Assessing the overlap of primary and secondary reactions by closed- versus open-system pyrolysis of marine kerogens

Abstract

The evolution of various petroleum fractions during programmed-temperature pyrolysis of immature samples from the Toarcian Shale (Germany) and the Duvernay Formation (Canada) at a heating rate of 5.0 K min −1 has been studied comparatively under closed- and open-system conditions using the micro-scale sealed vessel (MSSV) technique in the former and a modified MSSV technique in the latter case. Whereas open-system heating gives raise to continuous primary product generation, closed-system experiments are marked by the overlap of primary generation and secondary cracking reactions; the apparent degree of overlap, however, turns out to be different for different compound classes. Primary generated C 15+ (total C 15+ fraction) compounds, in particular, are subjected to severe secondary cracking from the beginning. However, at temperatures lower than 460°C for both samples, secondary cracking essentially transforms C 15+ compounds into C 6–14 (total C 6–14 fraction) compounds. Because these processes take place only within the C 6+ fraction the sum of closed-system C 6+ compounds is close to the quantities of C 6+ compounds generated in the open-system. At temperatures higher than 460°C secondary oil-to-gas cracking becomes obvious from the decrease of C 6+ concentrations. Because the open-system generation of C 6+ compounds has come to an end at this temperature, the overlap of primary and secondary reactions only affects the overall composition of the C 6+ fraction in the closed-system but not the timing of secondary gas formation.