Biodegradation of aromatic hydrocarbons in crude oils from the Barrow Sub-basin of Western Australia

Abstract

The compositions of aromatic hydrocarbon fractions isolated from four oils from the Barrow Sub-basin of Western Australia have been examined using capillary gas chromatography and gas chromatography-mass spectrometry. Each of the oils is derived from the same source rock formation, and the oils are of comparable maturity, but they contain very different distributions of aromatic hydrocarbons due to the effects of biodegradation and water washing. The susceptibility to biodegradation of the different aromatic hydrocarbon classes has been assessed and, in general, the rate of biodegradation is found to decrease with increasing number of aromatic rings and with increasing number of alkyl substituents. The rate also depends on the positions of the alkyl substituents, and it is very much lower if the aromatic hydrocarbon contains adjacent methyl substituents. Positional isomers of dimethylnaptthalenes are biodegraded at very different rates, with isomers having β-methyl substituents most susceptible to biodegradation. Ethylnaphthalenes are much less rapidly biodegraded than dimethylnaphthalenes. Trace amounts of mono- and triaromatic steroidal hydrocarbons were detected in all of the oils indicating that these compounds are ver resistant to biodegradation and thus can be used as maturity parameters even in severely biodegraded oils. There is evidence to suggest that C 21 and C 22 methyl-substituted triaromatic steroidal hydrocarbons have been removed from the severely biodegraded Mardie oil. A scale is proposed for assessing the extent to which an oil has been biodegraded based on the relative abundance of selected aromatic and saturated hydrocarbons.