Uranium mineralization and sedimentary facies of the Luohandong formation of the lower cretaceous in northwestern ordos basin

Based on the research of the sedimentary facies, it is concluded that the sedimentary systems are fluvial river-braided delta-lacustrine depositional system during the Luohandong period at the gentle slope zone in north-western Ordos basin. According to characteristic parameters C1/ΣC and C1/C2+ and discriminatory analyses (Z value method, Φ value method and discriminatory factor method) of acidolysis hydrocarbon, the hydrocarbon gas in Luohandong formation from the oil-type gas in lacustrine source rocks of Upper Triassic is demonstrated. By analyzing geochemical characteristics of oil-gas, and oil-gas reduced alteration characteristics and their relation to the uranium mineralization in the Luohandong formation, this paper demonstrates the discolored alteration (bleaching) in the Luohandong formation is the result of induced alteration of rocks due to acidolysis hydrocarbon gas infiltrating into this horizon, discusses the preliminary enrichment of uranium in braided delta during process of sedimentation-diagenesis, presents the sedimentary facies association and spatial configuration of oil-gas induced alteration zone are constraint factors of space orientation of uranium mineralization horizons in the late stage. (authors)