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Mechanisms of bedding fracturing in the Junggar Basin, northwest China: Constraints from in situ U-Pb dating and C-O-Nd isotopic analysis of calcite cements.

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Title: Mechanisms of bedding fracturing in the Junggar Basin, northwest China: Constraints from in situ U-Pb dating and C-O-Nd isotopic analysis of calcite cements.
Authors: Chen Zhang1,2 zhangchen2022@cdut.edu.cn, Xiangye Kong1,3 kongxiangye@cdut.edu.cn, Qiqi Wang4, Min She5,6, Feng Liang5,6, Yixin Dong1,2, Hao Xu1, Jianhua He1, Huadong Chen1,2
Source: Geological Society of America Bulletin. Jul2025, Vol. 137 Issue 7/8, p3037-3054. 18p.
Subject Terms: *Calcite analysis, *Isotopic analysis, *Rare earth metals, *Uranium-lead dating, *Shale oils, *Fluid inclusions, *Cement, *Radiocarbon dating
Geographic Terms: China
Abstract: Frequent natural fracturing along shale bedding interfaces facilitates the formation of bed-parallel fractures. However, these mechanisms are not yet fully understood. The middle Permian Lucaogou Formation in the Jimusar Sag of the Junggar Basin serves as a quintessential example of fractured shale. Complex geological processes have led to the development of numerous bed-parallel fractures, which significantly contribute to the formation's heterogeneity. In this study, we analyzed rare earth element, fluid inclusion, and C-O-Nd isotope data and in situ U-Pb ages of calcite cement to determine the timing and mechanisms of bed-parallel fracture formation. Our findings revealed two generations of bed-parallel fractures: type A and type B fractures. The crystallization age of calcite cement in type A bed-parallel fractures (group A calcites) ranges from ca. 254.1 Ma to 251.0 Ma. These calcites contain mainly aqueous inclusions with homogenization temperatures between 145.3 ℃ and 208.9 ℃. The δ13C, δ18O, and eNd(t) values for group A calcites range from -13.24‰ to -0.66‰, 4.22‰ to 12.93‰, and 6.14 to 8.61, respectively, indicating the calcite cement formed from high-temperature fluids related to postcollisional, mantlederived magmatism. In contrast, calcite cement in type B bed-parallel fractures (group B calcites) formed between ca. 15.6 Ma and 4.5 Ma, coinciding with the period of hydrocarbon generation in the Lucaogou Formation. These cements are characterized by the coexistence of hydrocarbon and aqueous inclusions, with homogenization temperatures ranging from 98.9 ℃ to 158.7 ℃. The δ13C, δ18O, and εNd(t) values of the group B calcites are -5.27‰ to 6.73‰, 12.09‰ to 21.64‰, and -3.35 to -1.99, respectively, like those of the surrounding shale, suggesting a connection between calcite cementation and hydrocarbon-generating fluids. We propose that continuous tectonism within the Junggar Basin during the late Permian resulted in N-S shortening, which led to fracturing of weak bedding planes. Fluid overpressure driven by late Permian magmatism triggered the opening of type A bed-parallel fractures. The formation of type B bed-parallel fractures was likely driven by overpressure from hydrocarbon generation and expulsion. Weak bedding planes were opened when the pore pressure reached the fracture threshold. Fracture analysis indicates that bed-parallel fractures are the most prevalent and welldeveloped fracture type within the Lucaogou Formation. Regions where both beddingparallel and non-bed-parallel fractures coexist are identified as optimal zones for shale oil accumulation. Our study underscores the importance of integrating petrological, fluid inclusion, geochronological, and geochemical data to understand the diverse mechanisms of bedding fracturing. The significant role of bed-parallel fractures in hydrocarbon accumulation, as revealed in this study, is crucial and should not be overlooked. [ABSTRACT FROM AUTHOR]
Database: Energy & Power Source
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DbLabel: Energy & Power Source
An: 186855512
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PubTypeId: academicJournal
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  Label: Title
  Group: Ti
  Data: Mechanisms of bedding fracturing in the Junggar Basin, northwest China: Constraints from in situ U-Pb dating and C-O-Nd isotopic analysis of calcite cements.
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  Data: <searchLink fieldCode="AR" term="%22Chen+Zhang%22">Chen Zhang</searchLink><relatesTo>1,2</relatesTo><i> zhangchen2022@cdut.edu.cn</i><br /><searchLink fieldCode="AR" term="%22Xiangye+Kong%22">Xiangye Kong</searchLink><relatesTo>1,3</relatesTo><i> kongxiangye@cdut.edu.cn</i><br /><searchLink fieldCode="AR" term="%22Qiqi+Wang%22">Qiqi Wang</searchLink><relatesTo>4</relatesTo><br /><searchLink fieldCode="AR" term="%22Min+She%22">Min She</searchLink><relatesTo>5,6</relatesTo><br /><searchLink fieldCode="AR" term="%22Feng+Liang%22">Feng Liang</searchLink><relatesTo>5,6</relatesTo><br /><searchLink fieldCode="AR" term="%22Yixin+Dong%22">Yixin Dong</searchLink><relatesTo>1,2</relatesTo><br /><searchLink fieldCode="AR" term="%22Hao+Xu%22">Hao Xu</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Jianhua+He%22">Jianhua He</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Huadong+Chen%22">Huadong Chen</searchLink><relatesTo>1,2</relatesTo>
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  Data: <searchLink fieldCode="JN" term="%22Geological+Society+of+America+Bulletin%22">Geological Society of America Bulletin</searchLink>. Jul2025, Vol. 137 Issue 7/8, p3037-3054. 18p.
