This dataset accompanies HyDRA Deliverable D4.1: Database and Site Characterisation for Underground Hydrogen Storage in Porous Reservoirs and provides the first harmonised, pan-European geological database focused on porous-rock sites relevant to underground hydrogen storage (UHS). The resource integrates data for 134 porous reservoir gas storage sites across Europe, including depth, temperature, pressure, lithology, porosity, permeability, salinity, depositional environment, formation names, basin definitions, operational status, and well information. The dataset supports microbial–geochemical risk assessment, laboratory experiment design, and strategic site selection for hydrogen storage. Contents Excel spreadsheet containing the HYDRA porous-reservoir attribute database, including: Site identifiers, operator and location data Working gas capacity and storage status Reservoir depth, temperature, and pressure (sourced or derived) Lithology and harmonised lithology categories Porosity, permeability, salinity Depositional environment, basin, formation Number of wells and operational notes GIS shapefiles providing spatial layers for: Reservoir depth Temperature and pressure Lithology and lithology category Porosity and permeability Salinity Basin and formation Site status and operator Supporting documentation summarising the methodology, harmonisation rules, normalisation procedures, data sources, assumptions, and limitations used to compile the dataset - please note due to limited data availability, many properties are unknown. Purpose and Use The dataset provides the geological baseline required to: characterise the range of conditions present in European porous-rock storage sites, identify representative lithologies and reservoir conditions for laboratory H₂–rock–fluid–microbe experiments, support EU-wide screening of UHS potential, inform microbial and geochemical risk assessments, develop boundary conditions for reactive transport and geomechanical modelling. It serves as a FAIR, reusable resource for researchers, industry operators, regulators, and stakeholders evaluating the suitability of porous reservoirs for hydrogen storage. Provenance and Funding This dataset was developed under the HyDRA Project – Diagnostic Tools and Risk Protocols to Accelerate Underground Hydrogen Storage, funded by the Clean Hydrogen Partnership and European Union (Grant Agreement No. 101192337). https://hydrahydrogen.eu/ Authors:Katriona Edlmann, Lubica Slabon, Ciaran Hemming, Andrew Kilpatrick (University of Edinburgh) Citation Please cite both the Zenodo record and HyDRA Deliverable D4.1 when using this dataset.