The Qingshankou Formation shale and its associated light oil in the Gulong sag were studied through detailed core observation, thin section analysis, field emission scanning electron microscopy (FE- SEM), energy- dispersive X- ray spectroscopy (EDS), X- ray diffraction (XRD), and gas chromatography. The study revealed that the shale contains abundant diatoms and diatom flocs, which played a significant role in the formation of the light oil. EDS analysis indicates that the silica content of diatoms and diatom flocs is exceptionally high, the Si/Al ratio ranges from 2. 88 to 15. 20, with an average of 6. 65, significantly higher than the Si/Al ratio of illite (1. 82, based on measurements of modern clay). This suggests that a substantial portion of the silica exists as biosilica, specifically biological quartz formed by diatoms through polyamine- induced biomineralization. The biosilica particles are extremely small, with diameters of several microns, and represent the primary source of ultrafine quartz particles (diameter＜3. 9 μm) in the Qingshankou Formation shale of the Gulong sag. The bitumen and oil in the Qingshankou Formation shale are rich in N, with concentrations ranging from 7. 26% to 46. 40%. This indirectly confirms the presence of abundant diatoms, as diatoms are rich in polyamines. Polyamines and fatty acids in diatoms undergo diagenesis transformations to form hydrocarbons: polyamines contribute to the formation of gasoline, while fatty acids contribute to the formation of diesel oil in the Gulong shale light oil. Specifically, diatomaceous polyamines undergo denitration to form N- dimethylpropylamine, which further undergoes denitration and demethylation polymerization to produce alkanes below C12, with C8 as the main peak. Another prominent peak in the Gulong shale light oil is C15, which is formed by the decarboxylation and reduction of C16- dominated fatty acids derived from diatoms. Diatoms also contain a significant proportion of C16 fatty acids, accounting for 55.4% of the total fatty acids. The C15 alkane (the main peak of diesel oil) can be formed from C16 fatty acid after decarboxylation and hydrogenation. In addition, C16 fatty acids can also be converted into C16 and C17 alkanes (45. 57% of the light oil) through methylation and ethylation, respectively. Octadecenoic acid and eicosenic acid in diatoms account for about 33. 4% of the total fatty acids, closely matching the paraffin ratio (30. 70%) of octadecane, nonadecane, eicosane, and heneicosane in the shale oil of Qingshankou Formation. Both qualitative or quantitative analyses demonstrate a strong correspondence between the diesel peak in Gulong shale light oil and the carbon number and composition of modern diatom unsaturated fatty acids. This provides reliable evidence that the diesel fraction of Gulong light shale oil is formed by diatom fatty acids. Gulong shale light oil in Qingshankou Formation is a kind of fractionated light oil and not pyrolytic light oil.