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 (R_o) 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 (E_a), 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 R_o 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 E_a) 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 E_a 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 R_o if the kerogen has had sufficient time to stabilize.

End_of_Article - Last_Page 451------------