Chapter VH 



— Ill — 



Activities in Deposits 



surface of the cores. Within core samples 50 cm. long, the organic content 

 near the lower end of the core fell to about two-thirds of that found near 

 the surface. The amount of organic matter in the lower portion of the 

 core was relatively constant. The vertical distribution of organic matter 

 corresponded with the abundance of bacteria. 



From his extensive studies of the organic content of marine sediments, 

 Trask (1939) concluded that by the time the sediments have been buried 

 to a depth of 30 cm., the quantity of organic matter seems to have de- 

 creased about 15 per cent, and by the time that they have been lithified, 

 some tens of millions of years after they have been deposited, the average 

 decrease in organic content is about 40 per cent. He points out that these 

 figures of 15 and 40 per cent are only rough approximations because the 

 loss of organic matter differs greatly in different sediments. Trask (1932) 

 believes that anaerobic bacteria are the only living things which can alter 

 organic matter after it has been buried to considerable depths. 



Further details on the transformation of organic matter are given in 

 Chapter X. 



The petroleum problem : — Since the majority of the known petroleum 

 deposits occur in ancient marine sediments, one is led to wonder whether 

 organic matter is being converted into petroleum or protopetroleum in 

 recent sediments. Assuming that petroleum is formed from plant or ani- 

 mal organic matter, there are many ways in which the activities of bac- 

 teria may contribute to the process. 



Field observations and laboratory experiments show that bacteria 

 tend to convert organic matter into substances which are more petroleum- 

 like. This they do by reducing the oxygen, nitrogen, phosphorus, and 

 sulfur content of organic matter. The average proximate analyses of 

 samples of organic matter representing different geological ages as com- 

 piled from various sources is as follows: 



Microbiologically produced hydrogen may be instrumental in the 

 hydrogenation of certain kinds of organic matter, thereby converting it 

 into substances which are more petroleum-like. Methane is known to be 

 produced by the bacterially-activated hydrogenation of CO2 (Barker, 

 1936a), and it is not impossible that higher hydrocarbons may be formed 

 by the hydrogenation of certain kinds of organic matter such as cellulose, 

 for example. H2S, which is a good reducing agent, may likewise con- 

 tribute to the hydrogenation or reduction of organic matter. 



Jankowski and ZoBell (1944) have found that certain sulfate-reduc- 

 ing bacteria are able to produce paraffin hydrocarbons ranging from Cio to 

 C36 from various fatty acids and carbohydrates. The common occurrence 

 of sulfate-reducing bacteria in oil-well brines (Bastin and Greer, 1930; 

 Gahl and Anderson, 1928) and the low content of sulfate in oil-well 

 brines is very suggestive of the significance of sulfate reducers in petroleum 

 formation. 



