Porosity Estimation of the Permo-Triassic Sherwood Sandstone Group Using BNRM and Petrophysical Models

Abstract. The Permo-Triassic Sherwood Sandstone Group is an important aquifer with potential for both shallow and deep geothermal energy use in the UK. This study investigates the hydrogeological properties of the shallow buried Sherwood Sandstone Group in Northern Ireland, with a focus on its porosity, using borehole nuclear magnetic resonance (BNMR) and petrophysical models (Archie and Waxman-Smits). BNMR and downhole geophysical logging (resistivity, EC, temperature and natural gamma) were carried out on three boreholes drilled into the lower Sherwood Sandstone Group aquifer at depths of about 100 m on Queen's University Belfast campus. The results showed that the porosity calculated from BNMR and the Waxman-Smits model are comparable, demonstrating the relationship between BNMR and petrophysical-derived porosity. The average porosity of the Sherwood Sandstone Group at this location ranges between 14.9 % and 17.6 %, with maximum values ranging between 33.7 % and 40.4 %. However, the results from the Archie model are significantly larger than those of BNRM, confirming its unsuitability for lithologies containing clays, even in small amounts. This study confirms storage capacities in the lower Sherwood Sandstone Group that make it suitable for ATES systems.