Abstract: Against the backdrop of the carbon peaking and carbon neutrality goals, the demand for the development and utilization of medium- and deep geothermal resources continues to increase, making the identification of deep geothermal reservoirs and target selection an important research focus in geothermal exploration. However, thick overburden, complex geological structures, and anthropogenic interference impose significant limitations on conventional single exploration methods in terms of investigation depth, resolution, and interpretation accuracy, making them inadequate for evaluating deep geothermal reservoirs under complex geological conditions. Focusing on deep geothermal reservoir target selection, recent advances in the application of seismic exploration, electromagnetic exploration, and hydrogeochemistry are systematically reviewed in representative geothermal fields worldwide. The applicability and limitations of these methods for identifying deep heat sources, characterizing reservoir structures, detecting fault-controlled fluid pathways, and analyzing geothermal fluid migration are comprehensively evaluated. On the basis of integrated exploration case studies from representative geothermal fields, the collaborative mechanisms of seismic imaging, electrical resistivity inversion, and hydrogeochemical constraints are summarized, and the technical characteristics and applicability of multi-source data integration under different geological settings are synthesized. The results indicate that seismic methods are effective in delineating fault distributions and deep structural features, electromagnetic methods exhibit a strong capability for identifying deep low-resistivity geothermal reservoirs and fluid-enriched zones, and hydrogeochemical methods provide effective constraints on reservoir temperature, fluid origin, and circulation processes. Individual methods are inherently constrained by geological conditions and their own detection capabilities, whereas integrated multi-method exploration fully exploits the complementary strengths of different techniques. By using seismic imaging to constrain the structural framework, electrical resistivity inversion to delineate the spatial distribution of geothermal reservoirs, and hydrogeochemical investigations to verify fluid circulation characteristics, comprehensive identification of the deep heat source–reservoir–fluid conduit system can be achieved. For geological settings characterized by thick overburden and strong interference, an integrated target selection strategy centered on the collaborative application of seismic exploration, electromagnetic exploration, and hydrogeochemical constraints is summarized. Corresponding combinations of exploration techniques and their applicable geological conditions are also synthesized, providing a practical technical framework for deep geothermal reservoir exploration in complex geothermal systems.