Formation of pristene, pristane and phytane: kinetic study by laboratory pyrolysis of Monterey source rock

The generation of pristane and phytane from Monterey kerogen under both open and closed system pyrolysis conditions can be modelled by an Arrhenius equation with an activation energy of 46.1 kcal/mol and a frequency factor of 1.1 × 1014/min. These data were derived from the pseudo-first-order reaction of early prist-1-ene formation (<60%) in open system pyrolysis. A reported maturity parameter—the Pristane Formation Index (PRI) has been evaluated; the results are: (a) the activation energy of 14.2 kcal/mol for pristane formation is much lower than the value of 46.1 kcal/mol obtained in our open system pyrolysis; and (b) the assumption that there is only one reaction generating pristane may not be correct. The generation of prist-1-ene in open system pyrolysis suggests at least two first-order reactions. In sealed tube pyrolysis, the early formation rates of pristane and phytane are the same, but the decomposition of phytane may be slightly faster than pristane. These results suggest that the ratio of pristane over phytane does not change with maturity for low maturity oils (before peak generation), but with continuing maturation, the ratio of pristane over phytane will increase. Finally, the reason for observing only pristenes in open pyrolysis and neither phytene or phytane are discussed based on a proposed kinetic scheme.