Isomerization and aromatization of hydrocarbons in sedimentary basins formed by extension

Abstract

Summary. The reactions involved in oil generation are of great economic importance, but remain to be studied in detail. We have investigated the rates of three reactions which occur before and during the early stages of oil formation, and have used the predicted thermal and subsidence history of stretched basins to estimate the six reaction constants. Two of the reactions are isomerization reactions, at C-20 in a sterane and at C-22 in a hopane hydrocarbon. The third reaction converts C-ring monoaromatic to triaromatic steroid hydrocarbons. No single measure of maturity can describe the progress of these reactions. In old basins, such as the North Sea, both isomerization reactions are almost complete before appreciable aromatization has occurred, whereas in young basins, such as the Pannonian Basin in Hungary, aromatization is almost complete before appreciable isomerization of the steranes has occurred. We show that the calculated progress of these reactions agrees well with that observed in both basins if the frequency factors and activation energies are 6 x 10-3 s-1 and 91 kJ mol-1, 0.016 s-1 and 91 kJ mol-1, 1.8 x 1024 s-1 and 200 kJ mol-1 for the isomerization of steranes, of hopanes, and the aromatization of steroid hydrocarbons respectively. The rate of conversion of the R to the S form was taken to be 1.174 and 1.564 times that of the reverse reactions for sterane and hopane isomerizations respectively, and the aromatization reaction was assumed to be irreversible. All three reactions were assumed to be first order and unimolecular. The aromatization rate is consistent with laboratory observations. The rate of hopane isomerization is not, and different reaction mechanisms probably dominate at different temperatures. The same constants can be used to predict the progress of the reactions in basins which have been uplifted by inversion, such as the Lower Saxony Basin in West Germany. The geochemical observations provide estimates of the amount and time of uplift which agree with those from geological studies. Geochemical observations from the eastern part of the Paris Basin suggest that this region has also been uplifted by between 1 and 2 km.