DISTRIBUTED ACTIVATION-ENERGY MODELS OF ISOMERIZATION-REACTIONS FROM HYDROUS PYROLYSIS

Distributed activation energy models of sterane and hopane isomerisati on have been developed using time series of hydrous pyrolysis experime nts at 300, 320 and 350-degrees-C and a formalised optimisation proced ure against field data. Frequency factor (s-1), mean activation energy (Ea, kcal/mol) and distribution factor (% of Ea) of hopane isomerisat ion are 1.0E8, 37.0 (+/-0.5) and 2.9%, those 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 r ates, which constitutes a considerable improvement on most previous mo dels. The mean (50% of the reaction completed) transformation temperat ure of the sterane isomerisation model at 2.5-degrees-C/Ma is 104 +/- 9-degrees-C; the corresponding temperature for hopane isomerisation is 91 +/- 3-degrees-C. Verification against an independent set of field data suggests furthermore that temperatures can be determined at an ac curacy of +/-7.5-degrees-C and that the model temperatures tend to be too low by approximately 5-degrees-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 mod els match the whole reaction interval better and respond in a more dif ferentiated way to complex thermal histories. Further refinement of th e models using sequential- or competing mechanisms would lead to impro ved performance only in case of thermal histories with strong emphasis on back reactions.