Applicability of the volumetric heat method in geothermal resource assessment

To improve the accuracy of geothermal resource assessment, it is necessary to define the applicability boundaries of the volumetric heat method for different types of geothermal systems and to propose corresponding improvement strategies. By tracing the origin and development of the volumetric heat method, this study begins with a fundamental comparison of hydrocarbon and geothermal systems in terms of resource attributes, reservoir characteristics, and utilization mechanisms. The applicability of the method to sedimentary basin-type and uplifted mountain-type geothermal systems was systematically analyzed, supplemented with case studies to assess its evaluation accuracy. The results show that the volumetric heat method is rooted in the static-closed-substance evaluation model of oil and gas systems, which inherently conflicts with the dynamic-open-heat characteristics of geothermal systems. This fundamental mismatch manifests as three specific limitations: (1) incompatibility between the static closed model and the dynamically renewable nature of geothermal resources, (2) inaccurate spatial characterization arising from oversimplified complex reservoir boundaries, and (3) tension between the single static evaluation mode and the diverse requirements of sustainable geothermal resource assessment. Based on these findings, this study proposes the applicability and adaptive improvement strategies for the volumetric heat method across different types of geothermal systems. For sedimentary basin-type geothermal systems, the volumetric heat method is applicable, and its estimation accuracy can be effectively improved by integrating three-dimensional geological modeling. For fault-controlled deep-circulation type geothermal systems, the volumetric heat method is unsuitable; instead, thermal flux evaluation should be prioritized. For volcanic-magmatic type geothermal systems, an improved version of the volumetric heat method based on magmatic heat estimation is recommended for resource assessment. The reservoir morphology and heat transfer modes of different types of geothermal systems jointly determine the quantitative expression of their resource potential. While fully leveraging the simplicity advantage of the volumetric heat method, it is necessary to select an appropriate evaluation strategy tailored to different types of geothermal systems.