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Application of time and temperature evaluations to maturation and hydrocarbon formation requires knowledge of the chemical kinetics of the maturation process. Lopatin and others have used first-order kinetics, assuming linear dependence of maturation on time for a given temperature and have derived a rate constant whose temperature dependence is governed by the Arrhenius equation. This model may be inadequate as maturation data used in such first-order kinetic equations have generally yielded Arrhenius factor activation energies which vary widely with temperature.
We report here a detailed kinetic analysis of the laboratory-simulated maturation of several distinct kerogens having different source organic compositions and utilizing data for the production of CO2, CH4, and higher hydrocarbon gases as a function of time and temperature. In all cases, the dependence of maturation on time departs from linearity. Empirically a dependence on t12/ gives the best fit to the data, indicating possible product inhibition of the maturation process. We develop a simple chain reaction model incorporating this feature for both short and long reaction times. The model yields an effective rate constant which should obey the Arrhenius equation and consistently gives temperature-independent composite activation energies of the same approximate magnitude as is implied by Lopatin's model. The nature of the mineral substrate present with the kerogen influences the rate of maturation, both directly by catalytic action in some cases and indirectly by adsorption of product.
Our results suggest a marked difference in kerogen maturation kinetics between closed and open systems, which must be considered in interpreting and comparing laboratory simulations and which may be of considerable significance for hydrocarbon genesis in the field. This may be reflected in a dependence of the kinetics on lithology and porosity of the source or rock unit.
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