Geothermal Resources in Iceland

Low & High Temperature fields

Increase in production of hot water from
1943 - 1990
Low-temperature fields are found in the vicinity of Reykjavik, yielding water at a temperature of below 150°C that can be used directly for space heating and washing.
Iceland's high-temperature fields are only found on the active volcanic rift zone that runs across the country, and yield water at temperatures in excess of 200°C, so rich in gases and minerals that it can not be used in the distribution system. Its high pressure and high thermal energy, however, make it well suited to heating fresh cold water which then can be used for space heating, and also for generation of electricity.

Geothermal energy in the Hengill area

The Hengill geothermal area is one of the largest high-temperature fields in Iceland. Geothermal activity in this part of south Iceland is linked to three volcanic systems. Reykjadalur and Hveragerdi are part of the Grensdalur system, which is now considered extinct. North of this is the Mt. Hromundartindur system, which last erupted some 10,000 years ago. Geothermal activity in the Olkelduhals area is connected to that system.
To the west of these fields is the Mt. Hengill volcanic system, with its fissure swarms running southwest to Innstidalur, Kolvidarholl and Hveradalur, and northeast via Nesjavellir to Lake Thingvallavatn.
The Hengill system has erupted a number of times in past glacial time, and occasional earthquakes are still felt at Nesjavellir, as elsewhere in the area. Some 2,000 years ago, the Nesjahraun lava field was created by a fissure eruption along Nesjavellir, which created the island of Sandey in Lake Thingvallavatn.
Studies have shown that some of the precipitation falling in the highlands north of Thingvellir permeates down to the bedrock and flows at a great depth along fractures to areas of lower altitude. Such water is heated there by contact with magmatic intrusions and on superheating is forced up through fractures below Mt. Hengill. Drilling has shown that heated water is found at depths of 1-3 kilometers.

Geothermal energy at Nesjavellir

General studies conducted in the Hengill geothermal area in 1947-49 included research into the thermal field of Nesjavellir. Studies and drilling of wells continued intermittently until 1986.
The greatest geothermal activity at surface level was found to be south of Nesjavellir. Exploration by drilling was therefore focused on that area. The extent of the area of geothermal distribution was also studied at a depth of 1-2 kilometers to the east, west and north. Extensive geological, geochemical and geophysical studies were also conducted at Nesjavellir.
The conclusions of these studies can be arranged in a model of the geothermal system which shows the flow of boiling water from under Hengill along Kyrdalshryggur on to Nesjavellir. Between Kyrdalshryggur and Koldulaugagil geothermal energy is found at a depth of one kilometer, but at a greater depth the temperature is highest in the Kyrdalshryggur. At still greater depths the geothermal system extends beneath Nesjavellir and Kyrdalur.

Strata & Reservoirs

Geologically speaking, the rock strata underlying Nesjavellir are relatively young as a result of intense volcanic activity. Palagonite turf predominates in the upper five hundred melees of rock, and there is basalt beneath them. Magmatic intrusions become more common at greater depths and become the predominant rock type at a depth of 1,400-1,600 meters and below.
Reservoirs often occur at the peripheries of intrusions, especially from intermediate acid rock. There is a fault at the Kyrdalshryggur which is indiscernible at the surface, although the wells clearly show that the valley floor has subsided.
The ambient rock temperature is highest at the Kyrdalshryggur, measuring around 100°C at sea level and increasing at greater depths, to reach a temperature in excess of 350°C below two kilometers.

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