In order to clarify the differences in hydrocarbon-generating precursor and mechanism of the shale oil between the upper and lower sweet spots of the Lucaogou formation,the source rocks of the Lucaogou formation in the Jimsar sag were characterized ultra-microbi-ologically using field emission scanning electron microscopy,electron probe,and Fourier transform infrared spectroscopy experiments.The results show that the main hydrocarbon-generating precursor of the shale oil in the upper sweet spot is lamalginite(Microcystis),with straight-chain aliphatic series in dominance,and the main hydrocarbon-generating precursor in the lower sweet spot is telalginite(Tasma-nian algae),which is rich in branched-chain aliphatic,aromatic,and sulfoxide functional groups.Due to the significantly higher activation energy required for the cleavage of long straight-chain saturated hydrocarbons than that for branched-chain hydrocarbons,as well as the lower bond energies of carbon-sulfur and carbon-nitrogen bonds,the activation energy of the precursor of the shale oil in the lower sweet spot is lower than that in the upper sweet spot.Consequently,early-stage hydrocarbon generation occurs,leading to the formation of high-density crude oil rich in non-hydrocarbon bitumen at low maturity,which is the primary reason for the relatively heavy and viscous nature of the crude oil in the lower sweet spot.
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