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<p>Lithofacies classification provides a fundamental basis for evaluating shale oil and gas reservoirs. However, the widely adopted ternary diagram method overlooks the volumetric importance of organic matter, leading to inaccuracies in classification when applied to organic-rich shales. In this paper, a quaternary diagram classification scheme is proposed, incorporating organic matter, silicate, carbonates, and clay minerals as the four end-members, with volumetric thresholds of 25% and 50% defining the nomenclature boundaries. It classifies shale into four main lithofacies types: ‘organic shale’, ‘siliceous shale’, ‘calcareous shale’, and ‘argillaceous shale’, with prefixes such as ‘organic-bearing’, ‘silicate-bearing’, ‘carbonate-bearing’, and ‘clay-bearing’. This method was applied to seven representative shale formations in China and the USA, and the results were compared with those obtained using the classical ternary diagram method. The findings revealed that shales with low organic matter content have consistent classifications derived from both methods. However, in shales with high organic matter content, the ternary diagram yielded results inconsistent with the petrological characteristics of the samples, whereas the quaternary diagram aligned well with observed lithological features. More importantly, the quaternary diagram scheme proved superior to the ternary diagram in assessing shale reservoir quality, gas content, and brittleness. This study advances shale lithofacies classification and highlights its importance in improving shale oil and gas reservoir evaluation.</p>

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Organic-Rich Shale Lithofacies Classification Scheme: Application and Discussion

  • 期刊论文

  • 发表日期：

    2025年8月20日

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  • Xinyu Jiang,
  • Nengwu Zhou,
  • Baizhi Li,
  • Yang Liu,
  • Shuai Zhao,
  • Wenbiao Li,
  • Guohui Chen,
  • Pengfei Zhang,
  • Shuangfang Lu

摘要

Lithofacies classification provides a fundamental basis for evaluating shale oil and gas reservoirs. However, the widely adopted ternary diagram method overlooks the volumetric importance of organic matter, leading to inaccuracies in classification when applied to organic-rich shales. In this paper, a quaternary diagram classification scheme is proposed, incorporating organic matter, silicate, carbonates, and clay minerals as the four end-members, with volumetric thresholds of 25% and 50% defining the nomenclature boundaries. It classifies shale into four main lithofacies types: ‘organic shale’, ‘siliceous shale’, ‘calcareous shale’, and ‘argillaceous shale’, with prefixes such as ‘organic-bearing’, ‘silicate-bearing’, ‘carbonate-bearing’, and ‘clay-bearing’. This method was applied to seven representative shale formations in China and the USA, and the results were compared with those obtained using the classical ternary diagram method. The findings revealed that shales with low organic matter content have consistent classifications derived from both methods. However, in shales with high organic matter content, the ternary diagram yielded results inconsistent with the petrological characteristics of the samples, whereas the quaternary diagram aligned well with observed lithological features. More importantly, the quaternary diagram scheme proved superior to the ternary diagram in assessing shale reservoir quality, gas content, and brittleness. This study advances shale lithofacies classification and highlights its importance in improving shale oil and gas reservoir evaluation.