608 KAPLAN AND RITTENBERG [CHAP. 23 



higher if based on the average 32S/34S ratio of SO42- in sea-water) can be at- 

 tained by such biological systems, under suitable conditions. 



5. Special Problems 



Basin sediments are thought to be particularly suited for the formation and 

 accunuilation of certain deposits commonly not present in other marine 

 environments. These include petroleum, base-metal sulfides and uranium. The 

 exact mechanism for their formation is unknown, and so far the evidence comes 

 mainly, if not in some cases entirely, from ancient marine sediments; a short 

 discussion of these topics is believed warranted in order to draw attention to 

 possible lines of study. 



A. Petroleum 



The origin of oil has been a much disputed question for the past 100 years. 

 Although a voluminous literature exists, no definitive theory has yet been 

 proposed satisfying all the questions arising from the complex properties of 

 petroleum and the special conditions needed to create them. At the present 

 time the majority of workers consider that most deposits have a marine origin. 

 Although a freshwater environment could likewise serve as a starting point, 

 such an environment is quantitatively unimportant compared with the marine 

 environment. It is generally agreed that the plankton serves as the source 

 material. 



The evidence for the marine environment is that petroleum is found in strata 

 which are either entirely or i^redominantly marine. The evidence for the bio- 

 logical origin stems from the direction of optical rotation and more important 

 from the presence of porphyrins (desoxophylloerythrin) in the petroleum. Since 

 these substances are sensitive to high temperatures and since petroleum has 

 been found in relatively young beds of the late Pliocene it has been concluded 

 that oil can form at low temperatures (< 150°C) and pressures. Complications 

 do arise, since the highly paraflftnic oils do not appear to contain porphyrins. 

 This has generally been interpreted as being the result of their destruction by 

 heat and pressure due to deep burial. However, parafhnic oils have been found 

 at shallow depth in stratigraphic traps which do not appear to show deep burial. 



This leads to the question of the nature of the organic substances from which 

 petroleum was derived. As mentioned in an earlier section, crude oil as such does 

 not exist in present-day basin sediments, although hydrocarbons have been 

 separated from the surface muds of both marine basins and freshwater lakes. 

 It would therefore appear that formation and accumulation begin soon after 

 deposition and perhaps even during sedimentation of the dying plankton. 

 The two theories suggested are (a) the accumulation of oil from residual im- 

 decomposed compounds such as lipids and fatty acids, (1)) the production of 

 hydrocarbons by diagenetic processes. The first proposal may account for the 

 low density asphaltic petroleums containing porphyrins, the second for the 



