Biodegradation of crude oil in the Aquitaine basin

Abstract

Two crude oils from the Aquitaine basin (SW France) have been subjected to in vitro biodegradation with the bacterium Pseudomonas oleovorans for periods ranging from five days to three months. A rapid disappearance of the linear, branched and isoprenoid structures is observed in the alkane fraction. The degree of alteration is highly dependent upon the quality of the initial material but the residual oils, obtained after biodegradation of either immature or moderately mature crudes, display a virtually identical alkane distribution. Such a distribution is also observed in natural asphalts i.e. heavily biodegraded crudes from the Aquitaine tar belt or in crude oils biodegraded in soils from well sites. This alkane distribution displays a high concentration of tri-, tetra- and pentacyclic alkanes. These polycyclic alkanes, organized into several ubiquitous families in the Aquitaine crudes, remain unchanged by bacterial attack either in the laboratory or in nature (soils, boreholes). They therefore represent several classes of compounds that should be of potential interest to correlate unaltered to biodegraded crudes in other basins. Among the families of polycyclics which are ubiquitous in the Aquitaine crudes, we find a novel group of 4 low mol wt steranes (a C21 and a C2) sterane, a C22 and a C23 4 methylsterane), a series of tricyclic terpanes (C19C26) a series of tetracyclic terpanes (C24C26) and the α-β hopane series (C27C35).