- Natural, shallow hydrothermal reservoirs
- naturally occurring hot water reservoirs, typically found at depths of less than 5 km below the Earth's surface where there is heat, water and a permeable material (permeability in rock formations results from fractures, joints, pores, etc.). Often, hydrothermal reservoirs have an overlying layer that bounds the reservoir and also serves as a thermal insulator, allowing greater heat retention. If hydrothermal reservoirs contain sufficient fluids (water or steam) at high temperatures and pressures, those fluids can be produced through wells to generate electricity or, for process heat.
- Human-engineered hydrothermal reservoirs
- typically created in conjunction with Enhanced Geothermal Systems (EGS).
- Liquid-dominated hydrothermal reservoir