The high-pressure mercury intrusion (HPMI) experiment is widely used to assess the pore architecture of tight sandstone reservoirs. However, the conventional analysis of the high-pressure mercury intrusion has always focused on the mercury injection curves themselves, neglecting the important geological information conveyed by the mercury ejection curves. This paper quantitatively describes the fractal characteristics of ejection curves by using four fractal models, i.e., Menger model, Thermodynamic model, Sierpinski model, and multi-fractal model. In comparison with mercury injection curves, we explore the fractal significance of mercury ejection curves and define the applicability of different fractal models in characterizing pore architectures. Investigated tight sandstone samples can be divided into four types (Types A, B, C and D) based on porosity, permeability, and mercury removal efficiency. Type D samples are unique in that they have higher permeability (>0.6 mD) but lower mercury removal efficiency (<35%). Fractal studies of the mercury injection curve show that it mainly reflects the pore throat characteristics, while the mercury ejection curve serves to reveal the pore features, and porosity and permeability correlate well with the fractal dimension of the injection curve, while mercury removal efficiency correlates only with the Ds' value of the ejection curve. The studies on the mercury ejection curves also reveal that the small pores and micropores of the Type C and Type D samples are more developed, with varying pore architecture. The fractal dimension D_S’ value of Type D samples is greater than that of Type C samples, and the dissolution of Type D samples is more intense than that of Type C samples, which further indicates that the Type D samples are smaller in pore size, rougher in surface, and with greater difficulty for the hydrocarbon to enter, resulting in their reservoir capacity probably less than that of Type C samples. In this regard, the important information characterized by the mercury ejection curve should be considered in evaluating the tight sandstone reservoirs. Finally, the Menger and Thermodynamic models prove to be more suitable for describing the total pore architecture, while the Sierpinski model is better for characterizing the variability of the interconnected pores.

The Hashan area, neighboring the Mahu Sag that is rich in the shale oil resources, showed commercial oil flow in the corresponding lacustrine shales of the Lower Permian Fengcheng Formation (P₁f) with reserve scale approximately 789 million tons, presenting great potential for oil exploration. Despite their geographical proximity, the hydrocarbon occurrence and oil-bearing capacity of shale in the Hashan area and Mahu Sag greatly differ owing to the complex tectonic evolution. Therefore, understanding the occurrence state and oil content of the P₁f in the Hashan area is crucial for ongoing shale oil exploration activities and the development of the northwestern margin of the Junggar Basin. In this study, an integrated investigation, including petrological observations, scanning electron microscopy (SEM) observation, analysis of nuclear magnetic resonance (NMR) T₁–T₂ spectra, and conventional and multistage Rock-Eval pyrolysis methods were conducted to evaluate the occurrence state and oil content of the P₁f shale in the Hashan area. The results indicate that plagioclase (average 30.7%) and quartz (24.1%) dominate the mineral compositions of the P₁f shale samples. A method involving quartz–plagioclase–carbonate minerals is proposed to conduct lithofacies classification. In the Hashan area, the organic matter abundance in the P₁f shale is scaled in fair to good range, the thermal maturity ranges from immature to early mature stage, and the primary organic matter types are Types Ⅰ and Ⅱ₁. Intergranular and dissolution pores are the two most common pore types. The free oil is mostly found in the pores and microfractures of the mineral matrix, whereas the adsorbed oil is mostly adsorbed on the surfaces of kerogen and clay minerals. The high organic matter abundance, quartz content, and porosity account for substantial increase in the oil content, the area rich in shale oil resources coincides with that rich in free oil. The most favorable lithofacies in the Hashan area is the calcareous mudstone/shale, which hosts the highest free oil content (average 2.49 mg), total oil content (15.02 mg/g), organic matter abundance (TOC = 1.88% and S₁ + S₂ = 20.54 mg/g) and porosity (5.97%).