Key geothermal resource parameters such as Area, Thickness, Temperature, and Recovery Factor play an important role in the characterization of resources, the identification of appropriate analogs, and the estimation of resource capacities. There are currently no industry-wide standards of these parameters in the context of geothermal resource assessments, resulting in inconsistent approaches to resource capacity estimations and confusion when communicating such parameters to both specialists and non-specialists alike. The definitions presented herein focus on a conceptual model approach to Area and a Thickness parameter rooted in the concept of heat-sweeping in a hypothetical or realized development. Supported by definitions and explanations of rules and exceptions for estimating Area, Thickness and Temperature, this study computes Recovery Factors for forty geothermal fields from a wide range of geologic settings. Additionally, an approach is presented to categorizing geothermal fields based on their Permeability Regime; a resource descrptor that is controlled by geologic setting and that influences parameters such as Area, well permeability, feedzone distributions, Recovery Factors, and post-development cooling trends. Four end-member Permeability Regimes are proposed to describe all naturally-occurring geothermal systems: Discrete, Limited Distributed, Enhanced Distributed, and Stratigraphic. The relationship of these permeability regimes to well productivity index values and Recovery Factors are described herein. This paper outlines an approach to standardize the documentation of key resource characteristics and Permeability Regimes for all geothermal systems, regardless of temperature or geologic setting. A companion paper reviewing the reservoir engineering characteristics of geothermal Permeability Regimes is provided by Murphy and Libbey (2026).

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