Abstract

Activation energy, a chemical kinetic parameter calculated for high-temperature pyrolysis reactions, cannot be extrapolated to low-temperature geologic systems via the Arrhenius equation, k = Ae−E/RT, for several reasons. The assumption of temperature independence of A and E is valid only over a short temperature range. Multiple reaction mechanisms may be competing, with one reaction controlling the kinetics in one thermal regime while a different reaction may be controlling in a different thermal regime. Temperature dependence of diffusion, and transition-state reversibility may also result in different kinetic parameters being applicable to a given reaction at different temperatures. Therefore, the observed lower activation energy of a geologic system (compared with laboratory simulations) may be due to theoretical, chemical, and physical variables and cannot be attributed solely or even significantly to clay catalysis. Estimation of geologic temperatures and other geologic parameters from pyrolysis kinetics data is similarly and equally perilous.

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