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<p>Mudstone caprocks are a primary controlling factor for the accumulation of natural gas in the high-temperature and overpressure environment of the Yinggehai Basin, South China Sea. Therefore, in-depth research on the displacement pressure of the caprocks provides a theoretical basis for the exploration of oil and gas in basins with similar temperature and pressure conditions. Combined with X-ray diffraction, scanning electron microscopy, high-pressure mercury intrusion and other petrological and physical analysis methods, we conducted research on the characteristics and controlling factors of displacement pressure in the Huangliu Formation I. The base depth of the mudstone caprock ranges from 2600 to 4100&#xa0;m, with displacement pressures between 1.91&#xa0;MPa and 34.73&#xa0;MPa (average 10.48&#xa0;MPa). While an overall increasing trend in displacement pressure with greater burial depth is observed, the correlation is weak. This suggests that within this depth interval, the rock has become relatively tight, and the influence of depth on displacement pressure is less pronounced than in shallower formations. The displacement pressure of the mudstone caprock exhibits a distinct negative correlation with porosity. When the total porosity ranges between 4% and 8%, the displacement pressure varies from 35&#xa0;MPa to 10&#xa0;MPa. Within this porosity interval, porosity exerts the strongest influence on displacement pressure. The displacement pressure of the mudstone caprock exhibits a strong positive correlation with clay mineral content. As the clay mineral content increases from 18 to 40%, the displacement pressure correspondingly rises from 2&#xa0;MPa to 35&#xa0;MPa. The clay mineral composition exerts considerable influence on displacement pressure. Specifically, elevated content of illite/smectite mixed-layer and kaolinite increases the displacement pressure of mudstone caprocks. Increased chlorite and illite content, conversely, reduces displacement pressure, consequently diminishing sealing capacity. Overall, robust sealing capacity in Yinggehai Basin caprocks correlates with significant thickness, deep burial depths, high clay mineral content, and favorable compositions (notably illite/smectite dominance). Conversely, elevated porosity and diapir activity reduce mudstone caprock sealing capacity. Therefore, caprock assessment must incorporate thickness, mineral content/composition, porosity, and diapir activity.</p>

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Characteristics and controlling factors of displacement pressure in mudstone caprock: insights from the Yinggehai Basin, South China Sea

  • 期刊论文

  • 发表日期：

    2025年8月8日

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  • Wei Duan,
  • Cheng-Fei Luo,
  • Lin Shi,
  • Qian Li,
  • Deyong Li

摘要

Mudstone caprocks are a primary controlling factor for the accumulation of natural gas in the high-temperature and overpressure environment of the Yinggehai Basin, South China Sea. Therefore, in-depth research on the displacement pressure of the caprocks provides a theoretical basis for the exploration of oil and gas in basins with similar temperature and pressure conditions. Combined with X-ray diffraction, scanning electron microscopy, high-pressure mercury intrusion and other petrological and physical analysis methods, we conducted research on the characteristics and controlling factors of displacement pressure in the Huangliu Formation I. The base depth of the mudstone caprock ranges from 2600 to 4100 m, with displacement pressures between 1.91 MPa and 34.73 MPa (average 10.48 MPa). While an overall increasing trend in displacement pressure with greater burial depth is observed, the correlation is weak. This suggests that within this depth interval, the rock has become relatively tight, and the influence of depth on displacement pressure is less pronounced than in shallower formations. The displacement pressure of the mudstone caprock exhibits a distinct negative correlation with porosity. When the total porosity ranges between 4% and 8%, the displacement pressure varies from 35 MPa to 10 MPa. Within this porosity interval, porosity exerts the strongest influence on displacement pressure. The displacement pressure of the mudstone caprock exhibits a strong positive correlation with clay mineral content. As the clay mineral content increases from 18 to 40%, the displacement pressure correspondingly rises from 2 MPa to 35 MPa. The clay mineral composition exerts considerable influence on displacement pressure. Specifically, elevated content of illite/smectite mixed-layer and kaolinite increases the displacement pressure of mudstone caprocks. Increased chlorite and illite content, conversely, reduces displacement pressure, consequently diminishing sealing capacity. Overall, robust sealing capacity in Yinggehai Basin caprocks correlates with significant thickness, deep burial depths, high clay mineral content, and favorable compositions (notably illite/smectite dominance). Conversely, elevated porosity and diapir activity reduce mudstone caprock sealing capacity. Therefore, caprock assessment must incorporate thickness, mineral content/composition, porosity, and diapir activity.