An integrated organic-inorganic geochemical characterization of Paleogene sediments in No.1 Structural Belt of the Nanpu Sag, Bohai Bay Basin, eastern China: implications for the origin of organic matter

Paleogene sediments, especially the third member of the Dongying Formation (Ed(3)) and the first and third members of the Shahejie Formation (Es-1 and Es-3), have been regarded as the most important source rocks in the Nanpu Sag. Organic and inorganic analyses, including Rock-Eval pyrolysis, gas chromatography-mass spectrometry, and element geochemistry, in 91 mudstone samples, were used to reconstruct the palaeoenvironmental conditions, such as palaeoclimate, palaeo-salinity and palaeo-redox conditions, and to recognize the origin of organic matter. The results show that Es-3 has a higher TOC content than Es-1 and Ed(3). Hydrocarbon genetic potential (S-1 + S-2) of the samples indicate fair to good hydrocarbon potential. The kerogen type of Ed(3) and Es-1 source rocks are Type II1-II2, while Es-3 source rocks are dominated by Type II2-III kerogens. Biomarkers and inorganic geochemical indicatives of source rocks, such as Pr/Ph, V/(V + Ni) and Cu/Zn, indicate a lacustrine environment with fresh to brackish water under suboxic to anoxic conditions during deposition. Ed(3) source rocks are characterized by low G/C30H (gamacerane/C(30)hopane) (<0.1), TT/C30H (tricyclic terpane/C(30)hopane) and S/H (serane/hopane), high Pr/Ph (pristane/phytane) and C24TeT/C23TT (C(24)tetracyclic terpane/C(23)tricyclic terpane), indicating mixed input of both algae and terrestrial higher plants, dominated by terrestrial higher plants. Es-1 source rocks display medium G/C30H, TT/C30H, S/H, Pr/Ph and C24TeT/C23TT, indicative of a mixed input of both algae and terrestrial higher plants. Es-3 source rocks are characterized by high G/C30H (>0.1), TT/C30H and S/H, low Pr/Ph and C24TeT/C23TT, typical of a mixed input of algae and terrestrial higher plants, with algal dominance. Ed(3), Es-1 and Es-3 source rocks were mostly deposited in semi-arid to humid-warm climate conditions, with an average temperature higher than 15 degrees C. This study suggests that suitable temperatures, a fresh to brackish lacustrine environment and suboxic to anoxic conditions could result in a high organic matter concentration and preservation, thus providing prerequisites for the formation of high-quality source rocks. Supplementary material: Tables S1-S3 are available at