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<p>The grain shoal facies dolomites of the Middle Permian Qixia Formation in the Sichuan Basin exhibit extensive distribution and diverse types, yet their reservoir properties vary significantly. This study elucidates the primary controlling factors for the differences in characteristics between platform-margin shoal and intra-platform shoal dolomite reservoirs. Based on core observations, thin section identifications, and geochemical analysis data, this study systematically compares the characteristics of dolomite reservoirs in the western Sichuan Basin (platform-margin shoal) and the central Sichuan Basin (intra-platform shoal) of the Qixia Formation. Results demonstrate that the platform-margin shoals in western Sichuan Basin were deposited in a high-energy environment, exhibiting greater single-layer thicknesses (average &gt; 5&#xa0;m), more intense and complete dolomitization (dolomite content &gt; 85%). The dolostones primarily underwent burial dolomitization, developing dissolution vugs and intercrystalline pores (average porosity of 2.9%). In contrast, intra-platform shoals in central Sichuan Basin were formed in a relatively lower-energy environment, characterized by thinner single dolostone layers (average &lt; 3&#xa0;m), relatively lower and incomplete degree of dolomitization (dolomite content 30%-60%). The dolostones are dominated by penecontemporaneous dolomitization. The reservoir space is mainly composed of residual intercrystalline pores (average porosity of 1.9%). This was owing to multiphase and thorough dolomitization processes that significantly improved reservoir properties (higher porosity and permeability) in western Sichuan Basin. Compared to intra-platform shoals, the platform-margin shoals exhibit higher depositional hydrodynamic energy, superior initial physical properties, and stronger fluid activity. These conditions provided more favorable diagenetic and reservoir-forming environments for multiphase dolomitization. Therefore, the sedimentary environment and tectonic activities during burial are key controlling factors on the quality differences in dolomite reservoirs between the platform-margin and intra-platform shoals in the Qixia Formation. This understanding holds significant implications for predicting favorable exploration zones in grain shoal facies dolomite reservoirs.</p>

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Dolomite reservoir differences in characteristics and controlling factors of grainstone shoal facies in Qixia Formation, Sichuan Basin

  • 期刊论文

  • 发表日期：

    2025年11月15日

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  • Benjian Zhang,
  • Hualing Ma,
  • Xiaojie Huang,
  • Xihua Zhang,
  • Xiao Chen,
  • Fujie Jiang,
  • Chen Xie,
  • Yangui Chen,
  • Ran Liu,
  • Wei Wang,
  • Haitao Sun

摘要

The grain shoal facies dolomites of the Middle Permian Qixia Formation in the Sichuan Basin exhibit extensive distribution and diverse types, yet their reservoir properties vary significantly. This study elucidates the primary controlling factors for the differences in characteristics between platform-margin shoal and intra-platform shoal dolomite reservoirs. Based on core observations, thin section identifications, and geochemical analysis data, this study systematically compares the characteristics of dolomite reservoirs in the western Sichuan Basin (platform-margin shoal) and the central Sichuan Basin (intra-platform shoal) of the Qixia Formation. Results demonstrate that the platform-margin shoals in western Sichuan Basin were deposited in a high-energy environment, exhibiting greater single-layer thicknesses (average > 5 m), more intense and complete dolomitization (dolomite content > 85%). The dolostones primarily underwent burial dolomitization, developing dissolution vugs and intercrystalline pores (average porosity of 2.9%). In contrast, intra-platform shoals in central Sichuan Basin were formed in a relatively lower-energy environment, characterized by thinner single dolostone layers (average < 3 m), relatively lower and incomplete degree of dolomitization (dolomite content 30%-60%). The dolostones are dominated by penecontemporaneous dolomitization. The reservoir space is mainly composed of residual intercrystalline pores (average porosity of 1.9%). This was owing to multiphase and thorough dolomitization processes that significantly improved reservoir properties (higher porosity and permeability) in western Sichuan Basin. Compared to intra-platform shoals, the platform-margin shoals exhibit higher depositional hydrodynamic energy, superior initial physical properties, and stronger fluid activity. These conditions provided more favorable diagenetic and reservoir-forming environments for multiphase dolomitization. Therefore, the sedimentary environment and tectonic activities during burial are key controlling factors on the quality differences in dolomite reservoirs between the platform-margin and intra-platform shoals in the Qixia Formation. This understanding holds significant implications for predicting favorable exploration zones in grain shoal facies dolomite reservoirs.