Thermal cracking of kerogen in open and closed systems: determination of kinetic parameters and stoichiometric coefficients for oil and gas generation

The purpose of the study is to compare artificial maturation of kerogens representative of the main types of organic matter (kerogen Types I, II, II-S and III) in open and anhydrous closed pyrolysis systems. The generated compounds are fractionated according to their thermal stability into six chemical classes: C1, C2---C5, C6---C14, C15+ saturates, C15+ aromatics and NSOs, which include resins and asphaltenes. In both systems, primary cracking of kerogen can be described by two main reactions: oil generation together with early gas generation followed by late gas production. Mass balances obtained in the two pyrolysis systems are reasonably similar, although some secondary cracking of the NSOs and underestimation of methane potential due to incomplete pyrolysis may occur in the open system. Kinetic parameters of thermal cracking are derived from experiments in the open system using an optimization procedure which determines a unique frequency factor and a weight distribution of chemical classes for a discrete series of activation energies. The resultant frequency factors are checked with those obtained for specific molecular tracers such as n-C12- and n-C24-alkanes generated from kerogens in open and closed systems under isothermal conditions. The results show that using a unique frequency factor as determined by optimization is correct for Type I, II and II-S kerogens. This factor is more questionable, however, when the distribution of activation energies is very broad, such as for Type III coals. Based on this study, a strategy for determination of kinetic parameters on any new source rock sample is proposed.