Giacomo Firpo

We present the latest results of play-based natural hydrogen exploration in the Semail Ophiolite of northern UAE, Ras al Khaimah. The study area constitutes the RAK South area which overlies the Aswad Block of the Semail ophiolite. The geological and geophysical database for the study area combines the results of geological fieldwork and sample analysis with regional and local potential field and seismic data. The geophysical database includes full tensor gradiometry (FTG) and aeromagnetic data acquired over RAK South in 2016 and re-processed in 2022. The FTG data have formed the basis for integrated interpretation and modelling resulting in a 3D static, spatial model for the ophiolite in RAK South. This forms the end-point for a geodynamic model that begins with ophiolite formation in the Mid-Cretaceous, obduction during the late Cretaceous/early Tertiary and post-obduction compressional tectonics in the Oligo-Miocene. The model depicts an overall folded geometry for the ophiolite imposed by post-obduction, out-of-sequence sub-ophiolite thrust reactivation. Internal structure is dominated by a network of vertical shear zones and faults formed initially during the high-temperature conditions of accretion and then by solid-state deformation during cooling associated with ophiolite detachment and emplacement (obduction). In order to maximise hydrogen generation from the source protolith the exploration philosophy adopted works on the basis of ‘high-temperature serpentinization’ (200 to 320OC). In the geodynamic history of the ophiolite two key scenarios are identified for hydrogen systems resulting from high-temperature serpentinization: - ‘Late’-phase Low-temperature serpentinization and hydrogen generation is known to be taking place in the Semail ophiolite by the action of meteoric water on ultramafic mantle rocks. For ‘high-temperature’ serpentinization to occur circulating groundwater needs to penetrate via faults or shear zones to mantle ophiolite at depths of at least 8km. Play elements for this hydrogen system have been derived from the static model. - ‘Early’-phase Earlier serpentinization and hydrogen generation was activated in the lower part of the ophiolite by water derived from de-hydration/metamorphism of subducting oceanic crust and/or de-watering of sub-ophiolite sediments. Play elements for this hydrogen system have been derived from the late Cretaceous thermal and structural history encompassed in the geodynamic model. The derived play elements have been analysed within the established geological and geophysical framework to define play fairways for focussing ongoing exploration efforts. Some of the key issues affecting the exploration and future exploitation of natural hydrogen in ophiolites will be highlighted.Co-auteur : Paul Swire (RAK Gas), Ian Hutchinson (NHSG), Steve Lawrence (NHSG), Jonathan Watson (Metatek), Andy Barnicoat (NHSG)).