Rare earth element geochemistry of dolostones and its indicative significance of the Permian Changxing Formation in Yuanba Gasfield,northern Sichuan Basin
Yu Xinya, Li Pingping, Zou Huayao, Wang Guangwei, Zhang Yi
College of Geosciences,China University of Petroleum(Beijing),Beijing 102249
Abstract：Rare earth element(REE)and Yttrium(Y)concents were determined from 49 carbonate samples of the Permian Changxing Formation of Yuanba Gasfield in northern Sichuan Basin,and the results show that the total content of rare earth elements(REE)is generally low(average 1.50×10-6),which indicates that rock samples were almost without any terrigenous clastics,and not be affected by the deep hydrothermal fluids. Shale-normalized REE characteristics of dolostone including light rare earth element(LREE)depletion,positive La anomalies,negative Ce anomalies(0.49 to 1.08,average 0.75),positive Eu anomalies(0.89 to 46.00,average 4.68) of different degrees and the higher Y/Ho ratios(28.82 to 63.47,average 43.04) recorded seawater-like REE and Y signatures,which show that the dolomitization fluids is reductive seawater derived,and the dolomitization occurred in a burial environment. The brecciated or zebra typical structure of hydrothermal dolostones were not observed in the cores and petrographic thin sections. Flattened structure and the horizontal continuous distribution of dolomite reservoirs and the higher δ18O value(-7.07‰ to-2.92‰,average -4.95‰)illustrate that no obvious hydrothermal activities occurred in the study area,and the higher degree of positive Eu anomalies have no relationship with the deep hydrothermal fluids. Many elemental sulfurs and sulfur-rich pyrobitumen originated from thermochemical sulfate reduction(TSR) can be observed by scanning electron microscope,which indicates that TSR created an extremely reducing environment of stratal seawater,and then caused the higher degree of positive Eu anomalies,which in turn explains that TSR had a certain dissolution on dolomite reservoirs.
Yu Xinya,Li Pingping,Zou Huayao et al. Rare earth element geochemistry of dolostones and its indicative significance of the Permian Changxing Formation in Yuanba Gasfield,northern Sichuan Basin[J]. JOPC, 2015, 17(3): 309-320.
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