Reply to comments by S. B. Nielsen and T. Barth on “A chemical kinetic model of vitrinite maturation and reflectance”

A vitrinite maturation model is described that performs well in its intended application of calculating reflectance changes in the subsurface. It is noted that it is not capable of reproducing the observed dependence of Rock-Eval T[sub max] vs. %Ro for samples of different geological maturity. The failure at low %Ro is described. The failure at high %Ro is not as well understood but also is less well characterized and probably less important. The VITRIMAT vitrinite maturation model was designed to simulate high pressure pyrolysis and is not expected to work under Rock-Eval conditions. It has been noted frequently in the literature that rapid pyrolysis at atmospheric pressure does not follow the natural maturation path on the van Krevelen diagram. Natural maturation first evolves mostly H[sub 2]O and CO[sub 2] with hydrocarbons following, while atmospheric pyrolysis takes a more direct course towards pure carbon. This is summarized schematically. Monthioux et al. (1985) were able to simulate natural maturation by using high confining pressures. This implies that, at a minimum, different subsets of a comprehensive kinetic mechanism are appropriate for atmospheric and high pressure pyrolysis. Because the kinetic mechanism was designed to simulate high pressure reactions, it would not be expected to work for atmospheric pressure pyrolysis, e.g., Rock-Eval. In other words, an independent parallel reaction model is not capable of describing pyrolysis of high oxygen-content kerogens at all temperatures and pressures. A more detailed discussion of the failure of the independent parallel reaction model has been presented elsewhere.