Saturation Model for Tight Sandstone Integrating Effective Medium and Equivalent Rock Element Theories

Tight sandstone reservoirs are pivotal unconventional hydrocarbon resources, yet accurate hydrocarbon saturation evaluation is severely hindered by their complex mineral composition and intricate pore-throat structure. Conventional shaly sand saturation models fail to decouple the joint effects of clay and calcite and lack the characterization of pore structure heterogeneity and fluid distribution nonuniformity, leading to significant errors in saturation calculation. To address these limitations, this study develops a novel water saturation model for tight sandstones by synergistically integrating effective medium theory and equivalent rock element theory. First, quantitative mineralogical analysis (QEMSCAN) and 3D pore structure characterization (FIB-SEM) were conducted on core samples from the Gaotaizi tight sandstone reservoir of the Qingshankou Formation in the northern Songliao Basin, revealing the coexistence of high clay/calcite content and a multiscale pore-throat system with coarse pores connected by fine throats. Based on the petrophysical characteristics, the effective medium theory was used to describe the multicomponent conductive behavior of the rock matrix, clay, calcite, and mixed pore fluids, while the equivalent rock element theory was incorporated to quantify the impact of pore structure on rock electrical conductivity by introducing the pore structure efficiency and fluid heterogeneity index. Key model parameters, including percolation rate, percolation exponent, pore structure efficiency, and fluid heterogeneity index, were calibrated via core experiments and the least-squares method. The proposed model was validated through core analysis data and field well logging applications, with comparisons to classic models and actual production test results. The results show that the proposed model exhibits superior consistency with core-measured water saturation and the calculated saturation profiles reliably guide oil–water layer identification, with interpretation results highly matching production test outcomes. This integrated model explicitly accounts for both complex mineral composition and intricate pore structure characteristics of tight sandstones, breaking the limitations of conventional models that only consider the clay conductive contribution. It provides a more accurate and reliable petrophysical tool for saturation evaluation of clay- and calcite-bearing tight sandstone reservoirs, and offers theoretical and technical support for the efficient development of unconventional tight oil resources.