Alteration Effects of Hydrocarbon Dissipation in the Dongsheng Uranium Deposit, Ordos Basin — Explanation for Green Alteration and Bleaching Phenomenon

Abstract

Obvious features of the Neopaleozoic natural gas dissipation have been known in the north part of the mid of Ordos Basin where the natural gas had concentrated. Because of mutual actions of the superficial blanket where the natural gas effused through the surrounding light ground, various reductive alterations were formed. Large scale of green-color alteration zone was formed in the Zhiluo formation of the north-easern basin, and bleaching phenomenon occurred on the top of the Yan'an formation. Studies have shown that green sandstone is molded by low organic carbon and sulfur, high ratio of Fe2+ / Fe3+ and Th / U (non-mineralized rock). Mineral containing abundant alkaline and reduced state of iron magnesium chlorite clay minerals is the internal cause of green. Fluid composition and carbon-sulfur isotope tests have shown that the fluid has the characteristics of organic matter (gas). From a geological point of view, green alteration containing oxidation rocks, residue can be seen with “tongue” or “roll-like” shape in interlayer oxidation zone. The bleached sandstones on the top of Yan'an Formation under Dongsheng deposit belt have low contents of iron, high ratio of Fe2+ / Fe3+ and Th / U. Minerals that contain abundant clay mineral kaolinite are the internal cause of white coloring. The study of fluid from Shanxi formation in north part of the basin indicated that the dissipative direction of natural gas was from south to north, and finally converged in northeastern part of the study area. Ultimately the gas leakage “window” formed in the Wulangeer paleohigh in northeast of the basin which was found to contain abundant oil seepages. Alteration zone located in the scale of hydrocarbon dissipation and closed to the leakage “window” region has a favorable reduction geochemical environment on the space. The time of hydrocarbon dissipation, alteration and uranium mineralization are consistent with the ages. Therefore the escaped hydrocarbon and its reductive alteration had a good coupling relation between time and space. According to the tests in the study area, high contents of aliphatic acid carbomethoxy compound were found in the organic material of uranium bearing sandstone of Zhiluo formation. Research has shown that they were formed by the reaction of dispersed Neopaleozoic natural gas and the groundwater solution containing uranylion. Similarly, the truth also proved that the north east part of Dongsheng area had experienced large-scale dissipative and reductive alteration effect caused by natural gas.