Water Vapor Adsorption Characteristics and Controlling Factor Analyses in Permian-Aged Organic-Rich Marine Gufeng and Dalong Formations in Northeastern Chongqing Area of China

Water plays a critical role in shale gas formations by directly influencing resource evaluation and production efficiency. A systematic understanding of the water distribution, sorption mechanisms, and interactions with shale components is essential for quantifying gas storage and extraction optimization strategies. This study examines a total of 11 organic-rich shale samples from the Gufeng and Dalong Formations in northeastern Chongqing, by analyzing their mineral composition, pore structure, and lithofacies to elucidate water vapor adsorption behavior. Methodologies include determining total organic carbon content, performing X-ray diffraction, N2 physisorption, field-emission scanning electron microscopy, and water vapor adsorption (WVA) experiments. The adsorption–desorption isotherms reveal two distinct hysteresis loop patterns: argillaceous/siliceous mixed shale (M-2) lithofacies exhibit an H2-type loop, while other lithofacies display mixed H2/H3 characteristics, indicating different adsorption mechanisms. WVA occurs in three distinct phases: (1) monolayer adsorption at low humidity (RH1 < ∼20%); (2) multilayer adsorption at intermediate humidity (∼20% < RH2 < ∼70%); and (3) capillary condensation at high humidity (RH3 > ∼70%). Clay mineral content positively influences initial adsorption energy but does not dictate total adsorption capacity. Micropores, predominantly hosted in organic matter, serve as the primary sites for water vapor uptake. Among the studied lithofacies, siliceous shale (S) exhibits the strongest adsorption, followed by carbonate-rich siliceous shale (S-1), while M-2 shows the lowest adsorption capacity. These findings provide key insights into fluid–shale interactions with implications in understanding water adsorption behavior and assessing shale gas formations.