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In addition to previously reported 25-norhopanoic acids with the 17α(H), 21β(H) and 17β(H), 21α(H) configurations, for the first time we report the identification of 25-norhopanoic acids bearing the 17β(H), 21β(H) configuration in a suite of heavily biodegraded oils from China. The 17β(H), 21β(H) stereochemistry is a thermally unstable biologically-derived structure, suggesting that the 17β(H), 21β(H) 25-norhopanoic acids may derive from petroleum-degrading microbes. Rather than representing a direct input, we propose that demethylation of 17β(H), 21β(H) hopanoic acids derived from bacteria in the reservoir may give rise to 17β(H), 21β(H) 25-norhopanoic acids, implying that microbial reworking of biomass is occurring alongside petroleum degradation.

Pristane (2,6,10,14-tetramethylpentadecane), frequently the most abundant biomarker hydrocarbon in crude oils and rock extracts, was thought originally to be derived directly from phytol or, later, from thermal degradation of tocopherols. We present new evidence supporting the hypothesis that pristane, under certain conditions, may result from the catagenic decomposition of methyltrimethyltridecylchromans (MTTCs) derived during diagenesis from condensation reactions between chlorophyll and alkylphenols. We show that phytol can condense, under mild conditions, with phenolic systems (in free or polymeric form) to produce free or bound structures that generate pristenes on flash pyrolysis. We suggest straightforward tests of this hypothesis.