Reconstruction of Paleoenvironment of the Eocene Source Rocks in the Xihu Sag, East China Sea, Based on Major and Trace Elements and Biomarkers

The Xihu sag of the East China Sea Shelf Basin is dominated by low- to ultra-low-permeability natural gas resources. The Eocene Gemstone Formation and Pinghu Formation are the two main source rock formations in the sag. Although previous studies have extensively addressed their geochemical characteristics and sedimentary facies, the differences in paleosalinity, evolution of redox conditions, and paleoclimate fluctuations are still unclear, which restricts the study of the correlation between low- to ultra-low-permeability resources and paleoenvironment. Here, the mudstone samples of Well N1 in the Pinghu slope zone of Xihu sag were taken as the research object. The main elements were analyzed by X-ray fluorescence spectrometry (XRF), the trace elements were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS), and the saturated hydrocarbon content of biomarkers was analyzed by gas chromatography-mass spectrometry (GC-MS). Based on the analysis results, the paleosedimentary environment of the source rocks of the Baoshi Formation and Pinghu Formation in Well N1 was reconstructed. The results show that Sr/Ba (<0.5), Rb/K and wax index jointly reveal that the sedimentary period of Baoshi Formation and Pinghu Formation is freshwater-brackish water environment. U/Th < 0.75, V/Cr < 2, δU < 1 and Pr/Ph > 2 synergistically indicate that the oxidation environment is dominant, and the elements are consistent with the biomarker parameters. It is confirmed that the oxidation is gradually enhanced from the Baoshi Formation to the Pinghu Formation; the climate indices C > 0.6 and Sr/Cu < 10 support warm and humid sedimentary paleoclimate, with vertical fluctuations. The ratios of steroids to terpenoids C₂₇R/C₂₉R and C₁₉TT/C₂₃TT indicate that the Pinghu Formation received more terrigenous input than the Baoshi Formation. Multivariate geochemical parameters provide important evidence for the reconstruction of paleoenvironment in the Eocene sedimentary period of Xihu sag. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202506060152.