Assessing the Geothermal Potential of the Lower Carboniferous Fell Sandstone

Abstract

After a hiatus of more than two decades, there is now a renewed interest in the exploration of deep geothermal resources in the UK as the country strives towards reaching net-zero greenhouse gas emissions. The Lower Carboniferous Fell Sandstone Formation (Mississippian) and laterally equivalent strata beneath Newcastle Upon Tyne was postulated to be one such deep geothermal system, heated by the inflow of high temperature brines along the Ninety-Fathom Fault and several of its footwall splay faults. In 2011 a consortium led by Newcastle University commenced with the drilling and testing of the Science Central Deep Geothermal Borehole (Science Central Borehole, recently renamed Helix) to examine whether low enthalpy geothermal energy might be recoverable from the Fell Sandstone. The Science Central Borehole proved the presence of the Fell Sandstone and a temperature of 73.3oC at 1740m however, low hydraulic conductivity within the formation did not allow for commercial production (Younger et al., 2016).
The findings of Younger et al. (2016) highlight the need to gain a better understanding of the reservoir properties of deep geothermal aquifers like the Fell Sandstone. This study further investigates the Fell Sandstone Formation across northern England, comparing petrographic data obtained from both outcrop and borehole samples. Well logs, optical microscopy, scanning electron microscope (SEM) analysis and statistical analysis are utilised to identify lateral changes in both the lithology and diagenetic history of the Fell Sandstone and examine what impacts these variations have had reservoir quality. An estimate of the geothermal resource within the Fell Sandstone Formation has been calculated based on these findings.
A lateral trend of decreasing net to gross down the paralic palaeo-river system combined with diagenetic compaction and pervasive cementation has resulted in significantly reduced porosities and permeabilities within the subsurface Fell Sandstone, compared to values obtained from outcrop. Despite these poor porosities and permeabilities the high temperatures recorded at Science Central prove that if future endeavours are able to intersect the Ninety-Fathom Fault, then there is still the opportunity for geothermal resource development.