Computer modeling of maturation processes is now a routine part of exploration programs. Sophisticated distributed activation-energy models of hydrocarbon generation from source rock kerogens, valid over a wide range of geological heating rates, are available. Methods for modeling vitrinite reflectance evolution still, in most instances, seem to be based on Lopatin's approach. Though widely used, this method is not applicable over all geologic heating rates and is of a very different numerical formalism to that being employed in the most sophisticated kinetic models of hydrocarbon generation.
I describe a new kinetic approach to the modeling of vitrinite reflectance based on calibration from direct chemical measurements of maturing vitrinite kerogens made by quantitative pyrolysis-gas chromatography. This approach uses distributed kinetic parameters to determine the evolution of real chemical systems, such as the change in phenolic and other aromatic species concentrations, in the evolving vitrinite macromolecule, as a function of thermal history. For example, the decrease in phenol precursor concentrations in vitrinite kerogens (as determined by py-gc) over the maturity range 0.45% Ro to 1.6% Ro can be modeled using a Gaussian distribution of activation energies with an average activation energy of 186 kJ/mole, a standard deviation of 7% of the mean, and a mean pre-exponential factor of 2.786 E22/m.y.
Predicted phenol concentrations are correlated to vitrinite reflectance using the calibration data set. The advantages of this new approach to the maturation modeling of rapidly subsiding basins in the United States Gulf Coast, mid-western Europe, and elsewhere are discussed. Numerically more intensive than Lopatin's method, the approach can be employed successfully on modern personal computers.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.