Sedimentary environment and a development model for source rocks rich in the green alga <italic>Pediastrum</italic>: a case study of the paleogene anjihaihe formation in the junggar basin, northwest china.

In this study, <italic>Pediastrum</italic> has been observed in the Anjihaihe Formation (E2+3a) shale through organic petrology studies. Additionally, analyses of total organic carbon and rock pyrolysis (Rock–Eval) revealed that the E2+3a shale predominantly comprises oil-bearing type I and II kerogen at a low-maturity stage. The organic matter (OM) in the shale is primarily contributed by <italic>Pediastrum</italic>, indicating excellent original hydrocarbon generation potential. Hydrous pyrolysis was conducted on a <italic>Pediastrum</italic>-rich sample, and the liquid product was analyzed using gas chromatography–mass spectrometry (GC–MS) of the saturated fraction. It was discovered that <italic>n</italic>-C27 might serve as a characteristic biomarker indicating <italic>Pediastrum</italic> development. During warm and humid climatic conditions, rivers transport significant amounts of freshwater and terrigenous organic matter (OM) into the lake, leading to an increase in felsic minerals and a relative decrease in carbonate minerals. This, in turn, reduces water salinity, resulting in a relative increase in the abundance of <italic>Pediastrum</italic>. Conversely, during earlier hot and arid climatic conditions, the input of terrigenous OM decreases, and evaporation leads to increased salinity. When the water’s salinity exceeds the “salinity critical point”, the Pediastrum content decreases. Finally, this study presents a formation and evolution model for the E2+3a shale in the Junggar Basin.Graphical Abstract: In this study, <italic>Pediastrum</italic> has been observed in the Anjihaihe Formation (E2+3a) shale through organic petrology studies. Additionally, analyses of total organic carbon and rock pyrolysis (Rock–Eval) revealed that the E2+3a shale predominantly comprises oil-bearing type I and II kerogen at a low-maturity stage. The organic matter (OM) in the shale is primarily contributed by <italic>Pediastrum</italic>, indicating excellent original hydrocarbon generation potential. Hydrous pyrolysis was conducted on a <italic>Pediastrum</italic>-rich sample, and the liquid product was analyzed using gas chromatography–mass spectrometry (GC–MS) of the saturated fraction. It was discovered that <italic>n</italic>-C27 might serve as a characteristic biomarker indicating <italic>Pediastrum</italic> development. During warm and humid climatic conditions, rivers transport significant amounts of freshwater and terrigenous organic matter (OM) into the lake, leading to an increase in felsic minerals and a relative decrease in carbonate minerals. This, in turn, reduces water salinity, resulting in a relative increase in the abundance of <italic>Pediastrum</italic>. Conversely, during earlier hot and arid climatic conditions, the input of terrigenous OM decreases, and evaporation leads to increased salinity. When the water’s salinity exceeds the “salinity critical point”, the Pediastrum content decreases. Finally, this study presents a formation and evolution model for the E2+3a shale in the Junggar Basin. [ABSTRACT FROM AUTHOR]