Which conditions make geothermal energy viable?

• A. Low heat flow, impermeable rocks, and no accessible reservoir near tectonically active zones.
• B. Moderate heat flow with variable permeability.
• C. High heat flow, permeable rocks, and an accessible reservoir near tectonically active zones.
• D. A geothermal system that relies solely on surface water.

Answer: (C)

Geothermal energy works best when there is a strong underground heat source, a way for fluids to move and transport that heat, and a reachable place to extract it. High heat flow provides enough heat to produce steam or hot water at practical temperatures and depths. Permeable rocks create the pathways for water or steam to circulate, forming a reservoir that can store and convey heat to the surface. Being near tectonically active zones often means both higher heat flow and more fractures or faults, which boost both the heat supply and the permeability, making it easier to develop and sustain production. An accessible reservoir is essential so wells can be drilled and heat can be pumped to the surface without excessive cost or drilling complexity.

Options with low heat flow, impermeable rocks, and no accessible reservoir lack the essential heat source and fluid pathways needed for a viable system. Moderate heat flow with variable permeability is less reliable because the heat supply and fluid flow can be inconsistent. A system that relies solely on surface water has no deep heat source to sustain long-term energy production.