Identifying potential geothermal reservoirs in Denmark Anders Mathiesen, Lars Henrik Nielsen and Torben Bidstrup Concerns about climate change have led to increased interest tion water to the surface and returning it to the subsurface in geothermal energy as one way of reducing the consump- in a closed system. The plants use absorption warmth pumps, tion of fossil fuels and thus limit CO2 emissions. Use of geo- which need steam and hence give rise to consumption of (fos- thermal energy is based on well-established technologies, a sil) fuel. Both Danish plants have two wells, a production high degree of security of supply, and little visual or noise well and an injection well in which the cooled formation inconvenience. More than one hundred plants have been es- water is returned to the geological reservoir at about 1 km tablished in Europe. away from the production point, in order to avoid mixing of There is a large potential for using geothermal energy from warm and cold water (Fig. 2). Geothermal energy can also be the Danish subsurface, as first pointed out by Balling (1976). used for electricity production, but Danish subsurface tem- Geothermal energy is highly suitable for district heating peratures are currently not believed to be sufficiently high to systems and is expected to cover a large part of the demand produce electricity directly. for district heating in the future. Two Danish geothermal Because geothermal energy is expected to play an increas- plants, the Thisted plant in northern Jylland and the Mar- ingly important role in the energy strategy of Denmark, the gretheholm demonstration plant near Copenhagen (Fig. 1), have shown that 8°E 10°E Frederikshavn Reservoir it is possible to produce large amounts Haldager Reservoir Gassum Reservoir of warm water for district heating. Only Skagerrak Reservoir 5–10% of the total energy output from Bunter Reservoir the plant is used to extract the heat from Reservoirs too shallow the subsurface by pumping warm forma- Gassum Res. too deep Bunter Res. too deep Major Fault Well 57°N Skagerrak–Kattegat StructuralPlatform High Thisted Geothermal plant Sorgenfrei–Tornquist Zone 12°E Sweden Viborg Danish Basin Jylland 56°N Margrethe- Fig. 1. The distribution of potential sandstone Ringkøbing – Fyn High holm reservoirs in Denmark with depths in the Øresund 800–3000 m interval and thicknesses above 25 m. Dark grey and black areas indicate where Sjælland the reservoirs are too deep (Gassum in northern Jylland; Bunter in western Jylland – both located in the central parts of the Danish Basin). Light grey areas indicate where reservoirs are absent 55°N (Ringkøbing–Fyn High) or too shallow (less Sønderborg than c. 800 m; northernmost Jylland). Hatched N o r t h G e r m a n B a s i n areas indicate two or more reservoirs with geo- 50 km thermal potential. Green areas: not mapped. © GEUS, 2010. Geological Survey of Denmark and Greenland Bulletin 20, 19–22. Open Access: www.geus.dk/publications/bull 19 Geothermal plant Consumer the Triassic – Lower Cretaceous succession in the Danish with heat exchanger Basin and the North German Basin, separated by the Ring- købing–Fyn High which shows a lower potential. Based on regional geological studies, four main stratigraphical units with a regional geothermal potential have been identified (Nielsen et al. 2004). Within these four main units, we have defined five geothermal reservoirs on the basis of their strati- graphical and spatial extent. Each reservoir comprises a large number of sandstone layers that are potential aquifers. The new assessment shows that large areas in both basins have a Reservoir good geothermal potential, as they contain several porous, water-bearing sandstone reservoirs in the economic interval 800–3000 m below the surface with formation temperatures Fig. 2. Schematic diagram illustrating the geothermal concept used in of 25–90°C (Mathiesen et al. 2009). Denmark. In the geothermal plant a production well pumps up warm wa- The principal challenge for successful geothermal explora- ter (red line) from the underground reservoir, and an injection well (blue line) returns the cooled water to the reservoir. tion is to assess whether good reservoir properties are present in terms of sufficient layer thickness, porosity, permeability, temperature and formation water geochemistry. These geo- Geological Survey of Denmark and Greenland (GEUS) and logical factors are used to evaluate whether the thermal en- the Danish Energy Agency have conducted a regional study ergy of the formation water can be exploited economically to update the assessment of the geothermal potential in Den- and used for heating purposes. By combining knowledge mark (Mathiesen et al. 2009). Based on existing well, seis- of the geographical distribution of the stratigraphical units mic and temperature data and the detailed knowledge of the with the potential reservoirs, their mapped burial depths and subsurface stratigraphy gathered by GEUS over many years, estimates of where the cumulative thickness of the reservoir