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<p>The fluid occurrence characteristics of lacustrine shale oil reservoirs present significant challenges for reservoir evaluation and exploration. In particular, it is difficult to determine the in-situ occurrence state of shale oil reservoir fluids because core samples often experience fluid loss during sampling and preparation. This study investigates fluid occurrence mechanisms in Yanchang Formation shale (Ordos Basin) using nuclear magnetic resonance and nitrogen adsorption tests on cores under different conditions (as-received, dried, extracted, saturated). Results reveal that lost fluids are primarily liquid hydrocarbons, with the sum of lost fluid and residual movable oil (T<sub>2</sub>O) showing a strong linear correlation with the movable oil index (S<sub>1</sub>/TOC) in both clayey and silty shales. In clayey shale, the lost oil content is higher than the residual movable oil and comparable to the residual movable water content, with movable oil occupying relatively larger pore sizes than movable water. In silty shale, the lost oil content far exceeds that of residual movable oil and water, dominating the effective porosity. Fractal dimension (DA) negatively correlates with S<sub>1</sub>/TOC, indicating meso- to macro-pore heterogeneity controls movable oil content, especially in silty shale. The findings offer insights into in-situ shale oil occurrence mechanisms.</p>

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Fluid Occurrence in Shale Oil Reservoirs Based on Fractal Analysis of NMR and N2 Adsorption

  • 期刊论文

  • 发表日期：

    2026年6月30日

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  • Yin Jintao,
  • Wang Aiguo,
  • Gao Chao,
  • Bai Yuan,
  • Chang Ying,
  • Feng Haoyu,
  • Zeng Lijun,
  • Xue Pei,
  • Wang Bianyang

摘要

The fluid occurrence characteristics of lacustrine shale oil reservoirs present significant challenges for reservoir evaluation and exploration. In particular, it is difficult to determine the in-situ occurrence state of shale oil reservoir fluids because core samples often experience fluid loss during sampling and preparation. This study investigates fluid occurrence mechanisms in Yanchang Formation shale (Ordos Basin) using nuclear magnetic resonance and nitrogen adsorption tests on cores under different conditions (as-received, dried, extracted, saturated). Results reveal that lost fluids are primarily liquid hydrocarbons, with the sum of lost fluid and residual movable oil (T2O) showing a strong linear correlation with the movable oil index (S1/TOC) in both clayey and silty shales. In clayey shale, the lost oil content is higher than the residual movable oil and comparable to the residual movable water content, with movable oil occupying relatively larger pore sizes than movable water. In silty shale, the lost oil content far exceeds that of residual movable oil and water, dominating the effective porosity. Fractal dimension (DA) negatively correlates with S1/TOC, indicating meso- to macro-pore heterogeneity controls movable oil content, especially in silty shale. The findings offer insights into in-situ shale oil occurrence mechanisms.