitized fossils of plant and animal organic matter 

 could not, of course, produce later a noticeable amount of biogenic hydrocarbons 

 when the sedimentary layers had been formed above it. 



Examination of a large nvmiber of analyses of the chemical composition of 

 bitumens and caustobiolites clearly reveals the rules governing the changes of 

 organic matter in the Earth's crust. The triangular baricentric diagram (Fig. 3) 

 shows two independent centres of primary organic matter. One of these centres 

 corresponds to the compositions of cellulose, wood, peat, and sapropel. The 

 figurative points of the composition of the changed organics of the coal series 

 (lignite and coal, anthracites, and other) and of the sapropelite series (sapropel 

 coals, combustible or bituminous shales, shungites) stretch in a narrow strip 

 from this centre to the pure carbon (graphite). Changes in the organics are 

 bovmd with losses of carbon dioxide, water, methane, and inevitably lead to 

 their dehydration rather than to hydrogenization. In this case, the ratio of the 

 number of hydrogen atoms to carbon atoms decreases from 1-67 to 0-2-0-5 

 receding more and more from the high ratio of H : C (i •7-2-1) characteristic 

 of petroleum. Another centre, being quite separate, located in the upper part 

 of the diagram, is formed by methane and other combustible gases, a complex 

 mixture of condensate pool hydrocarbons and methane oil. The diagram shows 

 that all the other oil bitumens are formed at the expense of dehydration and 

 oxidation of these compounds, forming naphthene and resin oils, oxyasphalts, 

 and others. 



Thus it is clear that changes in biogenic organic matter in the layers of the 

 Earth's crust, chiefly consisting in carbonization and elimination of hydrogen 

 and oxygen, cannot lead to the formation of petroleimi from animal and plant 

 organic substances, since petroleum consists of compoimds almost completely 

 saturated with hydrogen [44]. 



According to some conceptions, the source of oil is volatile derivatives, sepa- 

 rating when the change in the organic substances takes place, for example, 

 methane, which is formed together with CO2, CO, and H2O during the meta- 

 morphism of biogenic organic matter [45]. But as the study of the composition 

 of coal-mine gases shows, these gases, as a rule, do not contain any heavy hydro- 



