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.

Fossil charcoal from the Middle Jurassic of the Ordos Basin,China and its paleoatmospheric implications

The Yah＇an Formation of the Ordos Basin is a sequence of four members, consisting of si- liciclastic sediments deposited in alluvial, lacustrine and mire settings during the Middle Jurassic. Samples collected from Members Two and Four contain abundant blackened plant material identified through standard analytical techniques as fusain （fossil charcoal）. The occurrence of fusain in fluvial sandstones at multiple horizons in the outcrops, combined with the previously reported high concentra- tion of inertinite in the coals of Member One, indicates that paleowildfire was a common occurrence in the Ordos Basin during Yan＇an deposition. Sedimentary evidence from Yan＇an outcrops suggests that the paleoclimate was seasonal during deposition of Members Two through Four, which may have contributed to the wildfire frequency. The presence of fusain in the Yah＇an Formation indicates that atmospheric oxygen levels were clearly above the minimum required for sustained combustion during the Middle Jurassic. This conclusion contradicts previous geochemical models for paleoatmospheric composition, but supports more recent studies.