Comment: Suppression of vitrinite reflectance by bitumen generated from liptinite during hydrous pyrolysis of artificial source rock by Peters et al. (2018)

Abstract

Peters et al. (2018) provide new hydrous pyrolysis data in defining the coal-reflectance and the suppressed-reflectance trends that source rocks may follow during their thermal maturation. The difference in the two trends is between hydrous pyrolysis temperatures of 300 to 360 °C for 72-hour durations. As implied in their title the suppressed-reflectance of vitrinite is a result of bitumen generation from liptinite during hydrous pyrolysis. Their experimental results do show that coal reflectance is lower when mixed with isolated liptinite encased in G-class Portland cement. However, there are published and omitted experimental data that they did not include in their interpretations. These are addressed as concerns and include early experiments of mixtures of Liptinite in source rocks mixed with coal that do not show a reduced reflectance of the coal. Earlier publications show that some source rocks with oil-prone Type-II kerogens also follow the suppressed-reflectance trend. Resinites did not cause reductions in coal reflectance. The validity of using G-class Portland cement as an artificial rock is not established and is unlikely to be an appropriate representative of natural shales. Reflectance suppression by free radicals are also feasible and more research is needed to better define the causes of the suppression.