Formation of isohexyl alkylaromatic hydrocarbons from aromatization-rearrangement of terpenoids in the sedimentary environment: A new class of biomarker

Seven alkylaromatic hydrocarbons with an isohexyl and a methyl substituent group on adjacent ring carbons have been identified in a suite of crude oils derived from a range of locations, source types, palaeoenvironments, and ages. Hitherto unidentified series of isohexylalkylbenzenes and isohexylalkynaphthalenes have been observed with concentrations ranging from 10 to 565 mu g/g. Isohexylalkylbenzenes occur in crude oils ranging in age from Cambrian to Tertiary suggesting both a bacterial/algal and higher plant origin, whereas isohexylalkylnaphthalenes were restricted to crude oils of Permian to Tertiary age and are exclusively associated with specific higher plant precursors. The unique structure of these compounds has been shown to result from rearrangement and A-ring opening of diterpenoid and triterpenoid natural products during aromatization. The structural features that are necessary for this process to occur are a terpenoid A-B ring system with a gem-dimethyl group at C-4 and an angular methyl at C-10. Support for a ring opening process concomitant with aromatization was provided by laboratory experiments involving dehydrogenation of terpenoid natural products. Analysis of these reaction products showed that isohexyl alkylaromatic compounds, along with fully aromatized compounds with intact carbon skeletons were formed.