Hydrogen isotope ratios of aliphatic and diterpenoid hydrocarbons in coals and carbonaceous mudstones from the Liaohe Basin, China

Hydrogen-isotope compositions of the aliphatic and diterpenoid hydrocarbons were determined for five coal and carbonaceous mudstone samples collected from drilling cores (1531–1767 m depths) in the Liaohe Basin, China. The bulk organic materials were mainly derived from terrestrial higher plants. δD values for most of the n-alkanes varied from −150‰ to −220‰, and were not significantly different among the samples. Pristane was 34–69‰ depleted in D relative to phytane; both pristane and phytane, however, had the same trend of variation in δD from sample to sample. Diterpenoids were on average 49–81‰ depleted in D relative to the n-alkanes. Variations in δD also occurred between different diterpenoids, indicating a different source for these compounds. An enrichment process for the heavy hydrogen isotope was observed as expected when a compound was progressively altered through diagenesis (especially the dehydrogenation process). Overall, δD and δ13C showed distinct patterns between structurally different lipid classes, although possible hydrogen exchange cannot be completely excluded during maturation. Our results further support the notion that hydrogen isotopes of lipid biomarkers from ancient sediments can be used to assess the origin of the organic matter, to determine oil-source rock correlation, and perhaps to reconstruct the paleoenvironment under which the organic material was deposited.