## 2 Study Area
### Geological and Geomorphological Setting  
The Yela reservoir, a deep aquifer converted into an underground gas storage site in 2012, consists of a saline aquifer in a carbonate succession and covers an area of approximately 30 km\({}^{2}\). An aquifer can be considered suitable for gas storage if the porous sedimentary rock formation containing water is overlain with an impermeable cap rock. This situation allows the gas to remain confined in the reservoir, preventing possible leakage and spillage. Although the aquifer reservoir behaviour is comparable to one constituted by a depleted production field, its use for natural gas storage usually requires a larger amount of cushion gas (i.e., gas not available for withdrawal but needed to ensure a minimum adequate pressure for UGS activities). Furthermore, the aquifer reservoir allows a lower flexibility in injection and withdrawal rates. Availability rates can be improved by the presence of an active water system, which sustains reservoir pressure during injection and withdrawal cycles (the operation is described in the diagram in Figure 2B). The Yela reservoir is situated between 2300 and 2550 m below ground level. It resides within the 'Santa Barbara' formation, which is composed of Cretaceous carbonates (predominantly dolomite) and is characterised by a widespread network of fractures. This formation is capped and sealed by evaporitic facies (as shown in Figure 2B) [19]. The geological structure exploited during UGS activities is an anticline with two tops: (i) the reservoir, made up of dolomite, and (ii) the seal, consisting of anhydrites, both from the Upper Cretaceous (Table 2). The Yela facility is located in the north-eastern portion of the Madrid Basin, a Cenozoic basin bounded by the Central System intraplate mountain range to the North and the fold-and-thrust belt of the Iberian Chain to the North-East (developed in the Cenozoic era). It has a surface area of more than 20,000 km\({}^{2}\) and is characterised by a structure closely linked to the tectonic evolution of the Central System and the Iberian Chain, which allows for the presence of considerable hydrological resources. The Madrid Basin formed on a sedimentary layer composed of a Permian-Triassic clastic succession. This layer was subsequently overlaid by Middle to Upper Triassic carbonates, which include evaporitic sequences (notably, sequences from the Jurassic to the Lower Cretaceous are absent in the area) [20]. The sedimentary history of this basin is largely continuous from the Late Cretaceous to the Late Miocene. This continuity is indicative of its continental deposition environment, resulting in a diverse lithology that encompasses sandstones, clays, marls, limestones, and chalk (as illustrated in Figure 2). [PERSON] et al. [19] designed geological cross-sections from seismic reflection profiles of the southern boundary of the Central System and the Iberian Chain, which can recognise a complex thick-skinned thrust system developed during the Cenozoic [19]. Figure 3 shows the easternmost cross-sections of the Madrid Basin. During the injection process, natural gas is introduced into the porous rock, displacing the previously stored water. The presence of an active water system can improve the availability rates as well as sustain reservoir pressure during withdrawal operations. Therefore, using an active aquifer requires less cushion gas, allowing a higher quantity of working gas to be injected into the reservoir. The working gas capacity of the Yela UGS site is 1 billion m\({}^{3}\), and the total reservoir capacity is approximately double that. The delivery and distribution of natural gas take place thanks to the national network supervised by the control centre throughout the gas system based on the diverse demand in different areas of the country. Monitoring the pressure is necessary because a defined range of values