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<p>China possesses the most abundant nonmarine shale oil resources in the world; however, the strong heterogeneity of nonmarine shales has led to unclear mechanisms of shale oil accumulation. In this study, the lower submember of Member 1 of the Paleogene Shahejie Formation (Es<sub>1</sub><sup>L</sup>) in the Raoyang Sag, Bohai Bay Basin was investigated comprehensively using various methods, including organic and inorganic geochemistry, field-emission scanning electron microscopy, low-temperature nitrogen adsorption, mercury injection capillary pressure, and nuclear magnetic resonance. Results show that calcareous shale and calcareous hybrid shale have higher oil-bearing potentials, with average free hydrocarbon (S<sub>1</sub>) contents of 0.92 and 0.84 mg/g respectively, and are considered favorable lithofacies for shale oil accumulation. The accumulation of hydrocarbons in the favorable lithofacies is attributed to high total organic carbon contents (averaging 2.35% and 2.60%, respectively), laminated structures, a high volumetric proportion of macropores and microfractures (averaging 51.51% and 62.68%, respectively), and low fractal dimensions. These favorable lithofacies were deposited mostly in specific paleosedimentary environments characterized by limited terrigenous clastic input, relatively arid paleoclimate, moderate paleosalinity, high paleoproductivity, and strongly reducing water column. Furthermore, we established a shale oil accumulation model for the Es<sub>1</sub><sup>L</sup> by coupling shale lithofacies with paleosedimentary environments, pointing out that shale oil sweet spots in Raoyang Sag are mainly developed in the northwest gentle slope zone, with a burial depth of 3400–3700 m. Finally, a multi-scale and multi-factor accumulation model was proposed, revealing that Chinese nonmarine shale oil accumulates mainly in organic-rich hybrid sedimentary rocks, oil storing laminae (felsic or carbonate laminae), and near-source interbedded reservoirs (siltstone or carbonate interlayers) through three distinct mechanisms: self-generation and self-storage, micromigration and short-distance migration respectively.</p>

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Accumulation mechanisms of nonmarine shale oil in China: A case study of the Shahejie Formation in Raoyang Sag, Bohai Bay Basin

  • 期刊论文

  • 发表日期：

    2025年6月11日

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  • Yongbo Wei,
  • Quanyou Liu,
  • Shuangfang Lu,
  • Rixin Zhao,
  • Zhaojing Song,
  • Yicheng Mou

摘要

China possesses the most abundant nonmarine shale oil resources in the world; however, the strong heterogeneity of nonmarine shales has led to unclear mechanisms of shale oil accumulation. In this study, the lower submember of Member 1 of the Paleogene Shahejie Formation (Es1L) in the Raoyang Sag, Bohai Bay Basin was investigated comprehensively using various methods, including organic and inorganic geochemistry, field-emission scanning electron microscopy, low-temperature nitrogen adsorption, mercury injection capillary pressure, and nuclear magnetic resonance. Results show that calcareous shale and calcareous hybrid shale have higher oil-bearing potentials, with average free hydrocarbon (S1) contents of 0.92 and 0.84 mg/g respectively, and are considered favorable lithofacies for shale oil accumulation. The accumulation of hydrocarbons in the favorable lithofacies is attributed to high total organic carbon contents (averaging 2.35% and 2.60%, respectively), laminated structures, a high volumetric proportion of macropores and microfractures (averaging 51.51% and 62.68%, respectively), and low fractal dimensions. These favorable lithofacies were deposited mostly in specific paleosedimentary environments characterized by limited terrigenous clastic input, relatively arid paleoclimate, moderate paleosalinity, high paleoproductivity, and strongly reducing water column. Furthermore, we established a shale oil accumulation model for the Es1L by coupling shale lithofacies with paleosedimentary environments, pointing out that shale oil sweet spots in Raoyang Sag are mainly developed in the northwest gentle slope zone, with a burial depth of 3400–3700 m. Finally, a multi-scale and multi-factor accumulation model was proposed, revealing that Chinese nonmarine shale oil accumulates mainly in organic-rich hybrid sedimentary rocks, oil storing laminae (felsic or carbonate laminae), and near-source interbedded reservoirs (siltstone or carbonate interlayers) through three distinct mechanisms: self-generation and self-storage, micromigration and short-distance migration respectively.