Towards a new method of geochemical kinetic modelling:implications for the stability of crude oils

Usual geochemical models that describe the thermal decomposition of oils in reservoirs use first order rate laws with activation energies that are constant over the range 100–450°C. Because these empirical models cannot account for numerous observations such as hydrocarbon stability in high temperature reservoirs, we seek to develop a novel non-empirical method. This method describes cracking and alkylation stoichiometric reactions that account for the free radical nature of the reactions taking place in oils, and is thus not empirical. It is illustrated by a simple case that uses a simplified mechanism of hexane pyrolysis. It is shown that below 200°C, rate laws are of order 0.5 in reactant, contrary to what geochemical models imply. It is also shown that the activation energies of the stoichiometric reactions increase as temperature decreases, which is another reason why geochemical models are incorrect. Below about 250°C, the activation energies of cracking stoichiometries are about 70 kcal/mol, and those of alkylation stoichiometries are about 48.5 kcal/mol. The overall pyrolysis rate will also have an apparent activation energy of about 70 kcal/mol. It is shown that according to this model, hydrocarbons in reservoirs should be much more stable than previously thought: the half-life of pure hexane at 180°C is several hundred million years, and it is argued that, contrary to the predictions of usual oil decomposition models, hydrocarbons in crude oils should be stable to at least 180°C, and that the complete cracking of oil to dry gas at 200°C is not possible. A strategy is proposed to extend the present approach and to build a universal model of the secondary cracking of oils that will include about 10 000 reactions.