Quantitative geothermal interpretation of electrical resistivity models of the Rathlin Basin, Northern Ireland

Abstract

Abstract In the evaluation of low- to medium-enthalpy geothermal resources on the island of Ireland, some of the most interesting targets are the deep sedimentary basins of Northern Ireland. The deepest of these is the Rathlin Basin, where Permian and Triassic reservoir sediments are known to exist to at least 2300\u202fm depth. Two deep boreholes within the basin provide evidence of elevated temperatures at depth that are atypical within Ireland, prompting further geophysical exploration of the basin as one component of the IRETHERM project. The magnetotelluric (MT) method was selected as the investigative geophysical tool as it is capable of sensing and defining electrically conductive porous sediments beneath overlying resistive strata, in this case flood basalt sequences. MT data were acquired on a rectangular grid of 39 sites across almost half of the onshore basin to investigate the composition and spatial variation of the basin s formations. One-dimensional stochastic inverse modelling of the observed MT data was with a reversible-jump Markov chain Monte Carlo 1D inversion code, resulting in ensembles of models for each site. The use of model ensembles rather than single models avoids the pitfall of over-reliant interpretation on non-unique resistivity models, increasing the robustness of the interpretation. Interpreted models compare very favourably with nearby deep borehole records, and interpolation of the complete set of ensemble interpretations results in a conservative reservoir volume of approx. 32\u202fkm3 of combined Permian and Triassic sandstones beneath the MT survey. Based upon new, high quality temperature data available in the Ballinlea 1 borehole, an approximate estimation of thermal energy in place as a function of final reservoir temperature has been performed for the interpreted MT resistivity model volume. A final minimum temperature of 25\u202f°C (being the temperature that comparable estimates have been made for adjacent geothermal prospects) results in a minimum estimated Indicated Geothermal Reserve (IGR) of 2.9\u202f×\u202f1018\u202fJ beneath the MT survey area. The modelling results suggest that exploitation of the maximum volume of sediments would occur for a final temperature of ≈55\u202f°C.