Effect of depositional environment differences on micro-macro rheological behavior of sedimentary soft rocks

Although significant progress has been made in micromechanical characterization and upscaling of homogeneous materials, systematic investigations into deposition-controlled micro–macro rheological relationships in heterogeneous sedimentary soft rocks remain limited, particularly concerning time-dependent viscous parameter upscaling. This study investigates six typical fluvial and lacustrine microfacies from the Ordos Basin, China, including riverbed lag, natural levee, floodplain lake, point bar, sheet sand, and shallow lake mud. Mineral composition and microstructure are characterized, and nanoindentation creep tests quantify viscoelastic properties. A micro–macro upscaling method that transforms the time-domain Burger model into the frequency domain and utilizes three traditional homogenization schemes: dilute approximation, Mori-Tanaka, and self-consistent methods, for comparative estimation of macroscopic rheological parameters is proposed. Microstructural analysis demonstrates distinct fabric patterns controlled by depositional energy. Floodplain lake and sheet sand microfacies show superior rheological stability due to dense quartz skeletons, whereas riverbed lag and shallow lake mud perform poorly, caused by skeleton relaxation and clay-dominated slip, respectively. The point bar microfacies exhibits a “rigid-soft hybrid” behavior, with high long-term stability but reduced transient stability. Comparatively, the frequency-domain upscaling framework developed in this study, incorporating the Mori-Tanaka scheme, demonstrates satisfactory agreement with experimental data, validating its capability to predict macroscopic viscoelastic properties from microstructural features.