Characteristics Of Hydrocarbon Leakage In Northeastern Ordos Basin And Its Geological Effect

The study area lies in northeast of Ordos basin. Large areas on top of Yan'an Formation exhibit an extensive bleaching of sandstone. Dongsheng sandstone-type uranium deposit and Cretaceous oil-show in the Wulunge'er salience are distributed in this area. They are spatially related and suggest a genetic relationship. The cause of bleaching of sandstone can provide evidences for hydrocarbon migration and leakage. Therefore, bleaching of sandstone is a valuable index to hydrocarbon seepage and can act as a bridge communicating inorganic sandstone-type uranium deposit and organic energy mines. It is significant to discuss the genesis-enrichment environment of Dongsheng sandstone-type uranium deposit.Ordos basin has developed during middle to late Triassic and early Cretaceous. The reformation since late Cretaceous has made great influences on hydrocarbon migration, accumulation and leakage. Based on some crucial events during basin evolution and reformation, this paper firstly discusses gas seepage responding to events. Based on abundant literature research and field investigation, thin section identification, scanning electron microscope observation (SEM), cathodoluminescence, X-ray diffractions of clay minerals, chemical composition (XRF), trace elements (ICP-MS), vitrinite reflectance, organic matter, fluid inclusion, carbon and oxygen isotope of carbonate cement were used in this study. Study on geologic and geochemistry characteristics of Cretaceous oil-show and the maturity characteristics of Mesozoic and Palaeozoic coal-bearing strata, suggests that the oil-show is formed by Carboniferous-Permian coal bed methane escaping to the surface, which indicates that Neopaleozoic gas could migrate to the north margin of the basin. According to geochemical characteristics of hydrocarbon migration, and analyzing the shallow gas pool, it can be concluded that gas seepage is related to shallow gas pools in the northeast of basin. Gas seepage has been beginning since shallow gas pool formed, about 100 Ma BP. Secondly, study on bleaching of sandstone based on field characteristics, petrography, geochemistry and compared with other sandstone related, combined with geological setting of gas seepage in the area, reveals that natural gas leakage is responsible for bleaching of the sandstone on top of Yan'an formation. The bleaching resulted from interaction between the reductive fluid and the rock. At the same time the bleaching suggests that the northeast of basin has experienced large scale gas seepage. The formation of bleaching of sandstone is important in discussing the coupling relationship between Dongsheng uranium deposit and gas seepage. Thirdly, according to the petrography, clay mineralogy, geochemical characteristics of ore-bearing sandstone and alteration sandstone, and analysis of organic matter and fluid flow, simultaneously combining with spatial correspondence of uranium deposits, outcrops of bleached sandstone and oil-show, the time of natural gas seepage and the age of uranium deposits, the response of Palaeozoic gas leakage to mineralization environment of Mesozoic sandstone-type uranium deposit is discussed. Gas seepage is important to alter the oxidation-reduction potential (Eh) of the environment and can affect the pH of the uranium mineralization system. Such pH /Eh changes result in uranium mineralization.