Abstract

Paleogene sediments, especially the third member of the Dongying Formation (Ed₃) and the first and third members of the Shahejie Formation (Es₁ and Es₃), 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 Es₃ has a higher TOC content than Es₁ and Ed₃. Hydrocarbon genetic potential (S₁ + S₂) of the samples indicate fair to good hydrocarbon potential. The kerogen type of Ed₃ and Es₁ source rocks are Type II₁–II₂, while Es₃ 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. Ed₃ source rocks are characterized by low G/C₃₀H (gamacerane/C₃₀hopane) (<0.1), TT/C₃₀H (tricyclic terpane/C₃₀hopane) and S/H (serane/hopane), high Pr/Ph (pristane/phytane) and C₂₄TeT/C₂₃TT (C₂₄tetracyclic terpane/C₂₃tricyclic terpane), indicating mixed input of both algae and terrestrial higher plants, dominated by terrestrial higher plants. Es₁ 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. Es₃ 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. Ed₃, Es₁ and Es₃ source rocks were mostly deposited in semi-arid to humid-warm climate conditions, with an average temperature higher than 15°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 https://doi.org/10.6084/m9.figshare.c.5227684