Van Aarssen, Ben G.K.¹, Steven J. Fisher¹, Trevor P. Bastow¹ 
(1) Curtin University of Technology, Perth, Australia

ABSTRACT: Reconstructing the Original Composition of Biodegraded Oils using Aromatic Hydrocarbons

Combined insights into the formation and the biodegradation of aromatic hydrocarbons presents a powerful tool to reconstruct the original composition of biodegraded oils and to interpret mixtures of biodegraded and non-biodegraded oils. 
The distributions of the three major classes of aromatic hydrocarbons present in crude oils, i.e. benzenes, naphthalenes and phenanthrenes display characteristic trends with increasing levels of biodegradation. Generally, benzenes are more susceptible to biodegradation than naphthalenes, which are more susceptible than phenanthrenes. Furthermore, the greater the extent of alkylation within each of these classes, the greater is the resistance to biodegradation. Certain isomers amongst these are more resistant than others, and this information has enabled the development of a detailed scale for measuring the level of biodegradation. 
Most aromatic hydrocarbons present in oils are the result of chemical reactions taking place in the source rock during petroleum formation. Since the pathways that these reactions follow with maturity are universal and well understood, the distributions of these compounds in most non-biodegraded oils is predictable. In biodegraded oils, the abundance of biodegradation-resistant aromatic hydrocarbons can therefore be used to estimate the maturity of the original oil prior to biodegradation, thus enabling a complete reconstruction of the aromatic hydrocarbon distribution. Furthermore, in mixtures of oils biodegraded to different extents, predicted distributions will be distorted due to the enhanced abundance of resistant compounds. Unraveling the biodegraded signal and the non-biodegraded signal yields information about the levels of maturity, the extent of biodegradation, and the relative contribution of each of the component oils to the mixture.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.