PhD defence by Tingting Zheng

Shandong, one of China’s most populous provinces, faces growing pressure to shift to carbon-neutral energy to mitigate climate change. Tectonically located on the eastern margin of the North China Craton, Shandong has undergone substantial tectonic and magmatic activity throughout geological history, forming the foundation for extensive geothermal resources ideal for space heating and other direct uses. This PhD project applies an integrated geoscientific approach to improve understanding of the geochemical, hydrological, and reservoir engineering characteristics of representative systems and to support their sustainable utilization. Two main geothermal system types in Shandong Province were investigated. Firstly, convective geothermal systems hosted in intrusive rock in hot spring areas of the Shandong Peninsula (Eastern Uplift) feature active natural recharge and discharge through permeable fractures/faults (open boundaries). Geochemical and isotopic analyses indicate a dominantly meteoric origin, variably modified by seawater mixing, salt leaching, water–rock interaction, and conductive cooling during ascent. Two conceptual models are distinguished: (a) deep circulation (~2–5 km) associated with major faults and granite intrusions, characterized by higher reservoir temperatures and mixtures of reacted “old” and modern waters; and (b) shallow circulation systems controlled by minor fractures, displaying minimal water–rock interaction and local seawater influence. Secondly, sandstone hosted conductive systems in the Dezhou region (Northwestern Depression) that have limited natural recharge. Reservoir properties, reinjection performance, thermal breakthrough risk and sustainable yield of the Guantao sandstone reservoir were evaluated using long-term monitoring, reinjection experiments, lumped-parameter modelling, and volumetric energy-balance calculations. Hydraulic parameters measured in different tests indicate good injectivity. Long-term monitoring shows that heavy production caused large drawdowns, but after reinjection began the drawdown halted. Lumped-parameter modelling, combined with volumetric energy-balance calculations, shows that 90% reinjection can help sustain an average yield of approximately 1300 L/s during the heating season over a 100-year time frame, while satisfying constraints on minimum water level depth and minimal thermal breakthrough risk. These results highlight the critical importance of effective reinjection and sustainable management in maintaining reservoir performance in sedimentary geothermal systems with similar conditions. Heat-flux mapping across both regions clarifies the spatial distribution and controls of geothermal anomalies, guiding exploration toward structurally favourable targets for deeper, higher-temperature resources. The integrated workflow developed in this PhD project provides a transferable roadmap for early-stage geothermal assessment beyond Shandong. It outlines clear procedures for characterizing geothermal systems, incorporates a sustainability perspective that can guide resource assessment and prospect targeting in regions with similar tectono-thermal settings, such as Central Asia and Eastern Europe.