Distributed activation energy models of isomerisation reactions from hydrous pyrolysis

Organic Geochemistry(1993)

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Distributed activation energy models of sterane and hopane isomerisation have been developed using time series of hydrous pyrolysis experiments at 300, 320 and 350°C and a formalised optimisation procedure against field data. Frequency factor (s−1), mean activation energy (Ea, kcal/mol) and distribution factor (% of Ea) of hopane isomerisation are 1.0E8, 37.0 (± 0.5) and 2.9%, hose of sterane isomerisation are 3.5E7, 37.5 (±1.0) and 2.9% respectively. The models are able to predict the reactions at geological as well as experimental heating rates, which constitutes a considerable improvement on most previous models. The mean (50% of the reaction completed) transformation temperature of the sterane isomerisation model at 2.5°C/Ma is 104±9°C; the corresponding temperature for hopane isomerisation is 91±3°C. Verification against an independent set of field data suggests furthermore that temperatures can be determined at an accuracy of ±7.5°C and that the model temperatures tend to be too low by ∼ 5°C at low transformation ratios. The models correlate with apatite fission track (AFTA) temperatures at a coefficient of 0.91 and also tend to be too low at low transformation ratios. In contrast to single-activation-energy models distributed models match the whole reaction interval better and respond in a more differentiated way to complex thermal histories. Further refinement of the models using sequential- or competing mechanisms would lead to improved performance only in case of thermal histories with strong emphasis on back reactions.
sterane isomerisation,hopane isomerisation,hydrous pyrolysis,kinetic models,distributed activation energies,basin modelling
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