A Study on Heterogeneity Characteristics and Permeability Behavior of Porous Media Using Multifractal Dimension Analysis: A Case Study of Tight Conglomerate Reservoirs in the Baikouquan Formation, Mahu Sag

Abstract The tight conglomerate reservoirs in the Junggar Basin exhibit complex pore–throat structures and strong heterogeneity, which significantly affect reservoir seepage behavior and hydrocarbon recovery efficiency. However, the main controlling factors of their heterogeneity remain unclear, and systematic quantitative characterization is lacking. To address this, this study focuses on the Triassic Baikouquan Formation (T 1 b 1 ) tight conglomerate reservoirs in the Maxi slope area. Using X-ray diffraction (XRD) analysis, mercury intrusion, petrographic thin-section image analysis, and scanning electron microscopy (SEM), combined with grayscale image-based multifractal dimension calculations and finite element numerical simulation, the heterogeneity characteristics and seepage capacity of the reservoirs were systematically investigated. The results indicate significant heterogeneity differences among various lithologies of the target layer, with the intensity decreasing in the order: conglomeratic sandstone > sandy conglomerate > conglomerate. Among different pore types, intragranular dissolved pores exhibit the highest heterogeneity, followed by intergranular pores and microfractures. Clay minerals also exert varying influences on heterogeneity, decreasing in effect in the order: chlorite > illite–smectite mixed layer > illite > chlorite–smectite mixed layer > kaolinite. In contrast, displacement efficiency shows an inverse relationship with heterogeneity, with conglomerate exhibiting the highest efficiency (69.49%), followed by sandy conglomerate (65.32%) and conglomeratic sandstone (61.72%). Further analysis shows feldspar negatively correlates with heterogeneity and enhances storage and seepage, while clay positively correlates with heterogeneity and generally restricts permeability. The effects of different clay minerals on permeability and heterogeneity vary and cannot be simply classified as flow-inhibiting or heterogeneity-enhancing.