• Depositional environment significantly controls the microstructure and rheological behavior of sedimentary soft rocks, with floodplain lake and sheet sand microfacies exhibiting superior stability due to dense quartz skeletons.
• A novel frequency-domain upscaling method integrating the Mori-Tanaka scheme accurately predicts macroscopic viscoelastic properties from nanoindentation creep data.
• Riverbed lag and shallow lake mud microfacies show poor rheological performance due to skeleton relaxation and clay-dominated slip, respectively.
• The point bar microfacies displays a 'rigid-soft hybrid' behavior, combining high long-term stability with reduced transient stability.