Origin of prist-1-ene and prist-2-ene in kerogen pyrolysates

The principal isoprenoid hydrocarbons in flash pyrolysates from immature kerogens are prist-1-ene and, to a lesser extent, prist-2-ene. The precise origin of these compounds is still a matter of debate. Chemical degradation experiments performed on a sulfur-rich kerogen in this study show that at least two precursors can generate pristenes during pyrolysis of sulfur-rich kerogens: ether-bound precursors, probably tocopherols, and a sulfur-bound precursor. From pyrolysis experiments performed on mixtures of isolated kerogens with different clay minerals, it is concluded that prist-2-ene is likely formed by the double-bond isomerization of prist-1-ene. This isomerization of prist-1-ene into prist-2-ene depends on the amount of protons available for the formation of the intermediate carbonium ion. These protons can be derived from acidified aluminosilicates (e.g. the clay minerals montmorillonite or kaolinite) in the rock or from inorganic acids (e.g. HI, HCl). The degree of isomerization depends on the amount of protons available relative to the total organic carbon content.