In 2024, a significant breakthrough was achieved in well FSR-1, a scientific exploration well targeting deep geothermal resources in the Beibu Gulf Basin along the coastal region of South China. This well revealed the presence of hot dry rocks (HDRs)—weathered crustal granites with temperatures reaching up to 188.7 °C at burial depths exceeding 5123 m. In this study, high-quality log data were acquired from the target granites in the well, including natural gamma-ray (GR), caliper (CAL), density (DEN), compensated neutron log (CNL), sonic interval transit time (AC), dual laterolog (DLL) resistivity, temperature, borehole formation microresistivity imaging (FMI), and array acoustic logs. Using these datasets, this study characterized the lithology and the physical, thermodynamic, and electrical properties of HDRs in well FSR-1 and established log-based HDR identification criteria. Based on quantitative assessments of key geological and thermodynamic parameters, including lithology, mineral composition, porosity, fractures, in-situ stress, and specific heat capacity at constant pressure, this study established criteria for the selection of target intervals for fracturing. The research results indicate that the HDRs in well FSR-1 are weathered crustal granites characterized by generally poor physical properties and reservoir spaces composed of pores, vugs, and fractures. From the unconformity (weathered surface) downward, the reservoirs display a relatively systematic sequence: an eluvial zone, a completely weathered layer, a vertically fractured-vuggy vadose zone, a horizontally fractured-vuggy phreatic zone, and fractured bedrock. The horizontally fractured-vuggy phreatic zone is identified as the most favorable geothermal reservoir interval. Fracturing tests validated the log-based assessment, with the distribution of hydraulic fractures closely matching the predicted optimal fracturing interval vertically. Microseismic monitoring further indicates that during hydraulic fracturing, the initiation and propagation of hydraulic fractures were controlled by pre-existing natural fractures initially and were primarily governed by the orientation of the present-day maximum principal in-situ stress in the middle and late stages. The HDR identification criteria, the assessment methods for key geological and thermodynamic parameters, and the methods and criteria for fracturing interval selection established from well FSR-1 data, provide effective technical support for the exploration and exploitation of HDR resources in the Beibu Gulf Basin. • Logging responses and mechanisms of granitic DHRs. • Lithology, mineral composition, physical properties, fractures, in-situ stress and thermodynamic parameters for evaluation. • Criteria for selecting intervals for fracturing. • Hydraulic fracturing performance.
Liu et al. (Sun,) studied this question.