Volcanology and Geothermal Energy "d0e13151"

Hydrothermal Reservoirs

Hydrothermal reservoirs in Iceland are usually bounded by lava flow contacts or clay-rich hyaloclastite deposits. Water in aquifers can also pond when dikes act as barriers. By measuring the deuterium content of thermal waters, Arnoson (1976) has shown that the waters are of meteoric origin, although a few systems are charged with seawater. In most cases, water from the highlands percolates into bedrock and flows laterally for distances of as much as 150 km but more usually several tens of km. The water then rises to the surface along dikes or faults. Tables 6.1 and 6.2 summarize potential reservoir rocks and the flow rates through these rocks as sampled by drilling (Friedleiffson, 1975; Tómasson et al ., 1975).

Pillow lavas have a higher effective permeability than any other rock type encountered by drilling in Icelandic geothermal areas (Friedliefsson, 1978/79). Subglacial fissure eruptions produce elongate ridges (mobergs), as shown in Fig. 6.16, that are 1 to 5 km wide, tens of kilometers long, and a few hundred meters thick. The cores of these ridges consist of permeable pillow lavas, but the flanking hyaloclastite deposits can serve as aquitards. Subglacial eruptions are remarkable in that they are able to create both the reservoir and the caprock in one volcano.

Krafla volcano has three high-temperature geothermal fields, which are located

· within the 8- by 10-km Krafla caldera, where the thermal area is outlined by explosion craters, surface manifestations, and altered ground (Fig. 6.17). [Further exploration to define the reservoir included Schlumberger resistivity soundings and analyses of fumarolic gases. In this 35-km² thermal area, the temperature in a 2-km-deep drillhole reached 345°C (Stefánsson, 1981)];

· 6.2 km south of the caldera along the fissure swarm at Námafjall, where the geothermal area covers 4 km² ; and

· 5 km north of the caldera, also along the fissure swarm at Gjástykki; surface manifestations encompass an area 1 by 4.5 km parallel to the swarm.

Table 6.1. Potential Geothermal Reservoir Rocks in Iceland^a
*Tertiary*
Group 1	Stratiform horizons of pyroclastic rocks, including ignimbrites, sediments, and olivine tholeiite compound lava shields
Group 2	Local accumulations in central volcanoes of highly porous lavas, tuffs, agglomerates, and hyaloclastites^b (in caldera lakes)
*Quaternary*
Group 3	As in Group 1, plus primary and reworked subglacial hyaloclastites; hyaloclastite horizons reach maximum thickness over the eruptive sites
Group 4	Local accumulations in central volcanoes of hyaloclastites and subaerial eruptives, as in Group 2
^a From Fridleifsson (1975).
^b The term hyaloclastite is used here in a collective sense for all subaquatic volcanic products and thus comprises pillow lavas, pillow breccias, and tuffs.

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Table 6.2. Occurrence of Aquifers in Rock Types for Drill Holes (800- to 2045-m Deep) in the Reykir Thermal Area^a
	*Aquifers (Flow (I/s)*
*Rock Types*	£ 2	*>2 to 20*	*>20*	*Total Number of Aquifers*
Lavas	44	27	2	73
Hyaloclastites	29	12	4	45
Dolerites	—	1	1	2
Lavas and Hyaloclastites	53	38	20	111
Hyaloclastites and Dolerites	5	2	1	8
^a From Tómasson et al . (1975).

Drilling within the low-resistivity zone revealed three main rock units: hyaloclastite deposits, lava flows, and dikes, as shown in Fig. 6.17. Below a depth of 800 m, lava flows are dominant, but from a depth of 400 m to the bottoms of the deepest wells, dikes are more common. The transition depth from zeolite to greenschist facies metamorphism is at ~800 m (Stefánsson, 1981).

In his model of the Krafla field, Stefánsson (1981) identified two hydrothermal zones. The shallowest, extending to 1100-m depth, is water-dominated and has a maximum temperature of 205°C; the basal contact of this zone coincides with the base of a sequence of lava flows. Within the deeper zone—from 1100 to 2200 m (total depth for the deepest well)—the reservoir contains a mixture of

Fig. 6.16
Table mountain, or moberg, formed during eruption of basalt along a fissure under a glacier.
(Adapted from Jones, 1969.)

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steam, water, and CO₂ . The rocks, consisting of mostly lavas, a granophyre unit, and a dolerite sill, are intruded by multiple dikes. The two reservoirs are connected by a fault and/or a dike. Permeability in the upper zone is 10^-11 m² ; such permeabilities are reflected in the pressure increases and rises in water level during dike injection and eruption (Fig. 6.18). Pressure transients are not observed in the lower zone because the change is absorbed in the two-phase hydrothermal system.

Fig. 6.17
Schematic cross-section of the Krafla geothermal
field and the underlying magma body
(Adapted from Stefánsson, 1981.)

Stefánsson (1981) noted that a severe mistake was made in constructing a power plant before the drilling was completed and the field was tested. As a result, the proposed maximum capacity of 35 MW_e was not achieved. An eruption within the caldera also slowed construction work on the facility.

In Iceland, exploration techniques begin with detailed geologic mapping and dating of potential reservoir rocks. Determination of the eruption type is crucial; for example, by locating the vents and mapping facies within rocks erupted from a subglacial volcano, it is possible to evaluate not only the potential heat source, but also the location and extent of reservoir and caprocks. As noted earlier, pillow lavas are excellent reservoir rocks and the associated hyaloclastic carapaces are effective caprocks. All dikes and faults must be mapped for identification (or interpretation) of both thermal sources and potential aquicludes. After potential reservoir rocks have been evaluated at the surface, interpretative cross-sections can be prepared and evaluated by both geologist and hydrologist. An exploration drillhole can then be sited for further evaluation and temperature measurements. The primary targets in Iceland for direct-use purposes are highly permeable rocks that contain fluids with temperatures of > 100°C (Fridleifsson, 1978/79).