BIODEGRADATION OF CRUDE OILS IN THE PETROLEUM SYSTEMS OFFSHORE NORWAY: NEW MOLECULAR INSIGHTS FROM FUNCTIONALIZED PETROLEUM CONSTITUENTS

Biodegradation is an important and potentially ubiquitous process affecting both the chemical composition and physical properties of crude oil. Some of the influences of biodegradation on the molecular composition and physical properties of crude oils are empirically well-known from many studies of in-reservoir biodegradation, laboratory degradation studies, and studies of crude oil spills. For instance, hydrocarbons are selectively degraded in the order n-alkanes, isoprenoids, cyclic compounds, and the concomitant increase in polar compounds is manifested by decrease of API gravity and increase of viscosity. Nevertheless, while hydrocarbon biomarkers can provide specific information concerning the degree of alteration the actual processes taking place during degradation remain obscure. NSO compounds present means for further refining and extending these observations and to better understand the actual processes involved because of their partitioning behaviour in the system rock-oil-water and thus their bioavailability. We present results from a study on the petroleum systems offshore Norway. This area is a very complicated system where many of the oils are passed through long migration distances and affected by water washing and biodegradation. In this paper we utilise the behaviour of carbazoles, fluorenones and a novel class of petroleum constituents, the xanthones, to study these processes in case histories from the Viking Graben and Haltenbanken. One of them is the Gullfaks oil field, a well-known example for the effects of biodegradation in petroleum systems, which exhibits a gradient of increasing biodegradation from east to the west within the field. Based on generally accepted degradation parameters a comparative study of carbazoles, fluorenones and xanthones will be discussed for these case histories. Within these compound classes, certain isomers appear to provide a sterical hindrance for biodegradation which may lead to the observed relative increase of these isomers with increasing biodegradation. Since these isomers may also possess a shielded character they are suggested to be extremely promising molecular indicators of geochemical processes involving oil-water and oil-rock interactions.