H Lo ten Haven

Early diagenesis of terrigenous triterpenoids and its implications for petroleum geochemistry

Abstract The distribution of triterpanes in sediments and crude oils is commonly used for oil/oil and oil/source rock correlation in petroleum geochemistry while isomer ratios of such compounds are applied as maturity parameters. The triterpane pattern in terrestrially influenced geological samples is, however, remarkably less diverse than the triterpenoid skeletons occurring in the extant higher plant biosphere. Natural observations, laboratory experiments and molecular mechanics calculations have revealed that acid catalysed double bond isomerization and skeletal rearrangement reactions of terrigenous triterpenoids during early diagenesis are the key to understanding the triterpane patterns in more matured geological materials. Examples studied in detail are the double bond isomerization of oleanenes and the subsequent hydrogenation of the products into oleanane and 18α-oleanane, and the transformation of taraxer-14-ene and lup-20(29)-ene into oleanenes. Although the reactions are likely to be kinetically controlled, the main products formed and ultimately preserved are the thermodynamically most stable ones. The experimental results and natural observations have significant implications for the use of terrigenous triterpenoid compounds in petroleum geochemistry.

Characterisation of 24-nor-triterpenoids occurring in sediments and crude oils by comparison with synthesized standards

Abstract A series of 24-nor-triterpenoids; 24-nor-urs-12-ene, 24-nor-olean-12-ene and 24-nor-lupane have been identified in sediments and crude oils by comparison with standards obtained by synthesis. A further suite of 24-nor-oleanenes has also been identified by comparison with the acid-catalysed rearrangement products of 24-nor-olean-12-ene. The presence of these components in sedimentary organic matter points to an important new pathway of terrigenous triterpenoid diagenesis.