An integrated organic-inorganic geochemical characterization of paleogene sediments in no.l structural belt of the nanpu sag, bohai bay basin, eastern china: Implications for the origin of organic matter

摘要

Paleogene sediments, especially the third member of the Don-gying Formation (Ed_3 ) and the first and third members of the Shahejie Formation (E_(S1) and E_(S3) ), have been regarded as the most important source rocks in the Nanpu Sag. Organic and inorganic analyses, including Rock-Eval pyrolysis, gas chroma-tography-mass spectrometry, and element geochemistry, in 91 mudstone samples, were used to reconstruct the palaeoenviron-mental conditions, such as palaeoclimate, palaeo-salinity and palaeo-redox conditions, and to recognize the origin of organic matter. The results show that E_(S3) has a higher TOC content than E_(S1) and E_(d3). Hydrocarbon genetic potential (S_1 + S_2 ) of the samples indicate fair to good hydrocarbon potential. The kerogen type of E_d3 and E_S1 source rocks are Type Ⅱ_1-Ⅱ_2, while E_(S3) source rocks are dominated by Type Ⅱ_2 -Ⅲ kerogens. Bio-markers 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. E_(d3) source rocks are characterized by low G/ C_(30))H (gamacerane/C_(30)hopane) (＜0.1), TT/C_(30)H (tricyclic terpane/C_(30)ohopane) and S/H (serane/hopane), high Pr/Ph ( pristane/ phytane) and C_(24) TeT/C_(23)TT (C_(24)tetracyclic terpane/C_(23)tricyclic terpane), indicating mixed input of both algae and terrestrial higher plants, dominated by terrestrial higher plants. E_(S1) source rocks display medium G/C_(30)H, TT/C(30) H, S/H, Pr/Ph and C(24)TeT/C_(23) TT, indicative of a mixed input of both algae and terrestrial higher plants. E_(S3) source rocks are characterized by high G/C_(30)H (＞0.1), TT/C_(30) H and S/H, low Pr/Ph and C_(24)TeT/C_(23)TT, typical of a mixed input of algae and terrestrial higher plants, with algal dominance. E_(d3), E_(S1) and E_(S3) 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.