The IRETHERM Project: Magnetotelluric Assessment of Sedimentary Basins in Northern Ireland as Possible Geothermal Aquifers

Abstract

IRETHERM (www.iretherm.ie) is an academia-government-industry collaborative, SFI-funded research project to identify and evaluate sites within All-Ireland (south and north) possessing the greatest potential for deep, low-enthalpy, geothermal energy provision. Possible areas for geothermal potential include the Permian and Triassic sedimentary basins in Northern Ireland. These basins contain lithological sequences with relatively high primary porosity with viability for geothermal potential depending largely on the permeability distribution, which controls fluid flow and heat-exchange. The most promising of these lithologies is the Triassic Sherwood Sandstone Group, which has measured porosities and permeabilities of 8-24% and 2-1000 mD respectively from borehole core samples. Current research is focused on the Rathlin Basin in County Antrim, one of three onshore basins in Northern Ireland, where measurements in two independent boreholes show geothermal gradients of between 36 and 43 °C/km to depths of 1481 m. Previously published interpretations of gravity models across the basin attribute a thickness of 2000 m to the Sherwood Sandstone Group, with a maximum depth to the Permo-Triassic basement of 4000 m. Magnetotelluric data were acquired onshore across a 2-D grid of 57 sites with a 2 km site spacing, and on the nearby Rathlin Island on two profiles totalling 12 sites with an 800 m site spacing in order to image the thickness and continuity of the sediments in the north-eastern portion of the basin. Modelling results show the Permo-Triassic sediment fill as an electrically conductive body with a strong resistivity contrast to the surrounding resistive basal Dalradian metasediments. Analysis and modelling of the data has been performed to determine a resistivity model that maps the variation in thickness of the sediment fill and the truncation of the basin sediments against the Tow Valley Fault. Further synthetic testing of the model sensitivity to variation of the thickness of the Sherwood Sandstone Group within the sediment fill has also been performed, as the overlying sediments have lower porosities and permeabilities from core sampling. Future work, that will be presented, includes joint inversion of the data using constrains from gravity and possibly seismology.