Integrated evaluation and sustainable development implications of a Cambrian-Ordovician karst geothermal reservoir in the southern Fenwei Graben system, North China

Abstract

Low-to medium-temperature geothermal systems are important geological energy resources for clean heating and carbon-emission reduction, but their development in karst carbonate reservoirs is commonly limited by strong reservoir heterogeneity, uncertain groundwater circulation pathways, and risks associated with production-reinjection imbalance. In intracontinental rift basins of North China, deeply buried Cambrian-Ordovician carbonate reservoirs provide promising geothermal targets; however, the links among multi-stage reservoir formation, heat accumulation, well-group productivity and sustainable development remain insufficiently constrained. Taking the Linyi geothermal field in the southern Fenwei Graben System as a representative case, this study combines balanced-section restoration with outcrop and well-section observations, temperature logs, a newly compiled stratified production profiles from 12 wells and resource evaluation. Shallow-marine carbonate deposition provided the basic reservoir framework, whereas post-Ordovician exposure formed weathering-crust karst and unconformity-related storage. Later structural modification and Cenozoic fault subsidence adjusted burial depth, fracture connectivity and groundwater pathways. The effective reservoir is mainly developed in the Upper and Lower Majiagou formations, while the Yeli and Zhangxia-Xuzhuang formations contribute locally. Faults, unconformities and karst-fracture zones jointly control recharge, flow and heat accumulation, and may locally enhance vertical connectivity. Resource calculations indicate a recoverable geothermal resource of 1.33 × 10 9 GJ and annual usable heat of 1.67 × 10 7 GJ/a. These findings provide a geological basis for sustainable development of karst geothermal resources in the southern Fenwei Graben System and highlight the need to verify reinjection safety and long-term production–reinjection balance through tracer tests, reinjection tests and long-term monitoring of temperature, pressure, flow rate and hydrochemistry.