Petroleum from well sites in the Gifhorn Trough (Lower Saxony, northwest Germany) and the Maracaibo Basin (Venezuela) contained various types of microorganisms capable of degrading crude oils. Under substrate-limiting condition, selectivity of degradation was found, destroying medium-chain n-alkanes (C 20 , C 21 ) at a faster rate than long-chain n-alkanes (C 30 , C 31 ). During degradation the physical parameters of the crude oils (e.g. density, viscosity, average molecular weight) were altered and shifted into the direction of heavy oil. In vitro degraded oil is very similar to oil degraded in nature. Aromatic hydrocarbons and biomarker molecules (steranes and triterpanes) were not degraded. Pyrolysis-GC analysis of asphaltenes revealed no significant changes in the composition of pyrolyzates during biodegradation. There is sufficient evidence that heavy oils — besides some other effects — are generated by the in situ biodegradation of conventional oils.
