Carbon and oxygen isotopic composition of car- bonate cements of different phases in terrigenous siliciclastic reservoirs and significance for their origin: A case study from sandstones of the Triassic Yanchang Formation, southwestern Ordos Basin, China |
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作者单位: | WANG Qi 1,2,ZHUO Xizhun1,2,CHEN Guojun1,and LI Xiaoyan1,2 1 Key Laboratory of Gas Geochemistry,Institute of Geology and Geophysics,Chinese Academy of Sciences,Lanzhou 730000,China 2 Graduate University of Chinese Academy of Sciences,Beijing 100039,China |
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基金项目: | 国家重点基础研究发展计划(973计划),The Knowledge Innovation Proiect of Chinese Academy of Sciences,中国科学院基金 |
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摘 要: | Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope (their δ^18O values range from -0.3‰- -0.1‰) and lighter oxygen isotope (their δ^18O values range from -22.1‰- -19.5‰). Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later disso- lution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ^13C values ranging from -8.02‰ to -3.23‰, and lighter oxygen isotope with δ^18O values ranging from -22.9‰ to -19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous cal- cites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ^13C values ranging from -1.92‰ to -0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ^18O values ranging from -20.5‰ to -12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process.
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关 键 词: | 碳氧同位素合成 碳酸盐水泥 碳源 可分解水池 |
Carbon and oxygen isotopic composition of carbonate cements of different phases in terrigenous siliciclastic reservoirs and significance for their origin: A case study from sandstones of the Triassic Yanchang Formation, southwestern Ordos Basin, China |
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Authors: | Qi Wang Xizhun Zhuo Guojun Chen Xiaoyan Li |
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Institution: | [1]Key Laboratory of Gas Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China [2]Graduate University of Chinese Academy of Sciences, Beijing 100039, China |
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Abstract: | Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon
isotope (their δ18O values range from −0.3‰–−0.1‰) and lighter oxygen isotope (their δ18O values range from −22.1‰–−19.5‰). Generally, they are closely related to the direct precipitation of oversaturated calcium
carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability
of sandstones, preserving intragranular volume and providing the mass basis for later dissolution caused by acidic fluid flow
to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ13C values ranging from −8.02‰ to −3.23‰, and lighter oxygen isotope with δ18O values ranging from −22.9‰ to −19.7‰, which is obviously related to the decarboxylation of organic matter during the late
period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous calcites
in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx
and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ13C values ranging from −1.92‰ to −0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ18O values ranging from −20.5‰ to −12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed
marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic
environment at the mid-late stage would promote this process. |
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Keywords: | carbonate cement O and C isotopic composition clastic reservoir carbon source Ordos Basin |
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