China possesses vast and widely distributed oil and gas resources, characterized by significant development potential and rapid growth in their exploitation. However, the exploration and development of unconventional resources such as shale gas and tight oil remain at an early stage. As domestic demand for oil and gas continues to rise, the extraction of deep unconventional resources has consequently become a major focus in national energy engineering. A common practice in unconventional oil and gas production involves large-scale fluid injection into deep geological formations. Such injection disturbs the in-situ stress field and alters the stress state of subsurface faults, which may lead to fault instability, slip, and subsequently induced seismicity. Therefore, evaluating and mitigating such anthropogenic seismic risks has become a critically important issue for achieving safe and sustainable resource development. In recent years, seismic events have been monitored during the production phases of multiple deep energy projects worldwide. Post-earthquake analyses indicate a clear spatiotemporal correlation between these events and fault activation or instability triggered directly by fluid injection. Notable examples include geothermal projects in Pohang, South Korea, and Basel, Switzerland, which were suspended due to significant induced earthquakes. In fact, geological conditions in China's oil and gas fields are often more complex, making it difficult to accurately predict fault slip conditions, specific slip patterns, and displacement, or to determine optimal operational parameters near fault-developed zones. To balance benefits and risks in unconventional resource development, predicting and preventing the associated environmental geological issues and seismic hazards caused by deep energy extraction has become an urgent challenge. This study reviews typical cases of induced seismicity in energy projects globally, with a focus on analyzing the underlying stress conditions and the detailed triggering mechanisms of fault instability and slip at development sites. It systematically summarizes fault slip instability modes, established criteria for induced seismicity, estimated affected ranges, and current magnitude prediction models. The influence of both anthropogenic engineering factors and inherent environmental geological conditions on induced seismic events during development is also thoroughly discussed. Finally, this study summarizes the major unresolved issues and key technical challenges that need to be addressed in current research, and outlines potential future research directions through three primary approaches: numerical simulation, experimental methods, and enhanced field monitoring. The work contributes to a deeper fundamental understanding of human induced seismicity from oil and gas operations, and holds practical implications and considerable engineering value for mitigating or preventing potential seismic hazards.

For shale reservoir operations, assessing the brittleness of anisotropic shales is vital of optimizing wellbore stability analysis and fracturing design. Current brittleness indices lack effectiveness in characterizing shale brittleness anisotropy. Consequently, the experimental results reported in this paper derived from triaxial tests conducted on shale core samples from the Qingshankou Formation of the Songliao Basin. This study investigates the anisotropy of shale failure patterns and mechanical properties with respect to the bedding plane dip angle (θ), and quantifies the effect of confining pressure. Building on the cohesion weakening and friction strengthening (CWFS) theory, we established a novel triaxial brittleness index (B_t). This index uniquely combines the uniaxial brittleness index (B_u), reflecting inherent brittleness, with the brittleness weakening coefficient (B_w), quantifying the effect of confining pressure. Assessment of the anisotropic brittleness of shale based on B_t under varying confining pressures reveals that B_t first increases but then decreases with increasing θ. The brittleness peaks at θ = 0° and reaches its lowest point at θ = 60°, a trend that aligns closely with the observed variations in the failure patterns of shale. Furthermore, the ability of the confining pressure to decrease shale brittleness varies with θ. At θ = 0°, the uniaxial brittleness is the highest, but the confining pressure has the strongest weakening effect on shale brittleness. In contrast, the uniaxial brittleness at θ = 90° is second only to that at 0°, but the brittleness in this direction is least affected by the confining pressure. Compared with the five existing brittleness indices, the proposed index accounts for both inherent and apparent brittleness. It is more sensitive to internal lithological characteristics and external stress conditions and has strong potential for integration with geophysical data. This study provides valuable guidance for sweet spot identification, wellbore stability assessment, and fracturing scheme optimization in shale oil and gas exploration.