## 1 Introduction  
In recent years, China has achieved remarkable progress in the exploration of shale gas resources, making it the third-largest producer of shale gas in the world [1, 2]. In this process, scholars and the industry have not only continuously made breakthroughs in shale gas exploitation in major resource-bearing basins, including the Sichuan Basin, but also established a systematic geological understanding of shale gas enrichment and accumulation in China [3, 4]. With the continuous mining of medium and shallow shale gas resources, the development of deep shale gas reserves, at depths exceeding 3500 m, has become a crucial issue for the sustainable growth and long-term development of the shale gas industry [5]. The Wufeng-Longmaxi Formation shale reservoirs in the Sichuan Basin have great potential for deep shale gas exploration and development. It is noteworthy that deep shale wells in western Chongqing have exhibited good productivity capacities, showing promising development prospects [6]. However, the geological conditions of deep shale are more complicated, and the factors affecting the gas content of medium-shallow and deep shale gas fields are very different [7; 8]. Therefore, deepening the understanding of deep shale reservoir characteristics is a fundamental basis for the evaluation of potential reserves and the effectiveness of shale gas exploration. Pore structure parameters are critical indicators of reservoir quality, performing an essential role in the evaluation and prediction of shale gas resources [9; 10]. Shale gas reservoirs are characterized by complex nanoscale pore systems, and a comprehensive understanding of the pore system's structural characteristics is key to estimating gas content and assessing the productivity of the reservoir. Furthermore, the microscopic pore structure of deep shale reservoirs exhibits strong complexity and heterogeneity, which directly affects gas enrichment and is also an important influencing factor for the effectiveness of reservoir evaluation and prediction results [5]. The application of complementary methods and fractal theory is of great importance to form a comprehensive understanding of the particularity and heterogeneity of pore structure. NMR methods are widely used to acquire pore size distributions by measuring \(T_{2}\) relaxation time distributions [11; 12; 13; 14]. The measurement of shale pore structure using NMR methods has been proven to be a reliable technique. NMR is one of the precise methods for characterizing pore size distributions in natural porous media [15; 16]. Compared with high-pressure mercury injection and gas adsorption methods, NMR measurement is fast, accurate, nondestructive, and has a wide measuring range [14; 16]. Previous studies have shown that shale reservoir pores have a certain degree of self-similarity [17; 18]. Therefore, fractal theory can quantitatively analyze pore networks' complexity and heterogeneity and has gained widespread application in the analysis of rock pore structure [19; 20]. Shale reservoir pores are developed at both nanoscale and microscale, and fractal modeling can be used as a powerful tool for measuring their non-intuitive characteristics. In this study, a qualitative-quantitative combined investigation was conducted to reveal the pore structure and fractal characteristics of representative deep shale samples from the Wufeng-Longmaxi Formation in the western Chongqing area based on low-field nuclear magnetic resonance (NMR) and fractal modeling methods. Samples from different layers were selected to obtain information on the pore development degree, pore size distribution, and complexity of the pore system, and to compare the differences between layers. Field-emission scanning electron microscopy (FE-SEM) was employed to observe and examine the morphological characteristics of different types of pores at different scales.