An unusual hopane biodegradation sequence in tar sands from the Pt Arena (Monterey) Formation

• Published in: Nature (London) 1989-12, Vol.342 (6250), p.670-673
• Main Authors: Requejo, A. G, Halpern, H. I
• Format: Article
• Language: English
• Subjects: Biodegradation; Chemistry; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geology; Humanities and Social Sciences; Hydrocarbons; letter; multidisciplinary; Petroleum production; Science; Science (multidisciplinary); Sedimentary rocks
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/342670a0

HOPANES are ubiquitous in petroleum source rocks and crude oils. These compounds are thought to be derived mainly from a specific oxygenated precursor, bacteriohopanetetrol, found in the cell walls of prokaryotic organisms 1 , and commonly occur as a homologous series of structurally related isomers containing between 27 and 35 carbon atoms; hopanes containing more than 35 carbon atoms have also been reported 2 . Hopanes and other pentacyclic triterpanes are relatively resistant to microbial degradation 3–5 , making them useful in correlating moderately bio-degraded oils to their source rocks 5 . During the advanced stages of biodegradation, however, these compounds can be altered, being converted either to 25-normethyl homologues 4 or to other, as yet uncharacterized, products. Selective removal of individual compounds may occur; for example, 22 R extended homohopanes may be degraded whereas 22 S epimers are preserved 6–8 . Here we report an unusual biodegradation sequence of pentacyclic triterpanes in tar sands from the Pt Arena Formation, California, in which the 22 S and 22 R isomers of the 17α(H), 21β(H)–C 35 extended hopanes are selectively preserved. Demethylation at the C 25 position is observed for most hopanes in severely biodegraded samples, with the exception of the C 35 extended hopanes. Two commonly occurring trisnorhopanes 9 , designated Ts (18α(H), 21β(H)-22, 29, 30-trisnorneohopane) and Tm (17α(H), 21β(H)-22, 29, 30-tris-norhopane), also seem resistant to degradation. These findings, together with previous studies of crude-oil biodegradation, imply that biodegradative alteration of hopanes can proceed through numerous pathways, which can result in widely differing distributions.