## 2 Geological Setting  
The study area offers a wide view of the European margins from Macaronesia (SW corner: 24\({}^{\circ}\)15\({}^{\prime}\) N; 36\({}^{\circ}\)10\({}^{\prime}\) W) to the Black Sea and the Barents Sea (NE corner: 60\({}^{\circ}\)40\({}^{\prime}\) E, 90\({}^{\circ}\) N). This hydrate framework covers three great domains on the European continental margins: the Arctic, the northeast Atlantic Sea and the south European Alpine Belt (Figure 1). Hydrate systems in the Arctic province are located on the west Greenland, Svalbard and western Barents Sea margins. The east Greenland and west Svalbard margins were created during the Cretaceous to Paleogene continental rifting [23]. East Greenland is bordered by a wide continental shelf and deep basins. Gas seepage, bottom-simulating reflector (BSR) levels and pore water anomalies related to offshore Mesozoic sedimentary basins have been reported, associated with thermogenic gas migration along fractures [24]. The west Svalbard margin is composed of glacier debris flows, turbidites, hemipelagic sediments and contourites [25], where gas flares, seepage and BSR levels take place [26]. Hydrates are mainly represented in mud volcanoes in south Svalbard (e.g., Hakon-Mosby mud volcano--HM in Figure 1; [27]). Hydrocarbon gases are mainly thermogenic and locally biogenic [28], but abiogenic [29] contributions have been reported. The Barents Sea and northern Norwegian margins are composed of a complex structure of sedimentary basins (e.g., More and Voring; MB and VB in Figure 1, respectively) and structural highs resulting from the Cenozoic rifting [30]. The sedimentary basins are filled mainly with fine-grained hemipelagic sediments (Moicene-Pliocene) and glacierie debris flows and contourites (Plio-Pleistocene) [31]. The Storegga Slide (SS in Figure 1) affected a huge sediment volume (mainly of the More Basin) as a response to climatic variability, 8200 ya [32]. In the Barents Sea, hydrate indicators are mainly BSR levels, gas chimneys and seepage pipes associated with vertical fluid flow systems and shallow gas. However, no hydrates have been obtained. The nature of gases is mostly thermogenic, migrating through faults and fractures [35]. On the mid-Norwegian margin, hydrate samples were recovered during the TTR-16 cruise [36], and the indicators are BSR levels, bright spots, gas chimneys and pockmarks [37], all of them circumscribed to the header of the Storegga Slide. Here, gases have a microbial origin with thermogenic components [38]. The northwest British Islands margins (Figure 1) are composed of basins with thick Cenozoic series [39]. Several seepage and gas chimneys are present on the upper slope, controlled by unconformities and fractures and sourced from the Upper Carboniferous and Middle and Upper Jurassic successions [40]. No hydrates have been recovered. The hydrate province of the south European Alpine Belt (SEAB in Figure 1) is located on the southern Iberian and northwest African margins, in the eastern Mediterranean and in the Black Sea. These areas are located in the context of the Alpine orogen, owing to the convergence between the African and Eurasian plates [41]. On the southern Iberian and northwest African margins and in the Gulf of Cadiz and Alboran Sea (GC and AS in Figure 1), hydrate samples have been recovered in mud volcanoes [42,43], associated with other hydrocarbon fluid flow structures such as pockmarks and hydrocarbon-derived authigenic carbonate (HDAC) [44]. This fluid flow is controlled by fractures linked to a deep-rooted mud  
Figure 1: Main tectonic structures and geological domains on the European continental margins and adjacent areas. Location of the study area. HM, Hakon–Mosby mud volcano; VB, Vering Basin; MB, More Basin; SS, Storegga Slide; CGFZ, Charlie–Gibbs fracture zone; EEC, East European craton; Biscay Bay; GFZ, Gloria Fracture Zone; GC, Gulf of Cadiz; AS, Alborán Sea; SAEB, south European Alpine Belt, NAF, North Anatolian Fault; DD, Danube delta fan. (Taken from [23,33,34]).