Aryl isoprenoids and diaromatic carotenoids in Paleozoic source rocks and oils from the Western Canada and Williston Basins

Abstract

Aryl isoprenoids in the range C 13–C 31 have been identified in source rock bitumens and oils from several Paleozoic-age petroleum systems in the Western Canada and Williston Basins. The principal compounds possess the 2-alkyl-1,3,4-trimethyl substitution pattern characteristic of diaromatic carotenoids found in the Chlorobiaceae family of photosynthetic sulfur bacteria. Two C 40 perhydrodiaromatic carotenoids which are structurally related to the presumed precursors of the aryl isoprenoids have also been identified in oils and source rock bitumens of the Duvernay (Late Devonian) petroleum system. Pyrolysis/gas chromatograms of kerogens isolated from several Paleozoic source rocks show an unusual abundance of 1,2,3,4-tetramethylbenzene. It is proposed that this compound arises from β-cleavage of diaromatic carotenoid moieties which have been incorporated into the kerogen structure. With increasing maturation, concentrations of the aryl isoprenoids in Duvernay source rock bitumens increase to a point, whereas the C 40 perhydrodiaromatic carotenoids decrease steadily. Previous studies have suggested that aryl isoprenoids may be indicators of enhanced-salinity environments. This is not supported by the present work, which shows high concentrations in source rocks associated with presumed normal marine salinities. There is, however, a general association with intervals of condensed clastic or mixed clastic/carbonate sedimentation within carbonate depositional systems. From the association found and from parallels to modern habitats of photosynthetic bacteria, it can be inferred that these biomarkers indicate prolific bacterial growth in a stratified water column at and below the oxic-anoxic boundary layer in sub-tropical to tropical marine environments.