| Title | Renewing the Exploration Approach for Mid-Enthalpy Systems: Examples from Northern England and Scotland |
|---|---|
| Authors | Paul L YOUNGER, Michael E J FELIKS, Robert WESTAWAY, Alistair McCAY, Thomas L HARLEY, Thomas ELLIOTT, Gordon D.C. STOVE, Jenny ELLIS, Sean WATSON, Andrew WARING |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | exploration, geochemistry, geophysics, granite, radiometry, sandstone, structural, UK |
| Abstract | After a promising start in the 1970s and 80s, the UK rather fell behind other countries in the search for viable mid-enthalpy geothermal resources. This situation began to turn around in 2004, when the first of three deep geothermal exploration boreholes were drilled in northern England. What distinguished these from earlier drilling in Cornwall was the deliberate search for naturally-high permeability associated with major faults, especially those that have undergone strike-slip reactivation during the Caeonozoic. Boreholes at Eastgate in the North Pennines targeted buried radiothermal granite, whereas the 1,821m-deep Science Central Borehole in Newcastle upon Tyne targeted a postulated deep sedimentary aquifer (the Fell Sandstones), which were inferred to be connected laterally to the granitic heat source by a major fault (the reactivation of the Iapetus geo-suture). The drilling was in both cases rewarded with impressive heat flows, and in the case of Eastgate with what is believed to be the highest permeability yet found in a deep granite anywhere in the world. In parallel with these developments, a re-assessment was made of the pre-existing geothermal heat flow database for the UK, applying newly-standardised correction protocols for palaeoclimatic and topographic distortions, which were found to be particularly marked in Scotland (where only shallow boreholes had been used to establish geothermal gradients in the original 1980s analysis), providing new impetus for evaluation of geothermal prospects in Shetland, Aberdeenshire and the Midland Valley. In the case of Shetland and Aberdeenshire, surveys of radiothermal heat production in the Caledonian granites were undertaken by means of spectral gamma radiometry, revealing some very promising prospects. In the Midland Valley, the principal target is Devonian sandstones, such as the Kinneswood Formation, which are inferred to underlie areas with high heat demand, such as eastern Glasgow. Similar prospects in northern England (similar to that drilled at Science Central) are now the focus of commercial exploration efforts. Appraisal of fault dispositions relative to the present-day maximum compressive stress azimuth are being used to identify the most promising areas for intersecting fault-related permeability at depth. New geophysical tools – most notably atomic dielectric resonance scanning – are also being appraised for their ability to directly detect features (such as hot brines) which are indicative of localised convection in target fault zones and aquifers. |