TY - JOUR
T1 - Degradation of organic matter by radioactive radiation in black shales
T2 - An overlooked modification of organic molecular structures
AU - Bian, Leibo
AU - Zhao, Zhe
AU - Wang, Xiaomei
AU - Sanei, Hamed
AU - Chappaz, Anthony
AU - Dong, Jin
AU - Dong, Zhongtian
AU - Xie, Linfeng
AU - Schovsbo, Niels H.
AU - Goodarzi, Fariborz
AU - Zhang, Shuichang
AU - Zhao, Wenzhi
N1 - Publisher Copyright:
© 2025 Elsevier B.V.
PY - 2025/9/18
Y1 - 2025/9/18
N2 - Natural radioactive radiation emitted by uranium (U) in sedimentary basins continuously transforms organic matter and thus its degrading impact overprints the burial-related thermal maturation process. However, the systematic pathway for irradiated alteration of sedimentary organic matter and its influence remain poorly understood. This omission hinders a comprehensive understanding of organic matter evolution after burial. Our study investigated the artificially and naturally irradiated alteration of organic matter in the U- and organic-rich Cambro-Ordovician Alum Shale of Northern Europe and Triassic Chang 7–3 shale in the Ordos Basin of China. After exposure of samples to a Co-60 source, radiation accelerated the conversion of bitumen to natural gas with a lower gas dryness (C1/C1–5 < 0.8), while kerogen is converted to natural gas with a higher gas dryness (>0.8). Moreover, enhanced radiation dose facilitates the transformation from resins and asphaltenes to aromatics and leads to enrichments of carbon-13 isotope, when the radiation dose reaches the thresholds of approximately 100 and 2000 kGy, respectively. Natural radiation exposure in the Chang 7–3 shales, identified by spatial distribution and affiliated phases of U, may have influenced bitumen structures through accelerating transformation of macro- to micro-organic compounds and generation of wet gas that could have enhanced hydrocarbon mobility. Moreover, an addition to the standard model for organic matter burial and transformation is proposed with integration of natural radiation exposure. Based on the calculated Proterozoic-Phanerozoic radiation dose in sedimentary rocks, we hypothesize that the radioactive processes might have consistently modified the characteristics of retained bitumen in uranium-enriched shales, thereby affecting carbon cycle on Earth's surface environments.
AB - Natural radioactive radiation emitted by uranium (U) in sedimentary basins continuously transforms organic matter and thus its degrading impact overprints the burial-related thermal maturation process. However, the systematic pathway for irradiated alteration of sedimentary organic matter and its influence remain poorly understood. This omission hinders a comprehensive understanding of organic matter evolution after burial. Our study investigated the artificially and naturally irradiated alteration of organic matter in the U- and organic-rich Cambro-Ordovician Alum Shale of Northern Europe and Triassic Chang 7–3 shale in the Ordos Basin of China. After exposure of samples to a Co-60 source, radiation accelerated the conversion of bitumen to natural gas with a lower gas dryness (C1/C1–5 < 0.8), while kerogen is converted to natural gas with a higher gas dryness (>0.8). Moreover, enhanced radiation dose facilitates the transformation from resins and asphaltenes to aromatics and leads to enrichments of carbon-13 isotope, when the radiation dose reaches the thresholds of approximately 100 and 2000 kGy, respectively. Natural radiation exposure in the Chang 7–3 shales, identified by spatial distribution and affiliated phases of U, may have influenced bitumen structures through accelerating transformation of macro- to micro-organic compounds and generation of wet gas that could have enhanced hydrocarbon mobility. Moreover, an addition to the standard model for organic matter burial and transformation is proposed with integration of natural radiation exposure. Based on the calculated Proterozoic-Phanerozoic radiation dose in sedimentary rocks, we hypothesize that the radioactive processes might have consistently modified the characteristics of retained bitumen in uranium-enriched shales, thereby affecting carbon cycle on Earth's surface environments.
KW - Artificial and natural radiation dose
KW - Cambro-Ordovician Alum Shale
KW - Organic matter
KW - Proterozoic-Phanerozoic
KW - Triassic Chang 7 shales
KW - Uranium
UR - https://www.scopus.com/pages/publications/105013638091
U2 - 10.1016/j.coal.2025.104864
DO - 10.1016/j.coal.2025.104864
M3 - Journal article
AN - SCOPUS:105013638091
SN - 0166-5162
VL - 309
JO - International Journal of Coal Geology
JF - International Journal of Coal Geology
M1 - 104864
ER -