This paper is the result of mineral exploration engineering. Objective This study aims to ascertain the petrological, mineralogical, and geochemistrial signatures of the sandstone (mudstone) of the uranium bearing target layer in Erennur Sag, Erlian Basin, and to discuss uranium mineralization. Methods Research methods include field geological cataloging, polarized light microscopy, electron probe spectroscopy analysis, major trace elements, environmental geochemistry, and uranium valence analysis. Results (1) Uranium minerals exist in the form of adsorbed and independent uranium minerals, often co-existing (associated) with carbon chips and pyrite, with occasionally with presence of sphalerite. (2) The ore exhibits the characteristics of high organic carbon, S and CO₂. The average U⁶⁺/U⁴⁺ of mudstone is 1.01, and the average U⁶⁺/U⁴⁺ of sandstone is 0.57. (3) The CIA value of sandstone (mudstone) is 50.83 to 66.34. The characteristics of rare earth elements are consistent with those of granite in the erosion source area, the characteristics of main and trace elements all show that there is obvious water–rock interaction in the late sedimentation–diagenesis stage. Conclusions The following conclusions can be drawn: (1) The Hercynian granite is the dominant source of the sandstone (mudstone) of the Saihan Formation. The sedimentary environment is a semi–arid–semi–humid paleo–climate. (2) Adsorption and oxidation–reduction are the main mechanisms of uranium enrichment in the study area. Mudstone–type uranium enrichment is mainly in the form of adsorption, while sandstone–type uranium enrichment is mainly in the form of oxidation–reduction. (3)The deep fluid may have participated in the mineralization.