Unusual hexacyclic oleananes in Late Cretaceous/Tertiary terrigenous oils: NMR characterisation of the major hexacyclic oleanane in Niger Delta oil

Abstract

Four hexacyclic oleananes having characteristic mass spectra with an intense fragment at m/z 325 were found in Late Cretaceous/Tertiary terrigenous oils and immature source rocks containing 18α-oleanane and other saturated pentacyclic compounds of higher plant origin. They are sometimes as abundant as 18α-oleanane itself. They were not found in oils without oleanane. The major hexacyclic oleanane, comprising 50–80% of the hexacyclic oleananes, was isolated using high performance liquid chromatography (HPLC) and characterized using nuclear magnetic resonance spectroscopy (NMR) as 18α-oleanane with the Me group at C-10 shifted to C-9 and with the Me at C-14 creating a new methylene bridge to C-10. Hexacyclic oleananes were not visible in m/z 191 chromatograms. They are most effectively detected by way of GC-MS-MS using the m/z 410→325 transition. Since few other compounds produce any m/z 410→325 signal, they may sometimes be easier to detect than regular and rearranged oleananes in samples with a low concentration of angiosperm markers. Precursor natural products have not been identified among plant triterpenoids, but might be related to (poly)functionalized oleanoids. Oleanoids hydroxylated at C-27 are obvious precursors. Oxic/clay-rich depositional environments seem to favour the formation of hexacyclic oleananes. A series of 2-alkylated oleananes had previously been characterized using NMR. Their hexacyclic counterparts were tentatively assigned in this study.