OIL AND GAS 



479 



molecules having odd numbers of carbon atoms 

 conspicuously predominate among the heavy normal 

 paraffins (Bray and Evans, 1961). 



The great bulk of the organic carbon that occurs 

 in sedimentary rocks, like the total organic matter 

 and most of the hydrocarbons, is disseminated in 

 low concentrations through large rock volumes. On 

 the basis of many tens of thousands of rock samples 

 (Trask, 1937, p. 149; Trask and Patnode, 1942; 

 Ronov, 1958; Hunt, 1962; Gehmen, 1962), the or- 

 ganic carbon contents of unmetamorphosed sedi- 

 mentary rocks are known to range between a high 

 of 1.77 percent (average) for coastal-marine shales 

 of petroliferous regions and a low of 0.15 percent 

 (average) for nonmarine carbonate rocks of non- 

 petroliferous areas. The data of Trask and Patnode 

 (1942) and especially of Ronov (Ronov and others, 

 1965, p. 330; Ronov, 1968, fig. 7; 1958, figs. 1 and 2) 

 show that the distribution of organic carbon in 

 sedimentary rocks is not at all constant through 

 geologic time. Instead, a long-term increase from 

 very low concentrations in young Precambrian clay 

 strata of the Russian Platform (0.08 percent organic 

 carbon for 700-million-year-old Sinian strata) to 

 high concentrations in Mesozoic and Cretaceous 

 clays (0.41 percent organic carbon) is attributed 

 by Ronov and his coworkers ". . . to the overall 

 increase in the total mass of living substances in 

 the biosphere." Major fluctuations in this long-term 

 increase have been demonstrated (Ronov, 1958, 

 table 2 and figs. 1 and 2) from data from the Russian 

 Platform and the United States (Trask and Pat- 

 node, 1942). Figure 57 summarizes these data and 

 emphasizes the remarkable parallelism between the 

 Russian and the U.S. stratigraphic averages. Tap- 

 pan and Loeblich (1970), calling attention to the 

 worldwide contemporaneity of such geochemical 

 fluctuations with important marine and terrestrial 

 biologic events, suggested that changes in the quan- 

 titative distribution of marine phytoplankton are 

 the fundamental causative link connecting such 

 otherwise apparently unrelated correlations (see 

 also House, 1963). 



Knowledge about the uneven distribution of total 

 organic carbon in sediments of different grain sizes 

 and depositional environments and in sedimentary 

 rocks of different regions and ages is both sketchy 

 and problematic ; but it is vastly more complete and 

 satisfactory than knowledge about the hydrocarbon 

 compound whose carbon forms but a minute part 

 of the organic carbon. The relatively small quanti- 

 ties of hydrocarbon compounds, their mobility and 

 physicochemical lability, and the relative newness 

 of many of the analytical procedures and equipment 



U.S.S.R. 



Figure 57. — Age-dependent variations in the average con- 

 tent of organic carbon in sedimentary rocks from the Rus- 

 sian Platform and the United States. Adapted from Ronov 

 (1958, figs. 1 and 2). 



needed for quick reliable quantitative determinations 

 of most of them, combine with the enormous scope 

 of the undertaking to be more conducive to ignor- 

 ance than to knowledge. 



Hydrocarbon compounds have been identified from 

 some of the oldest Precambrian strata (3-billion- 

 year-old Fig Tree shale of the Swaziland System, 

 South Africa) and, among others, from unmeta- 

 morphosed carbonaceous middle Precambrian un- 

 doubted microfossiliferous chert (1.9-billion-year- 

 old Gunflint Iron Formation, Ontario, Canada). 

 However, critical investigations of the origins of 

 these compounds (aromatic hydrocarbons in the 

 older shale and normal alkanes and saturated and 

 unsaturated fatty acids in the younger chert) indi- 

 cate that the hydrocarbons are present because of 

 contamination that possibly is quite recent. They 

 are neither indigeneous nor syngenetic with the 

 mainly indigenous carbonaceous materials (Hoer- 

 ing, 1967, p. 99-100 ; Abelson and Hare, 1968 ; Smith 

 and others, 1970; Nagy, 1970). As Hoering (1967, 

 p. 105) so clearly stated in his remarkably far- 

 sighted treatment of such problems of hydrocarbon 

 contamination, "Vast quantities of geologically 

 younger petroleums are a part of our present en- 

 vironment and can appear in unsuspected places." 



However, the detection of chemically stable metal- 

 loporphyrins in an extract of 1.1-billion-year-old 

 Nonesuch shale from northern Michigan (Barg- 

 hoorn and others, 1965) points clearly to a biogenic 

 origin for the organic matter from which this relic 



