Bulk kinetic parameters and structural moieties of asphaltenes and kerogens from a sulphur-rich source rock sequence and related petroleums

Non-isothermal open system pyrolysis-gas chromatography and bulk petroleum generation experiments were performed on type II-S petroleum asphaltenes, source rock asphaltenes and related kerogens to evaluate petroleum composition and generation timing. The results show that, contrary to convention, there are significant differences in the reactivity between petroleum asphaltenes and their related kerogens in the studied type II-S petroleum system and these samples yield kinetic models that predict onset, peak, and completion of petroleum generation under different temperatures. Petroleum asphaltenes and source rock asphaltenes are less stable and show tighter petroleum formation windows than kerogens when extrapolated to geological heating rates. Source rock asphaltene, related kerogen, and oil asphaltene pyrolysates possess structural differences as revealed in the aromaticity and organic sulphur content. Aromaticity was highest in pyrolysates of source rock asphaltenes, intermediate in kerogen pyrolysates, and lowest in pyrolysates from petroleum asphaltenes. In addition, the highest organic sulphur content was identified in kerogen pyrolysates, whereas pyrolysates of both source rock and petroleum asphaltenes are similar in organic sulphur content. Thus, the amount of organic sulphur is not proportional to the reactivity of petroleum generation from asphaltenes at lower thermal stress. It is more likely that kerogens and asphaltenes possess different configurations and types of sulphur bonds resulting in cracking of covalent bonds at different levels of thermal stress.