A New Geothermal Play Type Catalog: Streamlining Exploration Decision Making

Abstract

Worldwide there is obviously a large range of geologic settings that host exploitable geothermal resources. This has resulted in various national approaches to assessment, classification, exploration and exploitation. It would be beneficial to standardize the global approach to the assessment framework, starting from the earliest stages. In the standard assessment process for hydrocarbon resources, the lowest unit in a bottom up approach is the geologically based play type, which leads to prospects and ultimately to reserves. A play type in petroleum geology represents a particular stratigraphic or structural geological setting, defined by source rock, reservoir rock and trap. Translated to geothermal systems, a play type might be defined by the heat source and the geological controls on heat transport and thermal energy storage capacity. Ultimately the geological setting not only controls the play type but also the most appropriate exploration and heat recovery technologies. A logical and consistent framework for a geothermal play type catalog needs to be simple enough to communicate important aspects of geothermal energy potential to both non-experts and the general public on the one hand, but comprehensive enough to provide a globally applicable geological framework to cover the whole range of possible geothermal systems. Plays can best help if they comprise meaningful natural groups that can be used both for reliable analog comparison and for exploration decision making in a specific geologic context. We propose a catalog of geothermal play types based on geologic controls. Key defining elements of this catalog are whether heat transfer is dominated by conduction or convection, and the characteristics of the heat source, reservoir and host rock, porosity-permeability structure and fluid types. This new catalog is explicitly not based on temperature, depth or hydrothermal versus petrothermal systems, because these criteria cannot be reliably known prior to exploration. Geothermal play types in convection-dominated systems are grouped into Magmatic, Plutonic and Fault-controlled in Extensional Domains, while geothermal play types in conduction-dominated systems are grouped into Intracratonic Basins, Orogenic Belts with Adjacent Foreland Basins and Basement types. The geologic controls on each play type are described and constrained by real-world case studies. This new catalog of geothermal play types provides a range of generic conceptual models that serve as basis for refinement through appropriate exploration methods.