Rapid expansion of shale gas development in China raises environmental and human health concerns. Several studies present related information on these concerns in the United States and Canada, yet they are few in China at present. This paper presents a series of original and published mineralogical and trace elements data from two gas shales (Niutitang shale and Longmaxi shale) considered as producing gas shale in China. Mineralogical and geochemical data surveyed can be applied to evaluate the potential environment pollutions during shale weathering and hydraulic fracturing. After compilations of the mineralogical and trace elements data, we can conclude that (a) there is generally more pyrite compared with carbonate for Niutitang shales, indicating that Niutitang shales are more prone to generate the hydrogen ions than Longmaxi shales; (b) many of the environmental hazardous trace elements considered here show an association with organic matter and/or pyrite, indicating that these trace elements are prone to mobilize and release during shale weathering and hydraulic fracturing; (c) some environmental hazardous elements like As, Ni, and Ba that have extremely high concentrations in some regions as compared with the screening limits for soil and drinking water should attract more attention; and (d) clay-rich shale with abundant organic matter may be the most favourable shale gas reservoir for the Niutitang shale, especially for those shales in Lower Yangtze region. Moreover, more mineralogical and geochemical data will be required for a comprehensive environmental impacts assessment. KEYWORDS environmental and human impacts, mineralogy, pyrite oxidization, shale, South China, trace elements
The Upper Ordovician-Lower Silurian mudstones (including the Wufeng, Guanyinqiao and Longmaxi Formations) in the Sichuan Basin are some of the most important shale gas plays in China. In order to enhance our understanding of the process of formation of organic carbon up to 10%, optical, microscopy and geochemical methods have been used to investigate the petrographic and geochemical characteristics of the formation. Firstly, three mudstone lithofacies were identified based on a wide variety of mudstone laminations. These are: (a) indistinctly laminated mudstone; (b) parallellaminated mudstone; and (c) nonparallel-laminated mudstone. Then, combining with the evidence from depocenter migration, Th/U ratios and total organic carbon, the abundant organo-minerallic fabrics suggest that organic carbon was preferentially deposited and preserved in anoxic, low energy and stagnant water conditions during deposition of the Wufeng and Longmaxi Formations. On the contrary, the Guanyinqiao Formation with poor organic carbon was deposited in oxic and high-energy water conditions.
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