With the aim of acquiring a better knowledge of the potential for unconventional gas resources in Spain, 134 rock samples were taken from 12 lithostratigraphic units in the Cantabrian Zone (north‐west Spain) and studied for a 3‐year period. The selected units gather, a priori, the basic conditions to constitute potential source rocks for unconventional hydrocarbons. Petrographic studies and total organic carbon analyses were carried out for all samples, whereas Rock‐Eval pyrolysis and vitrinite reflectance measurements with kerogen visual analysis were performed on selected samples. Lutites, litharenites, limestones, and slates are the predominant petrographic types. In most cases, the mean total organic carbon content is below the 2% limit for good quality potential source rocks. Only the Jurassic Rodiles and Tereñes limestones show total organic carbon values above this threshold. Rock‐Eval data reveal very low potential for hydrocarbon generation in almost all cases, with the exception of the dark mudstones of the Carboniferous Barcaliente Formation and the Rodiles Formation (fair and good potential for hydrocarbon generation, respectively). As regards thermal maturation, only the Ordovician formations are located in the peak of the gas generation interval. The Barcaliente and Rodiles formations are near the frontier oil/wet gas maturation areas, and the rest of the units are immature for gas generation.

Abstract. One of the concerns of underground CO2 onshore storage is the triggering of induced seismicity and fault reactivation by the pore pressure increasing. Hence, a comprehensive analysis of the tectonic parameters involved in the storage rock formation is mandatory for safety management operations. Unquestionably, active faults and seal faults depicting the storage bulk are relevant parameters to be considered. However, there is a lack of analysis of the active tectonic strain field affecting these faults during the CO2 storage monitoring. The advantage of reconstructing the tectonic field is the possibility to determine the strain trajectories and describing the fault patterns affecting the reservoir rock. In this work, we adapt a methodology of systematic geostructural analysis to underground CO2 storage, based on the calculation of the strain field from kinematics indicators on the fault planes (ey and ex for the maximum and minimum horizontal shortening, respectively). This methodology is based on a statistical analysis of individual strain tensor solutions obtained from fresh outcrops from the Triassic to the Miocene. Consequently, we have collected 447 fault data in 32 field stations located within a 20 km radius. The understanding of the fault sets' role for underground fluid circulation can also be established, helping further analysis of CO2 leakage and seepage. We have applied this methodology to Hontomín onshore CO2 storage facilities (central Spain). The geology of the area and the number of high-quality outcrops made this site a good candidate for studying the strain field from kinematics fault analysis. The results indicate a strike-slip tectonic regime with maximum horizontal shortening with a 160 and 50∘ E trend for the local regime, which activates NE–SW strike-slip faults. A regional extensional tectonic field was also recognized with a N–S trend, which activates N–S extensional faults, and NNE–SSW and NNW–SSE strike-slip faults, measured in the Cretaceous limestone on top of the Hontomín facilities. Monitoring these faults within the reservoir is suggested in addition to the possibility of obtaining a focal mechanism solutions for micro-earthquakes (M<3).

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