###### Abstract  
Unconventional oil and gas resources are indispensable, and shale oil is one of them. The Junggar Basin is a typical superposition oil and gas basin in China, with reserves of 100 million tons in many areas and various types of oil and gas reservoirs. The Permian Fengcheng Formation in Mahu Sag has great potential for oil generation, making the study of the Fengcheng Formation reservoir in Mahu Sag particularly important. Based on previous studies, the core sample from well Maye-1 is divided into four lithologies according to mineral composition: felsic shale, dolomitic felsic shale, clay-bearing felsic shale, and siltstone interlayers. Through core observation and description, it is found that the macroscopic porosity of each lithology is well-developed, with felsic shale exhibiting the highest macroscopic fracture density, followed by siltstone interlayers, and clay-bearing felsic shale showing the least development. Argon ion polishing scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) techniques show that the siltstone interlayer pore development is the best, with pore sizes ranging from 100 to 4000 nm. The fracture development of dolomitic felsic shale is the most significant, with fractures contributing up to 80.14%. The porosity of clay-bearing felsic shale is only 1.12%. The development of pores and fractures in the study area is related to sedimentary tectonic factors and diagenesis. It mainly exhibits three types of subfacies deposits, namely semi-deep lake subfacies, shallow lake subfacies, and lakeshoe lake subfacies, predominantly composed of felsic shale. Strong tectonic movements contribute to the formation of macroscopic fractures. Diagenesis plays a crucial role in the formation of microscopic pores. The Fengcheng Formation is primarily influenced by compaction, pressure dissolution, dissolution, and metasomatism. These various diagenetic processes collectively promote the formation of pores, ultimately leading to the development of a multi-scale porosity system in the Fengcheng Formation. pore development; multi-scale; SEM; NMR; core observation 2024201  
oil reservoirs, the pore structure of reservoir shale is of paramount importance [6]. Shale pores serve not only as storage spaces but also as migration channels. Therefore, relevant studies on shale reservoir pore structure are of significant importance [7; 8]. Lithofacies refers to the types of rocks and their combinations formed in certain sedimentary environments [9; 10; 11]. In different regions, due to variations in sedimentary environments and processes, there are often different divisions of lithofacies. Influenced by differences in material composition and diagenetic states, shale reservoirs of different lithofacies exhibit varying mineral compositions, organic matter contents, and pore structure distributions [11]. The research methods of reservoir pore structure are generally divided into two methods: direct observation and indirect tests. Direct observation is to observe and describe the samples directly by means of core description and observations using optical microscopy and scanning electron microscopy. The indirect test indirectly obtains the distribution of the pore structure inside the sample by calculating by filling the sample with fluid or gas, including high-pressure mercury injection, gas adsorption, nuclear magnetic resonance, and so on [12]. Pore development is generally affected by the reservoir mineral composition. Although different mineral compositions are affected by the same sedimentary environment during deposition, different pore structures are developed. On the other hand, in the process of diagenesis, pores, affected by mechanical compaction and cementation, will decrease, while diagenetic processes such as dissolution will make reservoir pores more developed [13]. Many scholars have studied the structure [14], sedimentation [15], rock [16], and minerals [17] in the Fengcheng Formation reservoir in Mahu Sag.