Research highlight

Unlocking the deep geothermal energy potential of the Carboniferous Limestone Supergroup

How understanding the subsurface beneath our towns and cities may allow us to access geothermal energy for heating homes and powering the UK.

28/06/2021
A pool of blue-green water surrounded by a pillared walkway made of yellow rock
Cool groundwater flows from the Mendip Hills through a Carboniferous Limestone aquifer and rises as hot springs in the city of Bath. This renewable energy source could be used for heating and energy to help meet net zero by 2050. Wikimedia Commons © Diego Delso, delso.photo, Licence CC-BY-SA.

The direct use of geothermal heat from the Carboniferous Limestone Supergroup (‘Carboniferous Limestone’) could make a significant contribution to delivering affordable, low-carbon heating to thousands of buildings, homes and offices through a district-heating system, as well as providing heating for agriculture and horticulture. Geothermal heat from the Carboniferous Limestone offers a clean alternative energy resource to help in the UK to achieve carbon net zero emissions by 2050.

Traditionally, the use of geothermal energy has focused on generating power using conventional steam-turbines in areas where high-temperatures (more than 160°C) occur at shallow depths (less than 500 m). Such settings are usually associated with plate boundaries and areas of active volcanism, as found in Iceland and Kenya. Now, due to technological advances in geothermal drilling, low-moderate temperature (over 60°C) deep geothermal energy can be utilised for industrial or residential (direct use) heating.

Deep geothermal energy is defined as the heat resource found at depths of between 500 m and 5000 m, where heat is transported by groundwater that is circulating within the pores of rocks in deep aquifers. These resources are exploited by doublet systems, which involve two deep boreholes, one for extracting the warm water and another one for reinjecting the cooled water back into the subsurface (Figure 1). As these fluids are pumped to surface they pass through a heat exchanger that transfers the heat into a district heating system (Figure 2). Such schemes have been developed in Belgium, the Netherlands and Germany.

In the UK, the Carboniferous Limestone is proven as a productive aquifer, hosting the circulation of deep hydrothermal fluids responsible for the thermal springs in Bath (Figure 3), Bristol, the Taff Valley and Buxton. The advantage of the Carboniferous Limestone as a resource is that it covers large areas of the UK subsurface and coincides with many large towns and cities. A number of schemes have successfully developed the Carboniferous Limestone geothermal resources for heating purposes, such as those in Belgium and the Netherlands.

A geological cross-section from the Mendip Hills on the right (or south-west) to Bath on the left (or north-east) showing the geology below the ground surface and the flow of water to the hot springs
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Figure 3: geological cross-section from the Mendip Hills to Bath illustrating the fluid flow model. Image based on Downing and Gray, 1986, adapted here by Gallois, 2007.  BGS © UKRI.

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The geothermal team at BGS has started a research programme looking at the utilisation of the deep geothermal energy held in the Carboniferous Limestone as a renewable heat resource for the UK.

During the Early Carboniferous period, the UK was situated near the equator experiencing a similar climate to that of modern-day tropics. Carbonate reefs developed across the northern and southern parts of the UK which, at that time, were separated by a strip of landmass called the Wales–Anglo–Brabant Massif (Figure 4). As a result of climate change 359–326 million years ago, sea levels dropped and these carbonate reefs became exposed to the rain (which acts as a very mild acid), causing the limestone to dissolve and creating underground drainage systems with sinkholes and caves. These features are preserved in the geological record as so-called palaeokarst.

These ‘karstified’ surfaces, along with solution-enhanced fractures and faults, provide connectivity in the rocks through which geothermal fluids can flow. BGS research focuses on identifying areas in the UK where we may find such deep flow systems (aquifers) and on assessing their potential for producing geothermal heat.

A map showing the disribution of Carboniferous Limestone across north-west Europe
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Figure 4: the distribution of Carboniferous Limestone (CL) platforms and shelves across north-west Europe and the UK. BGS © UKRI; based on Kombrink et al., 2010.

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Sustainable development goals

The BGS research into the utilisation of deep geothermal energy in the Carboniferous Limestone as an energy resource for the UK directly addresses a number of the UN Sustainable Development Goals (SDGs).

About the author

Darren Jones
Darren Jones

Petroleum geologist

BGS Edinburgh
Find out more

More information

Contact Darren Jones or visit geothermal energy for more information.

References

Abesser, C. 2020. Deep Impact: unlocking the potential of geothermal energy for affordable, low-carbon heating in the UK. British Geological Survey Science Briefing Note, September 2020. (Nottingham, UK: British Geological Survey.)

Abesser, C, Busby, J P, Pharaoh, T C, Bloodworth, A J, and Ward, R S. 2020. Unlocking the potential of geothermal energy in the UK. British Geological Survey Open Report OR/20/049 (unpublished).

Abesser, C, Schofield, D, Bonsor, H, and Ward, R. 2018. Who owns geothermal heat? British Geological Survey Science Briefing Note, November 2018 (unpublished).

Busby, J P. 2014. Geothermal energy in sedimentary basins in the UK. Hydrogeology Journal, Vol. 22(1), 129–141. DOI: http://dx.doi.org/10.1007/s10040-013-1054-4  

Downing, R A, and Gray, D A. 1986. Geothermal resources of the United Kingdom. Journal of the Geological Society, Vol. 143, 499–507. DOI: https://doi.org/10.1144/gsjgs.143.3.0499

Gallois, R W. 2007. The formation of the hot springs at Bath Spa, UK. Geological Magazine, Vol. 144(4), 741–747. DOI: https://doi.org/10.1017/S0016756807003482

Gray, D A, and Downing, R A (editors). 1986. Geothermal Energy: the Potential in the United Kingdom. (Nottingham, UK: British Geological Survey.) ISBN: 978-0118843669

Jones, D, Pharaoh, T C, Randles, T, Kearsey, T I, and Newell, A. Phase 1 resource assessment of the deep geothermal potential of UK Carboniferous basins. British Geological Survey Internal Report IR/21/007.

Kombrink, H, Besly, B, Collinson, J B, et al. 2010. Chapter 6. Carboniferous. 81–89 in Petroleum Geological Atlas of the Southern Permian Basin. Doornenbal, J H, and Stevenson, A G (editors).  (Houten, Netherlands: EAGE.)

Narayan, N, Gluyas, J, and Adams, C. 2018. Is the UK in hot water? Geoscientist, Vol. 28(9), 10-15. DOI: https://doi.org/10.1144/geosci2018-014

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