Xie and 7 co-authors reported in later 2025 that marine organic-rich shales frequently exhibit anomalously high uranium (U) concentrations, yet the mechanisms governing its enrichment in overmature formations like the Wufeng–Longmaxi shales in China remain unclear.
Vertical distribution and correlation of gamma ray (GR) and spectral uranium
(U) logs across the high-GR peak intervals of the Wufeng–Longmaxi shale
in the Sichuan Basin, as exemplified by well W#1. Labels A–C indicate the
three uranium-enriched intervals (Parts A–C).
The authors examined the distribution and enrichment patterns of uranium in the Wufeng–Longmaxi shales in typical wells through integrated geochemical and geophysical analyses, supplemented by natural gamma spectral logging data. Key findings include:
1) Multiple (up to three) uranium enrichment events are identified within the Wufeng–Longmaxi sequence, consistently corresponding to shale gas sweet spots.
2) Uranium content shows a clear dependence on organic matter (OM) type, with algal fragments being the primary host of uranium, likely due to incorporation during early diagenesis. Pore-water redox conditions and pH further govern the reduction of U (U⁶⁺) and its subsequent sequestration into organic phases.
3) The equivalent vitrinite reflectance (ERo) of uranium-rich shales is 0.11%–0.17% higher than that of non-uranium-rich shales, suggesting that uranium enrichment may slightly enhance OM thermal maturity.
4) Uranium distribution is collectively controlled by reducing conditions, volcanic eruptions (e.g., tuff layers), and OM type. Additionally, uranium enrichment provides chronostratigraphic markers that may aid in timing marine black shales.
These findings thus offer a mechanistic understanding of uranium enrichment in overmature shales, with direct implications for targeting productive intervals in shale gas systems.
The authors formally concluded that the distribution and enrichment mechanisms of uranium in the Wufeng–Longmaxi shales in China with an integrated analysis of geochemical and geophysical data, including spectral logging and laboratory measurements as follows:
1) One to three discrete uranium enrichment events are identified within the Wufeng–Longmaxi sequence. These high-uranium intervals correlate well with shale gas sweetspots, highlighting the value of uranium enrichment as a key indicator for sweetspot prediction.
2) The strong correlation between spectral uranium logs and gamma-ray (GR) curves confirms that uranium enrichment is the primary cause of high-GR anomalies. Uranium distribution is jointly controlled by reducing depositional conditions, volcanic activity (e.g., tuff layers), and organic matter type.
3) Uranium content (U content) shows a clear dependence on organic matter type, with algal fragments exhibiting a good positive correlation (R2 = 0.74), indicating their role as the main uranium host. Given the low uranium content in modern marine algae, uranium incorporation likely occurred during early diagenesis under favorable redox and pore water pH conditions.
4) All studied samples are overmature, with equivalent vitrinite reflectance (ERo) generally increasing with burial depth. However, ERo values in uranium-rich shale are 0.11%–0.17% higher than in non-uranium-rich shales, suggesting that uranium enrichment may have a slight enhancing effect on thermal maturity.
5) Uranium enrichment provides a practical chemostratigraphic marker for regional correlation and dating of marine shales, offering a simpler alternative to complex tuff-bed dating methods.
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