the assessment has documented a huge geothermal potential sandstones exceeds c. 25 m at depths of 800–3000 m, a useful in many parts of Denmark. The focus of the study was to indication of the regional geothermal potential is provided. evaluate (1) the potential of geothermal energy in Denmark Temperature and salinity of the formation water in the po- and (2) if it can contribute significantly to the Danish strat- tential reservoirs increase with increasing depth. Based on data egy for a safe, sustainable and reliable supply of energy. The from a number of wells a general temperature–depth relation specific potential in local areas was not evaluated in detail; has been established, showing a gradient of 25–30°C/km. The however, a well-defined and stepwise procedure to develop salinity of the formation water shows a general increase of local geothermal prospects by integrating existing and new about 10%/km burial depth, but large variations are found. data is suggested. Porosity and permeability decrease with increasing depth due to mechanical compaction and formation of diagenetic minerals that reduce the pore volume (porosity) and the con- Potential reservoirs and areas of interest nections between the pores (permeability). Several of these The Danish subsurface can be divided into five major struc- properties are directly related to the petrography of the sedi- tural parts: the North German Basin, the Ringkøbing–Fyn ment source areas and the grain size distribution of the ma- High, the Danish Basin, the Sorgenfrei–Tornquist Zone and terial supplied to the basins. Thus the various depositional the Skagerrak–Kattegat Platform (Fig. 1). These structural processes during the formation of the reservoirs and their divisions exert a decisive influence on the geothermal pro- subsequent burial depths determine their qualities as geo- spectivity of the Danish subsurface, as they essentially de- thermal reservoirs. However, the mutual dependency of the termine the distribution, thicknesses, facies types and burial various factors and processes is not fully understood, which depths of the potential reservoirs (Nielsen 2003; Nielsen weakens the predictive strength and reliability of the geologi- et al. 2004). The 1–10 km thick Mesozoic succession has cal models currently in use to identify areas of interest. been the target for hydrocarbon exploration since 1935 Figure 1 shows regions where sandstone-rich reservoirs and is thus relatively well known from about 60 deep wells are expected to have geothermal potential by combining geo- and seismic data acquired over many years, although with graphical distribution and burial depth information of the a highly variable data coverage and quality. The data show five reservoirs. The map also indicates where further detailed that the most promising geothermal reservoirs occur within research and investigations are required if geothermal ener- 20 gy production is to be further developed (Mathiesen et al. in the Danish Basin, and grades into the Skagerrak reservoir 2009). The central part of the Danish Basin is promising, towards the north-eastern basin margin (Fig. 2; Mathiesen et whereas the potential of areas located around the 800 m and al. 2009). The Bunter reservoir is dominated by fine-grained 3000 m cut-off limits is highly uncertain. Areas located along sandstones that were deposited in arid continental environ- the Ringkøbing–Fyn High are considered less prospective. ments with fluvial channels, aeolian dunes and with some marine influence. In southern Sweden, analyses of existing cuttings from old wells and data from new wells documented The need for further development the presence of a loose, medium- to coarse-grained, quartz- Interest in the use of geothermal energy has increased over itic sandstone composed of sub-rounded quartz and feldspar the past five years. So far, the Thisted and Margretheholm grains without overgrowth of minerals or other signs of cor- plants are the only working geothermal plants in Denmark. rosion and cementation. Data from the Swedish wells com- However, GEUS has carried out evaluations of the geother- bined with log correlation to Danish wells indicated the pres- mal potential in several local areas in Denmark with posi- ence of a c. 50 m thick, loose, conglomeratic sandstone unit tive conclusions. It is expected that new Danish geothermal in the basal part of the Bunter reservoir, and this unit was plants will be established in Sønderborg and Viborg within predicted to show high porosity and low degree of cementa- the next few years. The existing Thisted plant has produced tion at Margretheholm. heat from the Gassum Formation (the Gassum reservoir) In 2002 and 2003 the Margretheholm-1 and -2 wells were for almost 20 years without notable production or injection drilled to about 2700 m depth and confirmed the presence problems, and the newly established Margretheholm plant of several sandstone-rich aquifers in both the Gassum and produces heat from the Bunter Sandstone Formation (the Bunter reservoirs.