Karstified Carboniferous Limestones, Host To A Cryptic Uk Geothermal Resource?

Abstract

Karstified and fractured Carboniferous limestones host the UK’s only known hot springs at Bath, SW England. According to legend, the water which issues from the ground at 46°C was discovered by the mythical King Bladud in 836 BC. The validity of this legend is unknown. What is known is that Celtic people used the hot spring water before it was developed as a bathing complex by Roman invaders from 60-70 AD. Carboniferous limestone also hosts warm springs elsewhere in the UK, as well as being the petroleum reservoir for several oilfields including the nation’s first oil discovery at Hardstoft (1919) in Derbyshire, NE Midlands. Despite these occurrences, the geothermal potential of Carboniferous limestones in the UK has until now, been largely ignored in favour of evaluating granite bodies and Permo-Triassic deep, saline aquifers. The study reported here is based upon the hypothesis that permeable, karstified and fractured Carboniferous limestones could exist at substantial depth in the UK and could form a significant, cryptic geothermal resource. We have approached evaluation of the geothermal potential using both direct and indirect evidence. The temperature and fluid transmissivity of several known permeable Carboniferous limestones in UK onshore oilfields and their supporting natural aquifers, together with information from petroleum exploration wells that failed to find oil have been examined. Typically, temperatures of such known working systems lie in the range 30°C - ~70 °C and flow rates from individual wells are from <0.045 l s-1 to almost 3.47 l s-1. All these systems are pumped with no water injection and any recharge is due to natural aquifer inflow. Indirect evidence ofthe geothermal potentialistaken from twolinesofinvestigation. Tepid springs issuing from Carboniferous limestones in the UK were sampled and their geochemistry analysed. The ratios of certain solute pairs (for example: Na- K-Ca, Na-K, Na-Li in addition to quartz geothermometers) can be used to estimate the temperature at which equilibration took place between the water and its host rock. The data obtained suggest that despite the temperature at surface of these springs being in the range 20-30°C, they are derived and thus connected to geothermal systems at temperatures of ~>30°C to potentially greater than 100°C. We have also examined the seismic database maintained by the UK Onshore Geophysical Library and, from available 2D seismic lines, mapped the various unconformity surfaces that exist above the Lower Carboniferous limestones across much of the UK Midlands and parts of southern England. Unconformity surfaces of various ages from end-Early Carboniferous to Holocene have been identified as candidate sites for karstification of the limestones. The deepest and hence likely hottest of these surfaces occur in the east and West Midlands flanking the Pennine Dome. Substantial areas of likely karstified and fractured limestone are present at depths in excess of 1.5 km and therefore likely at temperatures in excess of 50°C and the deepest areas may contain geothermal water at over 100°C. Evaluation and risking of this substantial yet cryptic geothermal resource is still underway.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019