Novel tricyclic and tetracyclic terpanes in Tertiary deltaic oils: Structural identification, origin and application to petroleum correlation

摘要

Tricyclic terpanes of the cheilanthane series in crude oils and sediments are routinely used for geochemical correlation. The cheilanthanes are particularly useful to identify crude oils derived from marine-clastic or carbonate source rock as well as input to the source rocks from marine algal or terrigenous organic matter. However, they sometimes have limitations for the correlation of oils expelled from different source rocks that received similar organic matter, (e.g. oils from source rocks dominated by terrigenous organic matter). Novel non-cheilanthane tricyclic terpanes, consisting of C_(21) and C_(25) compounds and a C_(27) tetracyclic terpane were identified in oils from the Assam (India), Beaufort-Mackenzie (Canada), and the Niger (Nigeria), deltas. NMR data reveal that all three compounds have an oleanane-type structure with rings C, D and E intact, while rings A and B have been degraded, leaving a side chain attached to ring C in the case of the tricyclics and only a degraded A ring in the tetracyclic. On the basis of occurrence, NMR structure and co-variance with oleanane abundance, these new tricyclic and tetracyclic compounds are believed to form from A-ring functionalised oleanoid precursors in angiosperm (flowering) plants. The distribution of these compounds in representative oils from the Niger Delta accumulations permits separation of the oils into at least three distinct source rock organofacies.