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 keroge ns representative of the main types of organic matter (kerogen Types I , II, II-S and III) in open and anhydrous closed pyrolysis systems. Th e generated compounds are fractionated according to their thermal stab ility into six chemical classes: C-1, C-2-C-5, C-6-C-14, C15 + saturat es, C15 + aromatics and NSOs, which include resins and asphaltenes. In both systems, primary cracking of kerogen can be described by two mai n reactions: oil generation together with early gas generation followe d by late gas production. Mass balances obtained in the two pyrolysis systems are reasonably similar, although some secondary cracking of th e NSOs and underestimation of methane potential due to incomplete pyro lysis may occur in the open system. Kinetic parameters of thermal crac king are derived from experiments in the open system using an optimiza tion 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 obtai ned for specific molecular tracers such as n-C-12- and n-C-24-alkanes generated from kerogens in open and closed systems under isothermal co nditions. The results show that using a unique frequency factor as det ermined by optimization is correct for Type I, II and II-S kerogens. T his factor is more questionable, however, when the distribution of act ivation energies is very broad, such as for Type III coals. Based on t his study, a strategy for determination of kinetic parameters on any n ew source rock sample is proposed. (C) 1997 Elsevier Science Ltd.