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<p>The logging responses of organic-rich shale in large basins with stable deep-water environments significantly vary with burial depth. These variations can lead to substantial errors when extracting oil and gas information solely from logging curves. However, the mechanisms linking these responses to diagenesis and organic matter (OM) evolution remain poorly understood. This study integrated logging, mineralogical analysis, organic geochemistry, and pore characterization data from the Cretaceous Qingshankou Formation in the super large continental Songliao Basin to systematically investigate the relationship between logging responses and diagenetic processes. In addition, a depth-stratified logging prediction model was developed using the XGBoost algorithm. The results revealed that, with increasing burial depth, the logging responses of organic-rich shales exhibit distinct trends: gamma ray (GR) and resistivity (RT) exhibit increasing–decreasing–increasing patterns, whereas sonic (DT) values first decrease but then rebound. Neutron (CNL) values decline monotonically, and density (DEN) increases progressively. These trends are governed by (1) clay mineral transformation (smectite illitization), (2) OM properties, maturation, hydrocarbon generation, and expulsion, (3) porosity evolution, and (4) diagenetic processes including compaction, cementation, and dissolution. Shallow burial depths (&lt;1500&#xa0;m) exhibited high total organic carbon contents and porosities, and these characteristics are primarily controlled by low-maturity OM retention. Moderate burial depths (1500–2200&#xa0;m) reflected overpressure caused by hydrocarbon generation and pressure relief associated with hydrocarbon expulsion. Finally, deep burial depths (&gt;2200&#xa0;m) were characterized by organic pore development and gas cracking. A depth-stratified logging prediction model is proposed to optimize sweet-spot identification by emphasizing accurate resource prediction for shallow burial depths, coupling GR and RT to evaluate overpressure relief thresholds at moderate burial depths, and targeting high-porosity/brittleness zones as fracturing targets at deep burial depths. This study enhances the understanding of depth-dependent logging responses in continental organic-rich shales, provides a logging-based framework for lacustrine shale oil evaluation, and facilitates the effective reutilization of legacy wells.</p>

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Burial Depth-Dependent Logging Response Heterogeneity in Organic-Rich Shale: Qingshankou Formation, Songliao Basin

  • 期刊论文

  • 发表日期：

    2026年1月20日

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  • Lianxin Tao,
  • Ling Jiang,
  • Xuemei Tian,
  • Hongliang Dang,
  • Liyun Hou,
  • Pingchang Sun

摘要

The logging responses of organic-rich shale in large basins with stable deep-water environments significantly vary with burial depth. These variations can lead to substantial errors when extracting oil and gas information solely from logging curves. However, the mechanisms linking these responses to diagenesis and organic matter (OM) evolution remain poorly understood. This study integrated logging, mineralogical analysis, organic geochemistry, and pore characterization data from the Cretaceous Qingshankou Formation in the super large continental Songliao Basin to systematically investigate the relationship between logging responses and diagenetic processes. In addition, a depth-stratified logging prediction model was developed using the XGBoost algorithm. The results revealed that, with increasing burial depth, the logging responses of organic-rich shales exhibit distinct trends: gamma ray (GR) and resistivity (RT) exhibit increasing–decreasing–increasing patterns, whereas sonic (DT) values first decrease but then rebound. Neutron (CNL) values decline monotonically, and density (DEN) increases progressively. These trends are governed by (1) clay mineral transformation (smectite illitization), (2) OM properties, maturation, hydrocarbon generation, and expulsion, (3) porosity evolution, and (4) diagenetic processes including compaction, cementation, and dissolution. Shallow burial depths (<1500 m) exhibited high total organic carbon contents and porosities, and these characteristics are primarily controlled by low-maturity OM retention. Moderate burial depths (1500–2200 m) reflected overpressure caused by hydrocarbon generation and pressure relief associated with hydrocarbon expulsion. Finally, deep burial depths (>2200 m) were characterized by organic pore development and gas cracking. A depth-stratified logging prediction model is proposed to optimize sweet-spot identification by emphasizing accurate resource prediction for shallow burial depths, coupling GR and RT to evaluate overpressure relief thresholds at moderate burial depths, and targeting high-porosity/brittleness zones as fracturing targets at deep burial depths. This study enhances the understanding of depth-dependent logging responses in continental organic-rich shales, provides a logging-based framework for lacustrine shale oil evaluation, and facilitates the effective reutilization of legacy wells.