## 1 Introduction  
The extraction of natural gas from shale presents a highly promising and environmentally sustainable alternative to traditional fuel sources [1]. Shale gas, known for its relatively low carbon emissions, possesses the potential to address escalating global energy requirements. Therefore, the development of advanced shale gas extraction technologies has emerged as a prominent subject at the forefront of scientific and technological advancements [2; 3]. In order to efficiently extract shale gas from micro- and nano-scale gas-containing pores and fractures (Figure 1), hydraulic fracturing is required for shale reservoirs. The fracture network generated by hydraulic fracturing in shale reservoirs contains numerous microfractures. These microfractures serve as crucial pathways, allowing shale gas to flow from micro- and nano-scale pores towards macroscopic fracturing fractures and wellbores, thereby significantly improving the shale gas recovery rate [4; 5; 6]. However, the current academic and industrial focus is primarily on the evolution laws and affected volume of macroscopic fracturing networks, with less attention given to the microfractures connected to these macroscopic fracturing fractures [7; 8]. As a result, there is a lack of in-depth understanding regarding their formation mechanisms, distribution characteristics, and dynamic propagation processes, which severely limits breakthroughs in improving shale gas recovery processes [9; 10; 11]. Therefore, an in-depth exploration of the causes, characteristics, and propagation patterns of the microfractures generated during the process of hydraulic fracturing in shale holds urgent engineering practicality and significant research value. In addition to its practical significance in enhancing shale gas recovery [13], the investigation of microfracturing in rocks holds equal importance for the development of rock fracture theories. The theory of structural control in rock masses posits that the influence of rock mass structure on rock fracturing is significantly greater than that of rock materials, serving as a fundamental theory in rock mechanics. The microfracturing of rocks is similarly influenced by microstructural features such as mineral distribution, porosity, joints, cleavage, and fractures. Within rock masses, there exist abundant microstructures and microfractures, which, upon experiencing stress, undergo processes of extension, merging, and bridging, ultimately resulting in macroscopic fractures [14; 15]. Therefore, to gain in-depth insights into the fracture mechanisms of rock masses, the study of rock fracture processes from a microscopic perspective is imperative. Figure 1: (**a**) Organic matter pores in shale. (**b**) Microfractures connected to gas containing pores [12]. Compared to the research on macroscopic cracks, there has been relatively limited investigation into microfractures in shale, and the mechanisms behind microfracture formation remain unclear. The current state of research in this field is fragmented and lacks a systematic approach, with a dearth of comprehensive review articles outlining the progress made in the study of microfracturing in shale. Experimental testing serves as a fundamental means to conduct research, and the observation of microfractures in shale after hydraulic fracturing, as well as the continuous dynamic monitoring of microfracture initiation and extension in situ, have confirmed that shale fracturing occurs through the generation and connection of discontinuous fractures [16; 17]. What are the formation and evolution mechanisms of discontinuous fractures? What factors influence the generation of discontinuous fractures? How does the heterogeneity within shale affect the initiation and propagation of discontinuous fractures? In-depth studies on these issues require the acquisition of the micro-mechanical parameters of shale and the measurement of displacement, deformation, and energy during loading.