David Tierney

Natural hydrogen (also known as geological and/or ‘white’ hydrogen) is currently receiving a huge boost in scientific and commercial interest. To aid the understanding and to ultimately support the ranking of differing geological prospectivity, it helps to classify components of these systems into analytical stages based on source, migration, reservoir, trap and seal (as is done in mineral and petroleum systems). There are several formation processes (e.g. redox reactions between iron-rich rocks and water, radiolysis, pyrolysis of organic matter) which form the main ‘source rock’ targets. Serpentinization possesses a much quicker reaction rate of hydrogen generation relative to other formation processes, meaning that source rocks such as ultramafic (mantle) suites within ophiolites are considered prime targets for investigation. Given the expected high values of density and susceptibility within these iron-rich ultramafic rock types, we can use our gravity and magnetic data to map their surface and subsurface extents, model their properties and quantify possible variations in the degree of serpentinization laterally and/or with depth. This may predict specific locations hosting relatively ‘fresh’, unreacted mantle rocks which could still yield further natural hydrogen production. Furthermore, we can use these same potential fields data to map and model granitic bodies for potential of radiolysis. We can also leverage Getech’s predicted total organic carbon (TOC) models through geological history to assess the natural hydrogen potential derived through the heating/maturation of organic matter.The Vardar Zone (which is separated into Eastern (EVZ) and Western (WVZ) branches) represents the closure of the Vardar Oceanic Basin and the subsequent obduction of two ophiolitic suites from the Late Jurassic until the Early Cretaceous (Schmid et al., 2020). We have focussed on an area broadly covering Bosnia & Herzegovina, Serbia, Montenegro, Kosovo, Albania, and North Macedonia. Recent studies (Lévy et al., 2023) have investigated the spatial link between elevated hydrogen concentrations at surface and their proximity to outcropping EVZ and WVZ units, primarily attributing them to serpentinization of ultramafic rocks. This process is believed to be primarily taking place at depths of ~2 km through analysis of hydrogen isotopes (Lévy et al., 2023), which is also suggested by results of Getech’s magnetic vector inversion (MVI) over this region. The MVI models the 3D distribution of magnetisation amplitude and orientation which prove very illustrative when interpreting the subsurface distributions and varying degrees of serpentinization progress within these target ultramafic rocks. A 2D gravity and magnetic model has also been produced across this study area and provides further detail on the subsurface nature of this complex geological region. Source rock predictions (of ultramafics as well as other formation processes considered) can then be located within their geothermal regime setting to estimate their hydrogen generation potential. We can also incorporate fault dilatancy tendencies to model comparative fluid migration routes between different structures. All of these findings can be combined to map the predicted favourability of natural hydrogen systems which will further de-risk natural hydrogen exploration across the Balkans, and this workflow is replicable anywhere on the globe.