Geothermal systems in non-volcanic environments generally form low to moderate enthalpy or reservoir temperatures which reach temperatures of 200 degrees Celsius with varying depths. Geothermal potential in this non-volcanic field generally has a potential greater than 50 MW (Anonymous, 2010). Non-volcanic environments in western Indonesia are generally spread in the eastern part of the Sunda Expanse (Sunda Land) because the area is dominated by rocks which are constituents of the Asian continental crust (metamorphic rocks and sediments), for example, such as those in the Bangka Island region.
Meanwhile, in the eastern part of Indonesia, the non-volcanic environment is in the arms and legs of Sulawesi and the Maluku Islands to Papua which are dominated by granitic, metamorphic and marine sediments. The type of geothermal system in a non-volcanic environment can also be found on the island of Borneo including among them on the East Kalimantan border with Sabah (Malaysia). The non-volcanic geothermal system has the following assumed limits:
- The Earth’s heat system is not related to Quaternary volcanism;
- Found in sedimentary, plutonic, and metamorphic environments;
- Associated with tectonic processes;
- Geothermal manifestations are generally only characterized by the appearance of hot springs.
Lund (2007) and Hochstein and Browne (2000) found that the types of geothermal systems associated with non-volcanic environments are divided into 5 systems, namely:
- Geo-Pressure Geothermal System
- Geothermal System Sedimentary Basin
- Hot Dry Rock Geothermal System
- Radiogenic Geothermal System
- Heat Sweep Geothermal System
1. Geo-Pressure Geothermal System
The formation of a geo-pressure system is related to the inside of the sedimentary basin, in this case, the sedimentation process takes place so fast that it allows existing fluids to be trapped by the sedimentary layer which is impermeable at high pressure. Geothermal systems related to geo-pressure or those in sedimentation environments generally have very thick depression, with depths of 3 km to 4 km, at temperatures ranging from 90 degrees Celsius to 120 degrees Celsius, such as those found in geothermal systems in Louisiana Gulf Coast and Texas, United States. In Indonesia, geo-pressure systems can be found in the Duri field (Central Sumatra Basin), East Kalimantan (East Tarakan – Kutai Basin), East Java (Madura), Buru Island, and Papua (Manokwari).
2. Geothermal System Sedimentary Basin
This geothermal system is related to the formation of sedimentary basins which are filled quickly by sedimentation products so that the hydrothermal fluid formed is subjected to high pressure. Aquifers formed in sedimentary basins are partly filled with seawater, in this case, the marine sediments can contain up to 60% of seawater which can be trapped during the compaction and lithification process (rock formation). Sedimentation basins sometimes contain evaporates sequences which can increase the content of Cl and SO4. Not many of these systems have been explored, so understanding this system is still very limited.
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3. Hot Dry Rock Geothermal System
In principle, the Hot Dry Rock geothermal system uses heat stored in impermeable rocks, where to extract heat energy, the system is made to resemble a conventional system by making artificial cracks in rocks followed by injection of cold water in impermeable rock layers containing heat, so that water the cold is heated and used for electric power generation. This geothermal system has not been used in general, only a few countries have done it on an experimental scale, such as the United States (New Mexico) and Japan.
4. Radiogenic Geothermal System
The radiogenic geothermal system is related to the decay of radioactive elements such as uranium, thorium, and potassium which can produce heat sources. Generally, this geothermal system can be found in plutonic rocks (granite rock intrusion). Geothermal fields on Bangka Island are estimated to be the result of radiogenic processes.
5. Heat Sweep Geothermal System
Hochstein and Browne (2000) state that this geothermal system is related to a zone of fracture at a deep enough depth in areas that have high heat flow. The heat sweep geothermal system that occurs in plate collisions, the heat source is in the form of continental crust which experiences deformation (shearing). In this case, infiltration of rainwater and meteoric water enters through fractures and sweeps away the heat source, then flows towards the surface again. This system is found in many areas of Tibet, West and North Greece, and India.
Meanwhile, the heat sweep system on the active plate expansion path is located along with the Earth’s crust, where the heat source comes from intrusive rocks. The geothermal heat sweep system model on the active plate expansion path can be found in Northern Tanzania, Kenya, and Ethiopia.