Classification, controlling factors, and multi-scale characterization techniques in shale reservoir pores: A comprehensive review

The size, distribution, and connectivity of pores play a critical role in determining the characteristics and formation of shale reservoirs. Therefore, pore analysis is critical for the reservoir evaluation. Considering the lack of a comprehensive overview, this paper reviews advancements in pore classification schemes, the factors and mechanisms controlling pore occur, and multi-scale characterization methods. We present an integrated analysis of pore classifications based on various criteria, including pore size, occurrence, connectivity, genesis, distribution, morphology, and their relationship with pore fluids. Thermal maturity, organic matter abundance, mineral composition, diagenesis, and tectonic processes are identified as key geological factors that govern pore formation and development. This study also highlights the evolution of experimental techniques, which have greatly enhanced the ability to characterize shale pores accurately. However, a unified classification system for shale reservoir pores is still lacking. A precise quantitative analysis of the influence of each geological factors on shale pore development remains absent. Each pore characterization method, while offering distinct advantages, also has its own limitations in terms of range and accuracy. When integrating multiple methods for comprehensive shale reservoir characterization, issues like poor data compatibility and difficulties in integration still exist. Future research should prioritize the standardization of shale pore classification, the quantitative assessment of geological factors influencing pore development, and the establishment of a comprehensive pore characterization framework and set of standards. This paper seeks to advance the understanding of shale reservoir pores, providing valuable insights and references for subsequent studies.