Geothermal Geological Characteristics and Genetic Model of the Neogene Sandstone Geothermal Reservoirs in the Eastern Gushi Sag, Weihe Basin

The characterization of geothermal reservoirs and their genetic mechanisms is critical for understanding geothermal system evolution and evaluating geothermal resource potential. The eastern Gushi Sag of the Weihe Basin hosts three Neogene sandstone geothermal reservoirs, including the Gaoling Group, Lantian–Bahe Formation, and Zhangjiapo Formation; however, their reservoir characteristics and genetic mechanisms remain poorly constrained. This study integrates geological structures, geothermal well logging, core petrophysical properties, and hydrochemical data to characterize reservoir conditions and establish a genetic model. The results show that the Neogene reservoirs are mainly composed of feldspathic sandstone, with the Lantian–Bahe Formation identified as the primary geothermal reservoir due to its moderate porosity, low permeability, large sandstone thickness, and favorable continuity. The geothermal field exhibits an average geothermal gradient of 3.35 °C/100 m with a south-to-north decreasing trend. Hydrochemical evidence suggests that geothermal fluids originate mainly from meteoric water recharged from the northern Qinling Orogenic Belt and paleo-sedimentary water, with deep faults and pore networks controlling fluid migration and accumulation. The Quaternary strata and Zhangjiapo Formation provide effective sealing conditions. This study reveals the coupled controls of thermal conditions, reservoir architecture, fluid circulation, and preservation on sandstone geothermal systems, providing insights into geothermal resource assessment, exploration strategy optimization, and the formation mechanisms of similar sedimentary basin geothermal systems.