The Relationship between Jurassic Coal Measures and Sandstone-type Uranium Deposits in the Northeastern Ordos Basin, China

Outcrop and drill hole data show that the Jurassic coal measures in the northeastern Ordos Basin are composed mainly of the Yan'an Formation and the lowstand system tract of the Zhiluo Formation, and there is a regional unconformity between them. The Dongsheng uranium deposit is associated with the Jurassic coal measures. Research data indicate that the Jurassic coal measures in the study area have a certain hydrocarbon‐generating capacity, although the metamorphic grade is low (Ro=0.40%–0.58%). In the Dongsheng region alone, the accumulative amount of generated coalbed methane (CBM) is about 2028.29 × 10⁸ ?2218.72 × 10⁸ m³; the residual amount is about 50.92 × 10⁸ m³, and the lost amount is about 1977 × 10⁸ m³. Analysis of the burial history of the host rocks and the evolutionary history of the Dongsheng uranium deposit suggests that the Jurassic coal measures generated hydrocarbon mainly from Middle Jurassic to Early Crataceous, which is the main mineralization phase of the Dongsheng uranium deposit. By the Late Cretaceous, a mass of CBM dissipated due to the strong tectonic uplift, and the Dongsheng uranium deposit stepped into the preservation phase. Therefore, the low‐mature hydrocarbon‐containing fluid in the Jurassic coal measures not only served as a reducing agent for the formation of sandstone‐type uranium deposits, but also rendered the second reduction of paleo‐interlayer oxidation zone and become the primary reducing agent for ore conservation. Regional strata correlation reveals that the sandstone‐type uranium reservoir at the bottom of the Zhiluo Formation is in contact with the underlying industrial coal seams in the Yan'an Formation through incision or in the form of an unconformity surface. In the Dongsheng region with poorly developed fault systems, the unconformity surface and scour surface served as the main migration pathways for low‐mature hydrocarbon‐containing fluid migrating to the uranium reservoir.