– Name: Subject
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  Data: *<searchLink fieldCode="DE" term="%22Calcite+analysis%22">Calcite analysis</searchLink><br />*<searchLink fieldCode="DE" term="%22Isotopic+analysis%22">Isotopic analysis</searchLink><br />*<searchLink fieldCode="DE" term="%22Rare+earth+metals%22">Rare earth metals</searchLink><br />*<searchLink fieldCode="DE" term="%22Uranium-lead+dating%22">Uranium-lead dating</searchLink><br />*<searchLink fieldCode="DE" term="%22Shale+oils%22">Shale oils</searchLink><br />*<searchLink fieldCode="DE" term="%22Fluid+inclusions%22">Fluid inclusions</searchLink><br />*<searchLink fieldCode="DE" term="%22Cement%22">Cement</searchLink><br />*<searchLink fieldCode="DE" term="%22Radiocarbon+dating%22">Radiocarbon dating</searchLink>
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  Label: Geographic Terms
  Group: Su
  Data: <searchLink fieldCode="DE" term="%22China%22">China</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: Frequent natural fracturing along shale bedding interfaces facilitates the formation of bed-parallel fractures. However, these mechanisms are not yet fully understood. The middle Permian Lucaogou Formation in the Jimusar Sag of the Junggar Basin serves as a quintessential example of fractured shale. Complex geological processes have led to the development of numerous bed-parallel fractures, which significantly contribute to the formation's heterogeneity. In this study, we analyzed rare earth element, fluid inclusion, and C-O-Nd isotope data and in situ U-Pb ages of calcite cement to determine the timing and mechanisms of bed-parallel fracture formation. Our findings revealed two generations of bed-parallel fractures: type A and type B fractures. The crystallization age of calcite cement in type A bed-parallel fractures (group A calcites) ranges from ca. 254.1 Ma to 251.0 Ma. These calcites contain mainly aqueous inclusions with homogenization temperatures between 145.3 ℃ and 208.9 ℃. The δ13C, δ18O, and eNd(t) values for group A calcites range from -13.24‰ to -0.66‰, 4.22‰ to 12.93‰, and 6.14 to 8.61, respectively, indicating the calcite cement formed from high-temperature fluids related to postcollisional, mantlederived magmatism. In contrast, calcite cement in type B bed-parallel fractures (group B calcites) formed between ca. 15.6 Ma and 4.5 Ma, coinciding with the period of hydrocarbon generation in the Lucaogou Formation. These cements are characterized by the coexistence of hydrocarbon and aqueous inclusions, with homogenization temperatures ranging from 98.9 ℃ to 158.7 ℃. The δ13C, δ18O, and εNd(t) values of the group B calcites are -5.27‰ to 6.73‰, 12.09‰ to 21.64‰, and -3.35 to -1.99, respectively, like those of the surrounding shale, suggesting a connection between calcite cementation and hydrocarbon-generating fluids. We propose that continuous tectonism within the Junggar Basin during the late Permian resulted in N-S shortening, which led to fracturing of weak bedding planes. Fluid overpressure driven by late Permian magmatism triggered the opening of type A bed-parallel fractures. The formation of type B bed-parallel fractures was likely driven by overpressure from hydrocarbon generation and expulsion. Weak bedding planes were opened when the pore pressure reached the fracture threshold. Fracture analysis indicates that bed-parallel fractures are the most prevalent and welldeveloped fracture type within the Lucaogou Formation. Regions where both beddingparallel and non-bed-parallel fractures coexist are identified as optimal zones for shale oil accumulation. Our study underscores the importance of integrating petrological, fluid inclusion, geochronological, and geochemical data to understand the diverse mechanisms of bedding fracturing. The significant role of bed-parallel fractures in hydrocarbon accumulation, as revealed in this study, is crucial and should not be overlooked. [ABSTRACT FROM AUTHOR]
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RecordInfo BibRecord:
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    Identifiers:
      – Type: doi
        Value: 10.1130/B37973.1
    Languages:
      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 18
        StartPage: 3037
    Subjects:
      – SubjectFull: Calcite analysis
        Type: general
      – SubjectFull: Isotopic analysis
        Type: general
      – SubjectFull: Rare earth metals
        Type: general
      – SubjectFull: Uranium-lead dating
        Type: general
      – SubjectFull: Shale oils
        Type: general
      – SubjectFull: Fluid inclusions
        Type: general
      – SubjectFull: Cement
        Type: general
      – SubjectFull: Radiocarbon dating
        Type: general
      – SubjectFull: China
        Type: general
    Titles:
      – TitleFull: Mechanisms of bedding fracturing in the Junggar Basin, northwest China: Constraints from in situ U-Pb dating and C-O-Nd isotopic analysis of calcite cements.
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            NameFull: Chen Zhang
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            NameFull: Xiangye Kong
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            – D: 01
              M: 07
              Text: Jul2025
              Type: published
              Y: 2025
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              Value: 7/8
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