Hydrocarbon fluid feature of the Zhiluo formation sandstone in the Dongsheng Area, Ordos basin and its relationship to uranium mineralization

Based on the analysis for geological features of uranium deposits in the Dongsheng area of the Ordos Basin, this study conducted the acidolysis-desorbed hydrocarbon testing for 139 sandstone samples from Lower Section of the Zhiluo Formation in the Daying and Nalinggou deposits to understand the composition and origin of acidolysis-desorbed hydrocarbon and relationship of hydrocarbon fluids with sandstone-type uranium mineralization. The results show that the contents of CH4 and C2+ in the lower Zhiluo Formation sandstone are characterized by gradually decreasing from mineralized sandstone to gray sandstone and to gray-green sandstone, indicating close relation between uranium mineralization and hydrocarbon gas. Feature parameters, such as C1/ΣC, C1/C2+, C2/iC4, C2/C3, iC4/nC4, iC5/nC5, ln(C1/C2), ln(C2/C3) and δ13CCO2, of acidolysis-desorbed hydrocarbon gas show that hydrocarbon gas within the low Zhiluo Formation sandstone of the middle Jurassic is characterized by organic origin, with the gas at the stage of mature to high mature, and should be mixed gas consisting of cracked crude oil gas and associated gases. Based on the gas-source correlation, it can be inferred that gas, developed in sandstone of Lower Section of Zhiluo Formation, may derive from sapropelic source rock of deep-semi deep lacustrine facies in the Triassic Yanchang Formation. The mixture gas of cracking gas of crude oil and associated gases migrates mainly into sandstone of the Zhiluo Formation along deep faulting, micro fracture and other pathways. Besides, alteration of migrated hydrocarbon gas on surrounding rock can be defined by fading alteration and greenish alteration in the Lower Section of the Zhiluo Formation. The redox transition zone between above alteration sandstone, adjacent to fracture, can be regarded as the favorable target of sandstone-type uranium prospecting. © 2016, Science Press. All right reserved.