ORGANIC GEOCHEMISTRY OF VENEZUELAN EXTRA-HEAVY CRUDE OILS .2. MOLECULAR ASSESSMENT OF BIODEGRADATION

The extent of biodegradation of extra-heavy crude oils from the Eastern Venezuelan basin (Orinoco "Tar" Belt) has been assessed using biological marker compounds which include steranes, triterpanes, aromatic steroids and phenanthrenes (analyzed by GC-MS) and petroporphyrins (analyzed by HPLC). The analyses indicate that biodegradation takes place in two regions, namely "surface or close to surface" and "subsurface", with a different pattern of biodegradation taking place at each type of location. Some oils show patterns of biomarkers which may be the combined result of degradation at both main sites of biodegradation. However, two empirical biodegradation parameters are proposed to achieve this classification. The classification of crude oils in terms of degree of biodegradation is further complicated by the identification of "mixed crudes".