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

Abstract

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 organisms¹, 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 reported². 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 rocks⁵. During the advanced stages of biodegradation, however, these compounds can be altered, being converted either to 25-normethyl homologues⁴ or to other, as yet uncharacterized, products. Selective removal of individual compounds may occur; for example, 22R extended homohopanes may be degraded whereas 22S 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 22S and 22R isomers of the 17α(H), 21β(H)–C₃₅ extended hopanes are selectively preserved. Demethylation at the C₂₅ position is observed for most hopanes in severely biodegraded samples, with the exception of the C₃₅ extended hopanes. Two commonly occurring trisnorhopanes⁹, 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.