Water vapor sorption behavior of shale organic matter with various types and maturation

To investigate the water vapor adsorption (WVA) behavior of shale organic matter (OM), we conducted a series of WVA experiments integrated with gas physisorption, Fourier transform infrared spectroscopy (FTIR), and small-angle neutron scattering (SANS) on six OM samples with varying types (I, Ⅱ, and Ⅲ) and maturities (R_o: 1.21–3.56 %). The adsorption process was analyzed using the Dent and Freundlich models. Results indicate that capillary condensation accounts for more than 50% of the total WVA capacity at RH 0.95. Layered adsorption is primarily governed by pore surface properties: Type Ⅲ OM, contains carboxyl groups, exhibits the strongest adsorption strength, whereas overmature Type I OM, despite exhibiting rough pore surfaces and abundant adsorption sites, shows comparatively weak adsorption strength. Pore volume (PV) and specific surface area (SSA) provide the spatial basis for adsorption. Moreover, the presence of strongly hydrophilic functional groups (particularly carboxyl) and enhanced pore connectivity can extend the effective adsorption pore size range, further facilitating WVA. Among the OM types, Type I exhibits the greatest pore development and connectivity but lacks hydrophilic functional groups, while Type Ⅲ shows the opposite characteristics. Maturity also exerts a significant influence on both pore structure and surface properties. These findings highlight the coupling effects of pore structure and surface properties in controlling WVA and fill the research gap into the adsorption behavior of shale OM with different types and maturities.