Geochemical Fossils in Crude Oils and Sediments as Indicators of Depositional Environment and Geological History

Abstract

Petroleum contains a diastereomeric doublet for each of the C25 to C29 tricyclic terpanes due to stereoisomerization at C-22, where the elution order of the 22S and 22R epimers is unknown. Geometry-optimized molecular mechanics models for each pair of epimers show similar calculated total energies, indicating similar thermal stability. Similar stability explains the nearly equivalent size of the 22S and 22R chromatographic peaks for each doublet in nonbiodegraded petroleum. Molecular mechanics MM + and COMPASS force-field calculations indicate an abrupt conformational change between the C28 and C29 tricyclic terpanes, corresponding to a discontinuity on plots of molecular mass versus log of gas chromatographic retention time. The second-eluting peak in each C26 to C29 doublet is more readily biodegraded, with (Alberdi, M., Moldowan, J.M., Peters, K.E., Dahl, J.E., 2000. Stereoselective biodegradation of tricyclic terpanes in heavy oils from Bolivar Coastal Fields, Venezuela. Submitted to Organic Geochemistry) or without microbial demethylation to form 17-nor-tricyclic terpanes. Factors controlling the chromatographic elution order of epimers are not fully understood. However, elution order can be inferred if one assumes that epimers with greater calculated surface areas are more susceptible to microbial attack, as for the extended hopanes where C-22 epimer elution order is known. Surface areas of 22R epimers exceed 22S for C25 to C29 tricyclic terpanes, suggesting that the 22R epimers elute after 22S. Proof of elution order will require co-injection of authentic standards. Four epimers are possible for each of the C30 and C31 tricyclic terpanes. Molecular mechanics and high-resolution chromatography suggest that all four peaks occur in petroleum, but only two are normally observed due to co-elution. Complete resolution of these epimers will require improved chromatographic methods. (C) 2000 Elsevier Science Ltd.