Reservoir characteristics and effect on methane adsorption capacity in marine–continental transitional shale: The Carboniferous Yanghugou Formation in the Weiningbeishan area (eastern North Qilian orogenic belt), China

Marine–continental transitional (MCT) shale from the Yanghugou Formation in the Weiningbeishan area represents a promising yet underexplored target for shale gas development. This study systematically investigates the reservoir properties and methane adsorption behavior of MCT shale samples from the Well ZK03 through integrated analyses, including total organic carbon (TOC), Rock-Eval pyrolysis, maceral composition, vitrinite reflectance ( R O ), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low-pressure nitrogen adsorption, and high-pressure methane adsorption. The results reveal that the shale is rich in type II 1 and II 2 organic matter with high TOC content and has entered the dry gas generation window. A key finding is the predominance of well-developed clay mineral-hosted pores, in contrast to the scarcity of organic matter-hosted pores—a distinctive feature compared to typical marine shales. The complex pore structure is dominated by meso–macropores in terms of volume, whereas micropores contribute most significantly to the specific surface area. Methane adsorption capacity shows positive correlations with both TOC and clay content, underscoring the synergistic role of organic and clay components in controlling gas adsorption. By clarifying the specific mechanisms governing methane adsorption in MCT shales of the Yanghugou Formation, this work provides novel insights into the unique gas enrichment patterns of transitional shales, addressing a critical gap in the current understanding of their reservoir characteristics.