Changes in the molecular structure of a Type II-S kerogen (Monterey Formation, U.S.A.) during sequential chemical degradation

Flash pyrolysis and sequential chemical degradation were combined to study the molecular composition of an immature Type II-S kerogen from the Miocene Monterey Formation. Firstly, base hydrolysis was performed in order to hydrolyse ester bonds, in the second step aliphatic ethers were cleaved and in the third step sulfur–sulfur and sulfur–carbon bonds in the kerogen were broken. Linear and isoprenoid alkanes and alkenes were partially released upon cleavage of ether-bonds and are probably derived from n-alkyl and isoprenoid algaenans, respectively, biosynthesized by marine algae. The precursor moieties of the alkylthiophenes generated upon pyrolysis were released upon cleavage of sulfide-bonds indicating that their precursors, probably sugars, were sulfur-bound to the kerogen. Upon ether- as well as sulfur-bond cleavage, alkylpyrroles were released indicating that their precursors, probably tetrapyrrole pigments, occur as ether- as well as sulfur-bound in the kerogen. Furthermore, prist-1-ene as well as tocopherols were removed from the flash pyrolysate after ether-bond cleavage, indicating that ether-bound tocopherols are probably a major source of prist-1-ene in kerogen pyrolysates. Furthermore, our degradation approach resulted in the recognition of specific ester-, ether- and sulfur-bound low-molecular-weight biomarkers attached to the kerogen. However, the release of these biomarkers into the extract has hardly any impact on the residue pyrolysate and thus most of the compounds identified in the extracts are probably less important constituents of the kerogen.