Research progress and implications of experimental techniques and occurrence patterns of pore water in high- to over-mature shale reservoirs

The distribution and occurrence characteristics of pore water have a significant impact on shale gas occurrence and enrichment, but the understanding of pore water in existing research is insufficient, which limits the research on shale gas enrichment mechanisms and exploration deployment. This study systematically reviews the current research progress on experimental testing techniques and occurrence patterns of pore water in shale reservoirs, and proposes key research directions for the future based on exploration practices. The study shows that: ① the evaluation of water saturation in shale reservoirs mainly relies on logging calculation methods and experimental testing methods. The characterization of microscopic occurrence state and mobility of pore water is mainly conducted using nuclear magnetic resonance (NMR) experiments and molecular simulation techniques. ② The occurrence states of pore water in shale reservoirs could be divided into structural water, interlayer water, bound water (water film bound water and capillary bound water), and free water. ③ Pore water in shale reservoirs is preferentially distributed in hydrophilic pores. The distribution of pore water in organic/inorganic pores is affected by various factors such as medium interface wettability, rock mineral composition, and pore structure. ④ High- to over-mature shale is usually characterized by ultra-low water saturation. However, due to graphitization and structural adjustment, shale with high water content can also be formed. In the future, it is necessary to optimize the evaluation methods for shale water saturation under insitu conditions, develop automated experimental technology and equipment, improve analytical efficiency, and promote the standardization of experimental procedures. Additionally, it is recommended to strengthen research on the occurrence states under in-situ conditions and the multi-scale distribution characteristics of shale pore water, deepen the understanding of the genetic mechanisms of high water content in high- to over-mature shale, reconstruct the differential evolution pathways of pore water occurrence, and develop a shale gas geological evaluation parameter system that considers pore water. This can provide a theoretical basis for the exploration deployment and development decision-making of deep and ultra-deep shale gas.