Shale fractures, serving as conduits for the flow and storage space of shale oil, play a critical role in resource exploration and evaluation. Shale oil is considered a highly promising unconventional natural gas resource in the 21st century, offering a significant supplement to conventional energy sources and playing an important role in addressing the growing global energy demand. Based on a comprehensive synthesis of existing research on shale fractures, this paper proposes a novel classification system aimed at investigating the formation mechanisms, developmental characteristics, and primary controlling factors of different fracture types. Rooted in geological origins, shale fractures are categorized into six major classes and eleven subcategories, including structural fractures, diagenetic fractures, bedding fractures, fluid pressure fractures, structural diagenetic fractures, and structural fluid pressure fractures, with each type's formation mechanism and development process discussed in detail. The factors influencing fracture development include both internal and external elements. The controlling factors for different fracture types vary. For instance, structural fractures are primarily influenced by tectonic stress, while diagenetic fractures are related to changes in rock properties due to diagenesis. Through an in-depth exploration, this paper reveals the causal mechanisms underlying each fracture type. Structural fractures typically arise when tectonic stress exceeds the rock's fracture strength; diagenetic fractures result from changes in rock structure and mechanical properties induced by diagenesis; bedding fractures primarily result from tectonic uplift, leading to the fracturing of bedding planes through a series of diagenetic processes; fluid pressure fractures are caused by sudden abnormal fluid pressures in the formation, leading to overpressure and subsequent fracture formation.

Moreover, the development of structural rock fractures is primarily controlled by a combination of lithology, rock structure, tectonic activity, and fluid pressure, typically formed through the coupled effects of tectonic activity and diagenesis. The formation of fluid pressure fractures, on the other hand, is facilitated by the interaction between tectonic activity and fluid pressure. This paper further analyzes the underlying causes of fracture development and systematically describes the main controlling factors, providing an essential theoretical foundation and guidance for shale oil exploration. In light of the above research, this paper also identifies several future research directions, including the establishment of unified classification standards, the refinement of micro-fracture scale characterization methods, and the further quantification of composite fracture formation mechanisms. A more in-depth and accurate understanding of fracture development processes will provide a scientific basis for shale oil exploration and development, offering both theoretical support and practical guidance for the efficient utilization of resources.

The Fengcheng Formation in the Hashan area, located on the northwestern margin of the Junggar Basin, represents a saline-alkaline lake facies with fine-grained mixed sedimentation. This formation is rich in alkaline minerals and serves as a high-quality source rock for hydrocarbon generation in alkaline lakes. However, its lithology is complex, and the origins of the salt minerals remain unclear. This study focuses on the salt minerals in the Fengcheng Formation of the Hashan area. Using core observation, thin section identification, scanning electron microscopy, electron probe micro-analysis, trace and rare earth element analysis, stable isotope analysis, fluid inclusion analysis, and zircon U-Pb dating, the sedimentary age of Fengcheng Formation and the mineralogical and geochemical characteristics of salt minerals were systematically studied. The salt minerals identified in the Fengcheng Formation include calcite, dolomite, eitelite, northupite, shortite, reedmergnerite, and Na-carbonate. According to the different types of salt minerals, the different contact relations between minerals, the different production positions and production styles of mineral combinations, the salt mineral assemblage in the study area is classified into three categories: The combination of calcite, dolomite, shortite, and reedmergnerite, The combination of Na-carbonates, eitelite, shortite, and reedmergnerite, The combination of dolomite, eitelite, shortite, and northupite. Two zircon U-Pb ages, 307.8 ± 2.7 Ma and 308.5 ± 3.5 Ma, span the Carboniferous-Permian boundary, corresponding to an interglacial period within the Late Paleozoic Ice Age, aligning with the development of salt minerals. Salt minerals have the formation modes of sedimentation, replacement and hydrothermal transformation. Terrestrial weathering products, atmospheric, volcanic and hydrothermal processes, residual seawater, clay mineral transformation, thermal evolution of organic matter and tuffaceous alteration are material sources. The salt-forming fluid has the characteristics of weak acid-alkaline, medium-low temperature and high salinity, and is mainly driven by subduction zone high pressure, magmatic heat and gravity. The burial depth, temperature and CO₂ concentration required for the formation of salt minerals were clarified, and the evolution sequence of salt-forming fluids from sedimentation to diagenesis and accompanied by hydrothermal (hot water) activities was summarized. The evolution model of salt minerals controlled by different genesis from the first member to the third member of Fengcheng Formation was established. The research findings are significant for understanding the paleoenvironment of the Fengcheng Formation, the formation mechanisms of high-salinity lakes, and the salt formation models.