Degradation of organic matter by radioactive radiation in black shales: An overlooked modification of organic molecular structures

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/C_1–5 < 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.