Magnetic Geophysical Modeling of Ophiolitic Bodies in Northwest Saudi Arabia – Implications for Natural Hydrogen Exploration

Abstract Saudi Arabia, located on the Arabian tectonic plate, is a promising area for exploring sites for natural hydrogen production and accumulation due to the broad existence of ophiolitic ultramafic rocks, especially serpentinites. These rocks, recognized as accreted forearc fragments, are exposed in the western and northwestern segments of the Precambrian Arabian Shield. The present study aims to model the geometry and depth of two ophiolitic suites in NW Saudi Arabia by analyzing airborne magnetic data. The airborne magnetic data were analyzed using edge-detection algorithms (magnetic field derivatives) to study the distribution of lineaments corresponding to rock contacts and tectonic structures, and a lineament density map was produced. The magnetic susceptibility of rock samples collected during field surveys in the broader study area was measured in the lab. The measured values were used as constraints for the 3D inversion of the magnetic data. A 3D geophysical model was constructed and used to determine the tectonic regime of the subsurface in relation to the distribution of the ophiolitic bodies. The lineament map presented the distribution of the tectonic structures, while the lineament density map indicated highly fractured areas. These areas are suitable for the infiltration of rainwater, which creates aquifers into the ophiolitic bodies. These highly fractured areas could serve as water carriers for serpentinization processes at suitable temperatures and provide conduits for vertical migration to possible traps. Serpentinization, a hydrothermal reaction between water and ultramafic rocks, typically occurs at temperatures greater than 200°C because of the oxidation state of reduced iron in the minerals. Considering an average regional temperature gradient, the suitable depth for serpentinization in the area is estimated to be more than 7 km. The 3D models made by constraining their boundaries with respect to the surrounding rock formations show the distribution and geometries of the ophiolitic masses continuing to the required depths, exhibiting fractures suitable for hydrogen generation and subsequent upward migration. However, hydrogen accumulations within highly fractured areas within the ophiolitic bodies cannot be ruled out. This study provides insights into the potential for natural hydrogen production and migration from ophiolitic massifs and could be an analog for areas where thick sediments cover the Arabian Shield, providing traps for potential hydrogen accumulation. The results highlight the importance of integrating geologic knowledge with geophysical insights to model the distribution of these masses and can aid future hydrogen exploration projects in Saudi Arabia.