With renewable power projects such as solar and wind, we are used to defining the available energy resource as a pretty-much fixed amount (albeit with some statistical uncertainty) before building a business case. We can measure these energies at the surface, where they’ll be collected and converted.
Geothermal resources are very different. They lie hundreds or thousands of metres below our feet and, like oil and gas fields, their sizes can only be estimated based on a relatively few sampling points (where we drill wells). Not surprisingly therefore, you’ll hear terms describing geothermal resources which are similar to those you hear around oil and gas exploration. Some of these are indicated on the chart below.
As a project proceeds and the underground resource is better understood, ‘possible’ or ‘probable’ reserves can become ‘proven’. And as technologies improve, reserves which were previously uneconomic to exploit can start to make business sense to drill for. So the energy resource that can be tapped becomes a changeable feast.