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

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 degrees 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 degrees 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 bur not the timing of secondary gas formation. (C) 2000 Elsevier Science B.V. All rights reserved.