Origin of coarse-crystalline calcite cement in Early Ordovician carbonate rocks, Ordos basin, northern China: Insights from oxygen and carbon isotopes and fluid inclusion microthermometry |
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| Authors: | Hairuo Qing Guoxiang Chi Shaonan Zhang |
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| Institution: | aState Key Laboratory of Oil/Gas Reservoir Geology and Exploitation, Chengdu University of Technology Chengdu, Sichuan, China;bDepartment of Geology University of Regina, Regina SK, Canada S4S 0A2 |
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| Abstract: | The calcite cement in the Lower Ordovician Majiagou Formation in the Ordos basin in northern China can be subdivided into three groups based on preliminary results of oxygen and carbon isotopes and fluid inclusion microthermometry. Group 1 has low oxygen isotopes (− 14‰ to − 18‰), low Th values (92–103 °C), and low salinities (1.7–4.9 wt.% NaCl equivalent) and is interpreted to have precipitated during early burial from porewater influenced by meteoric water. Group 2 has much higher oxygen isotope values (− 5‰ to − 8‰), which, coupled with the higher Th values (136–151 °C), suggest that the calcite was precipitated from fluids that were significantly enriched in 18O, possibly resulting from fluid–rock reaction during burial. Group 3 occurring along fractures is characterized by high salinities (21–28 wt.% NaCl equivalent) and is interpreted to have been precipitated from locally preserved residual evaporitic brines. The occurrence of primary hydrocarbon inclusions and its low carbon isotopes (− 11‰ to − 15‰) suggest that precipitation of group 3 calcite took place in the presence of hydrocarbons. |
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| Keywords: | Calcite cement C– O isotopes Fluid inclusions Ordos basin |
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