Oleanane or lupane? Reappraisal of the presence of oleanane in Cretaceous-Tertiary oils and sediments

摘要

Nonpolar GC-columns are normally used for GC-MS analysis of saturate fractions from crude oils. Under these conditions lupane coelutes with oleananes and some minor C-30 compounds of non-terrigenous origin having lupane-like mass spectra. Because any peak eluting fractionally earlier than 17alpha,21beta(H)-hopane in the m/z 191 mass chromatogram is routinely assigned to oleanane, the presence of lupane may sometimes have been overlooked. Lupane and oleananes are easily separated using reverse phase HPLC. Triterpane concentrates from 10 crude oils known to contain oleananes were HPLC-separated. Lupane was unambiguously identified in six of the oils by full-scan GC-MS analysis of fractions having the HPLC-retention time of authentic lupane. GC-separation of lupane and oleananes is possible using polar GC-columns having a polyethylene glycol stationary phase [Organic Geochemistry, 23 (1995) 211, allowing estimation of the lupane/(lupane + oleanane) ratio. High ratios were measured in the Marraat oil, West Greenland (0.8) and the Amauligak oil from the Beaufort-Mackenzie Delta (0.26). The results suggest that lupane may be more frequently occurring than previously thought, and that this compound is preferably associated with high latitude samples. High concentrations of 28-nor- and 24,28-bisnortriterpanes (oleananes, lupanes and taraxastanes) were detected in the Marraat oil. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 81]