Distinguishing Geometric and Apparent Relaxation–Response Fractal Parameters in Fuyu Formation Tight–to–Low–Permeability Sandstones: A Comparative MICP–NMR Framework

Fractal analysis has been widely applied to characterize the complexity of pore systems in tight sandstone reservoirs. However, fractal parameters derived from different experimental techniques are frequently interpreted without sufficient consideration of their distinct physical meanings. In this study, tight sandstones from the Fuyu Formation in the Songliao Basin were investigated using cast thin sections, scanning electron microscopy, high–pressure mercury intrusion porosimetry, and nuclear magnetic resonance. The objective was to establish a comparative interpretation framework that distinguishes capillary–pressure–controlled geometric fractal dimensions from NMR–derived apparent relaxation–response fractal parameters (ARR fractal parameters). The pore–throat system is dominated by medium– to fine–sized throats, with the dominant pore–throat radii concentrated between 0.05 and 0.15 μm. Mercury–intrusion–derived fractal parameters primarily characterize the geometric heterogeneity of relatively large and fine pore–throat systems, whereas NMR–derived parameters mainly reflect the apparent relaxation response associated with pore–scale fluid occurrence rather than strict geometric complexity. In the present MICP–NMR subset, comparative regression trends indicate that the geometric fractal parameter representing fine pore throats is more responsive to median pore–throat radius and movable–fluid saturation, whereas the NMR–derived apparent relaxation–response parameter associated with longer relaxation times shows a relatively closer relationship with log–transformed permeability. These relationships should be regarded as exploratory response trends rather than universal predictive models. These findings demonstrate that the two types of fractal parameters provide complementary rather than interchangeable information for reservoir evaluation. The proposed comparative framework offers a physically constrained basis for integrating pore–throat geometry, relaxation response, fluid occurrence, and seepage properties in the characterization of tight sandstone reservoirs.