Biodegradation of aromatic land-plant biomarkers in some Australian crude oils

Land-plant-derived aromatic hydrocarbons with a range of susceptibilities to reservoir biodegradation have been used to assess the accumulation history of crude oils from two Australian sedimentary basins. The compounds used in this study (retene, 9-methylretene, 6-isopropyl-2-methyl-1-(4-methylpentyl)naphthalene and 6-isopropyl-2,4-dimethyl-1-(4-methylpentyl)naphthalene) are thought to originate from land-plants and are the result of reactions of their natural produce precursors, involving aromatisation, rearrangement and methylation in the sediments. They are therefore suggested as markers for land-plants in severely biodegraded oils in which other biologically derived compounds cannot be recognised. The order of biodegradability of the methylated compounds was assessed relative to their non-methylated counterparts, namely 6-isopropyl-2-methyl-1-(4-methylpentyl)naphthalene and retene. In both cases the methylated homologue is less susceptible to biodegradation. These compounds were used to assess the accumulation history of a crude oil that was previously reported to contain a mixture of a severely biodegraded and a non-biodegraded crude oil.