Achieving a thorough understanding of how primary sedimentary granularity drives considerable heterogeneity in internal reservoir attributes of terrigenous fine-grained deposits is of great significance. We investigated the quantitative differentiation and its corresponding driving forces of physical reservoir properties and pore-structure characteristics of silty-mud sediments in the Upper Triassic Xujiahe Formation (SW China) using a multi-method approach. The results show that the micro-mesopore volume and surface area of mudstones/shales are apparently higher than those of silty mudstones and a remarkable threefold rise in average permeability also presents. Extensively distributed bitumen pores occurring mostly along brittle mineral grains or forming clay-organic complexes make considerable contributions to shrinking microcracks. Furthermore, an evidently higher concentration of clay minerals in mudstone/shale reservoirs is primarily responsible for development of the two types of clay inter-crystalline pores distributed along grain aggregates and between well-oriented platelets. These two major causes facilitate the formation of micro-bedding fractures/non-bedding microfractures and connected fracture and pore-fracture networks, and also high-quality argillaceous reservoirs by strongly enhancing storage spaces and seepage capacities. Finally, a conceptual model is established for interpreting a differential reservoir-forming mechanism and corresponding two-sided effects on petrophysical and reservoir quality properties for continental silty-mud sediments.