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If a first order reaction can be assumed for kerogen maturation during burial diagenesis, then its reaction rate constant is k = -ln(f)/t, where f is the fraction of kerogen transformable to hydrocarbon remaining after some functional reaction duration, t. The fraction of reactive kerogen is estimated from Tissot and Espitalie's model of vitrinite reflectance (Ro) evolution. A method for calculating the functional reaction duration is suggested by kerogen maturation experiments that show hydrocarbon generation proceeds by concurrent reactions with successively higher activation energies (Ea), which at a given temperature: (1) are already complete and not generating products; (2) are generating significant products; or (3) are slow and will not genera e significant products in geologic time. The general correlation of Ro with maximum temperature suggests that at a given temperature, only a limited suite of reactions control hydrocarbon generation, and increased time at that temperature will not make the slower (high Ea) reactions geologically significant. Thus, the functional reaction duration cannot exceed the time necessary for the controlling reactions to essentially complete hydrocarbon generation (to the 99% level). Geologic field data, and kerogen maturation experiments extrapolated to geologic time and temperature ranges, suggest this occurs in 106-107 years.
When plotted on an Arrhenius diagram (ln k versus l/T), reaction rate constants calculated for 80 cases of kerogen maturation at maximum temperature show a strong linear relationship (r = 0.77). The pseudo Ea of the overall kerogen maturation reaction is about 9 kcal/mole, and its frequency factor is 10-11 sec-1. This curve provides a method of assessing maximum paleotemperature from Ro if the kerogen has had sufficient time to stabilize.
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