Free and sulphurized hopanoids and highly branched isoprenoids in immature lacustrine oil shales

Abstract

A study of the solvent extracts of four samples from two immature oil shales from Tertiary lacustrine basins, Ribesalbes and Campins (southern European rift system), deposited under reducing conditions, has allowed the identification of S-containing hopanoids and novel highly branched isoprenoids (HBI). HBI derivatives occur only in the Ribesalbes shales where two new structural types of C25 S-containing HBI isomers have been identified. These compounds probably have different precursor organisms from those in recent marine systems. They are present as thiophenes, thiolanes and thiolane sulphoxides in one case and only as thiolanes in the other. 17,21(H)-, 17,21(H)- and 17,21(H)-thienyl- and methylthienylhopanes have been found in the shales from the two basins. In the Campins samples these compounds occur together with thiolanyl- and methylthiolanylhopanes. Distributions of 17,21(H)- and 17,21(H)-hopanes, hop-17(21)-enes and 2-methylhop-17(21)-enes (Ribesalbes only) are also encountered. Comparison of the hopane distributions from both solvent-extractable aliphatic and thiophene fractions and the sulphur- and oxygen-bound macromolecular matter in Ribesalbes Basin shows a remarkable similarity between all fractions. This resemblance points to rather uniform inputs and depositional processes for the accumulation of these compounds both as small molecules or bound to macromolecules, e.g. early bonding of bacteriohopanepolyols to sulphur and to the macromolecular organic matter. The good correlation among the macromolecular fractions in Ribesalbes Basin is reinforced when comparing the 13C composition of phytane, C20 and C25 HBI and the C35 hopane homologues in the different fractions.