Modelling petroleum formation from heterogeneous source rocks: the influence of frequency factors on activation energy distribution and geological prediction

For terrestrial and heterogeneous marine source rocks the frequency factors systematically increase with increasing level of transformation, while such an increase was insignificant or absent when homogeneous source rocks were studied. By applying geological heating rates it was possible to illustrate the significance of the extended calculation of individual frequency factors. The predicted onset temperature of petroleum formation was displaced to 60-70 °C in case of lignites/coals and type II-S source rocks, which was up to 30-40 °C lower than predicted by the use of a conventional single frequency factor model. To a lesser extend such a shift was also documented for some marine shales (e.g. Draupne Formation and the Duvernay Formation). The reason for this phenomenon is that an average single frequency factor suppresses the initial formation of petroleum at geological heating rates because high activation energies are needed to maintain the kinetic equation at low generation temperatures. In contrast, correct activation energies derive from individual frequency factors which keep the activation energies low at low levels of organic matter transformation and result in a more reliable description of the low energy bonds in the heterogeneous organic matter types.