Geological and Geochemical Characteristics of Chang 7 Uranium-Enriched Hydrocarbon Source Rocks in Ordos Basin and the Possible Reasons for its Formation

The Ordos basin Chang 7 hydrocarbon source rocks are the most important rock strata of premium oil source formed during the largest lake flood period of the terrestrial lacustrine. Our study found that the bottom of the rocks are rich in radioactive uranium. Element geochemical studies showed that the rocks are also rich in C, S, P, Cu, V, Mo and other pro-life elements but not rock-forming minerals-related lithophile elements Li, Zr, Sr, etc. Their uranium content has obvious positive correlation with phosphorus, sulfur, iron, organic carbon content, but not with major rock-forming elements. Petrography and scanning electron microscopy revealed the rocks are rich in pyrite, phosphate rock and striped organic matters. These features indicate that uranium enrichment and the presence of these minerals are closely related. The significantly positive correlation of the ratio of uranium to trace elements such as U/ (U+Ni) and U/Sc indicates that anoxic environment played an important role in uranium enrichment. Electron microprobe analysis found for the first time uranium ore and independent titanium-containing uranium minerals in phosphate rocks in the area. α radiation photography and sequential chemical extraction found that 50% of uranium present as independent uranium and its isomorphic minerals in the phosphate rocks, 20% is absorbed to pyrite and organic matter, and less than 10% is absorbed on clay minerals.