Stereoselective biodegradation of tricyclic terpanes in heavy oils from the Bolivar Coastal Fields, Venezuela

Gas chromatography–mass spectrometry (GC–MS) and GC–MS–MS analyses of heavy oils from Bolivar Coastal Fields (Lagunillas Field) show a complete set of demethylated tricyclic terpanes. As is the case for the 25-norhopanes, the demethylated tricyclics are probably formed in reservoirs by microbially-m...

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Bibliographic Details
Published in:Organic geochemistry 2001-01, Vol.32 (1), p.181-191
Main Authors: Alberdi, M., Moldowan, J.M., Peters, K.E., Dahl, J.E.
Format: Article
Language:English
Subjects:
Applied sciences
Biodegradation
Crude oil, natural gas and petroleum products
Crude petroleum
Earth sciences
Earth, ocean, space
Energy
Exact sciences and technology
Fuels
Gas chromatography
Geology and geochemistry. Geological and geochemical prospecting. Petroliferous series
hopanes
Hydrocarbons
Mass spectrometry
oil fields
Paraffins
Prospecting and exploration
Prospecting and production of crude oil, natural gas, oil shales and tar sands
Sedimentary rocks
sterane
tricyclic terpanes
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Summary:Gas chromatography–mass spectrometry (GC–MS) and GC–MS–MS analyses of heavy oils from Bolivar Coastal Fields (Lagunillas Field) show a complete set of demethylated tricyclic terpanes. As is the case for the 25-norhopanes, the demethylated tricyclics are probably formed in reservoirs by microbially-mediated removal of the methyl group from the C-10 position, generating putative 17-nor-tricyclic terpanes. Diastereomeric pairs of tricyclic terpanes are resolved above C24 due to resolution of 22S and 22R epimers, but the elution order of the 22S and 22R epimers is unknown. Early-eluting diastereomers (EE) predominate over late-eluting diastereomers (LE) (C25–C29) in the heavily degraded oils, indicating a stereoselective preference for the LE stereoisomers during biodegradation. Conversely, the LE diastereomers predominate over the EE diastereomers in the 17-nor tricyclic series (C24–C28), indicating that tricyclic terpanes and 17-nor-tricyclic terpanes are directly linked as precursors and products, respectively. A good correlation exists between the destruction of steranes and the demethylation of hopanes and tricyclic terpanes. This suggests that terpane demethylation occurs during sterane destruction and hopane demethylation, although the rate is slower, indicating that tricyclic terpanes are more resistant to biodegradation.
ISSN:0146-6380
1873-5290
DOI:10.1016/S0146-6380(00)00130-3