Fractionation of benzocarbazoles between source rocks and petroleums

Abstract

The concentrations of the [a] and [c] isomers of benzocarbazoles in a number of crude oils and source rock bitumens were obtained by “in house methods” and from reported published data. The changes in benzocarbazole concentrations and the benzocarbazole [a]/([a]+[c]) ratio relating to source maturity, facies and migration are described. With increasing source maturity in North Sea oils and source rocks, up to 0.85% vitrinite reflectance equivalent, benzocarbazole concentrations increase due to generation followed by a decrease above 0.85% vitrinite reflectance equivalent due to expulsion/dilution by newly generated hydrocarbons. Comparison of benzocarbazole concentrations between reservoired petroleum and related source petroleum revealed a concentration bias in favour of source petroleum where concentrations of benzocarbazoles reached up to 10 times that found in equivalent reservoired petroleum. The differences in benzocarbazole concentrations between related oils and source rocks indicate that the major fractionation is introduced during primary migration and expulsion, because when secondary migration is limited, as in the Miller field, we see extensive benzocarbazole fractionation between reservoired petroleum and associated source petroleum. The relationships between source rock TOC content and petroleum benzocarbazole concentrations and [a]/([a]+[c]) ratios indicate that source organic matter exerts a strong influence on benzocarbazole distributions. The introduction of a reservoir source fractionation parameter (RSFP), defined as the ratio between the concentrations of benzo[a]-+benzo[c]carbazole in reservoired petroleum versus the concentration of benzo[a]-+benzo[c]carbazole in source petroleum, enables description of the relationship between reservoired petroleum and source petroleum from different petroleum systems. Petroleum deriving from clastic source rocks demonstrated high levels of benzocarbazole fractionation between reservoired petroleum and source petroleum during primary and secondary migration compared to petroleum deriving from carbonate systems. The RSFP parameter may also be used to examine fractionation effects during migration between different compound types. The more polar benzocarbazoles are fractionated more strongly than dibenzothiophene and phenanthrene between reservoired petroleum and source petroleum. Whereas sometimes source maturity signals do dominate the benzocarbazole signals in migrated oils, in other circumstances, often associated with clastic source rocks, no clear maturity signal is transferred to migrated oils.