The total amount of geothermal energy on earth has been estimated to be 3,5x 1015 TWh, which is more than enough to cover the world's annual energy consumption for the next 500 000 years. The challenge is to access this energy and make it useful. First, let's looks at the difference between ground source energy and deep geothermal energy.
Deep geothermal energy is heat found more than 300 m below the earth's surface. This heat is produced naturally through the radioactive decay of rocks. It is this natural chemical process of rock decomposition that that converts mass to energy.
Ground source geothermal energy, involves tapping the energy stored up to 300m below the surface. The main source of this ground source energy is solar radiation. Ground source geothermal energy or "heat pumps" are commonly used and are referred as rock-sourced, water-sourced or earth-sourced heat pumps.
Two kinds of reservoirs
There are, generally speaking, two types of geothermal systems: "Hot Dry Rock (HDR)" and "Hot Wet Rock" (HWR). The HDR system can be used when the bedrock has low permiability, so that closed system can be installed by drilling holes or by artificially creating fractures into the rock, by a technique called hydraulic fracturing. The HWR system is suitable when there permeable rocks or a localized systems of fractures, making use of the hot fluids found naturally in the basement rock.
In general, the bedrock in the Norway impermeable, and suitable for HDR-system, but in some areas, permeable rocks or fractures systems where the HWR-system would be used.