Algal-derived polycyclic aromatic hydrocarbons in Paleogene lacustrine sediments from the Dongying Depression, Bohai Bay Basin, China

The sources and origins of polycyclic aromatic hydrocarbons (PAHs) in thirty eight Paleogene organic-rich shales and mudstones from the Shahejie Formation in the Dongying Depression, Bohai Bay Basin have been assessed. Phenanthrene, retene, cadalene, fluoranthene, pyrene, benz[a]anthracene, triphenylene, chrysene, benzofluoranthenes, benzo[e]pyrene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, benzo[ghi]perylene and coronene were identified in the sediments. The symmetrical 4-ring triphenylene was unequivocally identified by comparison with mass spectra and a synthetic standard. Key PAH ratios including fluoranthene/(fluoranthene + pyrene), benzo[a]anthracene/(benz[a]anthracene + chrysene), indeno[1,2,3-cd]pyrene/(indeno[1,2,3-cd]pyrene + benzo[ghi]perylene) and benzofluoranthene/(benzofluoranthene + benzo[e]pyrene) indicate a mainly diagenetic/catagenetic origin for the PAHs. The relative low abundance of the 6-ring indeno[1,2,3-cd]pyrene and benzo[ghi]perylene and the 7-ring coronene suggest the absence of larger, high temperature wildfires, either due to inadequate seasonality of the humid-arid climate, or limited terrigenous organic matter input. Phenanthrene and alkylphenanthrenes are likely derived from phytoplankton, based on the correlation of the relative abundance of C₂₇ regular steranes and 1-methylphenanthrene and 9-methylphenanthrene. The methylphenanthrenes, triphenylene, alkyltriphenylenes and benzo[e]pyrene may share a common source as their occurrences are closely correlated. Four groups of samples were differentiated on the basis of the relative abundance and distribution patterns of benz[a]anthracene, triphenylene and chrysene, showing that these compounds are useful PAH markers in petroleum geochemistry. Thermal maturation effects are the major control on the relative abundance and distribution of the non-combustion derived PAHs (e.g. biphenyl, fluorene, fluoranthene, pyrene, benz[a]anthracene, triphenylene) in lacustrine sediments. Benz[a]anthracene decreases in relative abundance while triphenylene increases in relative abundance with burial depth. Correlation of the newly defined triphenylene-chrysene ratio (TCR; triphenylene/(triphenylene + chrysene) with burial depth and biomarker maturity parameters was observed, suggesting that it may be a useful maturity indicator for lacustrine oils and sediments in the Dongying Depression.