Understanding and predicting the main controls on the sealing capacity of carbonate cap rocks is of great significance for ultradeep carbonate reservoir exploration and production. This study focuses on revealing the pore networks and sealing capacity of the Ordovician carbonate cap rocks in the Tarim Basin by analyzing samples from outcrop analogues using optical and scanning electron microscopy and a combination of mercury intrusion capillary pressure and nitrogen gas adsorption. Three classes of cap rocks are defined here according to their pore-throat structure, fractal dimension, and sealing capacity. These carbonate cap rocks are dominated by limestones and dolomitic limestones. Four pore types are identified: microfractures, intragranular, intercrystalline, and intracrystalline. Six pore structure types show multiscale variability from macropores to micropores. The pore structures present multiple fractal behaviors, with fractal dimensions showing an increasing trend as the pore diameter decreases. The cover coefficient, a parameter that allows characterization of the caprock sealing performance, shows an increasing trend along with increasing the fractal dimension of pore structure. The average cover coefficients of six pore structure types not only show good correlations (either exponential or linear) with certain fractal dimensions, but they also demonstrate a strong positive correlation with the average fractal dimension. These results suggest that the sealing capacity of the studied rocks increases with increasing fractal dimension. The sealing performance of cap rocks significantly decreases with an increasing amount of macropores. This work provides a relevant case study for further understanding of pore structures and sealing capacity of carbonate cap rocks.

中文翻译：

---

奥陶纪碳酸盐盖层孔隙网络的分形特征和封闭能力-以塔里木盆地露头类群为例

认识和预测碳酸盐盖层封闭能力的主要控制手段，对超深层碳酸盐岩储层勘探开发具有重要意义。这项研究的重点是通过使用光学和扫描电子显微镜，结合汞侵入毛细管压力和氮气吸附技术，分析露头类似物的样品，揭示塔里木盆地奥陶系碳酸盐盖层的孔隙网络和封闭能力。根据盖层的孔喉结构，分形维数和封闭能力，将其分为三类。这些碳酸盐盖层岩以石灰岩和白云质石灰岩为主。确定了四种孔类型：微裂缝，粒内，晶间和晶内。六种孔结构类型显示了从大孔到微孔的多尺度变化。孔结构表现出多种分形行为，随着孔径的减小，分形维数显示出增加的趋势。覆盖系数是允许表征盖岩封闭性的参数，随着孔隙结构的分形维数的增加，其趋势呈增加趋势。六种孔隙结构类型的平均覆盖系数不仅与某些分形维数显示出良好的相关性（指数或线性），而且与平均分形维数也显示出很强的正相关性。这些结果表明，研究岩石的封闭能力随分形维数的增加而增加。盖层岩石的封闭性能随着大孔数量的增加而显着降低。这项工作为进一步了解碳酸盐岩盖层孔隙结构和封闭能力提供了相关的案例研究。