A Burnol
email: a.burnol@brgm.fr

I Thinon

L Ruffine

J-M Herri

Influence of impurities (nitrogen and methane) on the CO 2 storage capacity as sediment-hosted gas hydrates -Application in the area of the Celtic Sea and the Bay of Biscay

Keywords: CO 2 storage capacity, Impurities, Gas hydrates, Deep offshore, Marine sediments, French EEZ

Deep saline aquifers are the most important potential storage reservoirs for CO 2 under supercritical conditions. Another option is the trapping of CO 2 in deep-sea sediments at low temperature, either in liquid state or as gas hydrates. The Negative Buoyancy Zone (NBZ) and the Gas Hydrate Stability Zone (GHSZ) are used to calculate the theoretical storage volume.

It depends essentially on the geothermal gradient and on the quality of the injected CO 2 . A sensitivity analysis shows that the storage volume in the Celtic Sea and the Bay of Biscay area is one order of magnitude lower if the injected fluid contains impurities like nitrogen or methane. Overall, the storage capacity in the French Exclusive Economic Zone (EEZ) exceeds 100 years of storage of the current French CO 2 emissions from large point sources.

The conservative estimate of the EEZ storage capacity is found to be higher than in the deep saline aquifers of the Paris Basin. Moreover, the CO 2 storage capacity in this area is doubled when considering the zone beyond 200 miles from the shores. The implications of these results are potentially important for the long-term deployment strategy for CO 2 storage in France and in Western Europe.

Introduction

The capture of CO 2 emissions from large point sources followed by its storage in deep geological reservoirs (CCS) is considered as an option in the portfolio of actions to mitigate the current global warming [START_REF] Metz | IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change[END_REF]. In the first injection tests (e.g. Lacq since 2010, Sleipner, North Sea since 1996 and Snøhvit, Sea of Barents since 2009), CO 2 is injected under supercritical conditions (high pressure and high temperature). Because the density of supercritical CO 2 is lower than that of pore water, the storage safety depends on the impermeability of the cap rock. Conversely, there is no need of such a cap rock when considering the case of a deep offshore CO 2 storage in deep-sea sediments as the storage integrity is indeed insured by the difference of density: beyond a certain depth, the injected fluid is denser than the seawater and is gravitationally trapped in the deep-sea sediments [START_REF] House | Permanent carbon dioxide storage in deep-sea sediments[END_REF][START_REF] Levine | Gravitational trapping of carbon dioxide in deep sea sediments: Permeability, buoyancy, and geomechanical analysis[END_REF][START_REF] Goldberg | Carbon dioxide sequestration in deep-sea basalt[END_REF][START_REF] Goldberg | A global assessment of deep-sea basalt sites for carbon sequestration[END_REF][START_REF] Slagle | Evaluation of ocean crustal Sites 1256 and 504 for longterm CO2 sequestration[END_REF][START_REF] Eccles | Global CO 2 storage potential of self-sealing marine sedimentary strata[END_REF]. The thickness of this Negative Buoyancy Zone (NBZ) depends on the local geothermal gradient. In addition, the CO 2 can be trapped in gas hydrates, which can further reduce the permeability by clogging the porous volume [START_REF] House | Permanent carbon dioxide storage in deep-sea sediments[END_REF][START_REF] Tohidi | CO2 Hydrates Could Provide Secondary Safety Factor in Subsurface Sequestration of CO2[END_REF]. Due to the presence of both gravitational and gas hydrate barriers ("self-sealing process"), the long-term safety of the storage option at low temperature is a priori higher than that of the storage option at high temperature.

Submarine venting of natural liquid CO 2 has been previously observed only at a few submarine volcanoes [START_REF] Lupton | Submarine venting of liquid carbon dioxide on a Mariana Arc volcano[END_REF][START_REF] De Beer | Saturated CO 2 inhibits microbial processes in CO 2 -vented deep-sea sediments[END_REF]. The study in laboratory of the dynamics of the gas hydrates in sedimentary medium (namely formation, accumulation and destabilization processes) is still at its beginnings. Thus, the technology for a CO 2 storage at low-temperature is not mature yet and there is a need for a precise description of the physicochemical behavior of the injected fluid with respect to the geological matrix, particularly in the Gas Hydrate Stability Zone (GHSZ). However, the technological issues could be overcome in the medium-long term owing to the development of the hydrocarbon industry:

(1) oil and gas companies are targeting very deep-offshore drilling for hydrocarbon extraction in the near future (beyond 3000 m water depth); (2) the injection of CO 2 in its liquid form is under investigations and would reduce the costs of conditioning in a hostile marine environment [START_REF] Vilarrasa | Liquid CO2 injection for geological storage in deep saline aquifers[END_REF]; (3) the potential formation of CO 2 hydrate in the injection well or in the vicinity of the borehole may be avoided by implementing a heating jacket on flexible risers similar to that used for the deep-offshore gas transport.

From an economic point of view, such a deep offshore storage would increase the costs of both transport (due to longer distances, see Figure 1) and injection (due to the hostile marine environment). For instance, the maritime transport would be preferred because it is cheaper than offshore pipelines for distance longer than 500 km [START_REF] Decarre | CO2 maritime transportation[END_REF]. These higher costs have been recently estimated by [START_REF] Eccles | Economic evaluation of offshore storage potential in the US Exclusive Economic Zone[END_REF]: the comparison in the specific case of the United States shows that deep offshore storage may be three times more expensive than onshore storage. However, a comprehensive comparative study should consider all the associated costs due to potential usage conflicts (e.g. with geothermal exploitations), the applicable regulations related to social acceptability and finally the monitoring from the end of the injection through the entire life of the storage. In the offshore case, CO 2 trapping at low temperature could be economically viable in hydrate-bearing sediments if it is coupled with the production of natural gas as an energy resource. On May 2012, the US DOE (Department of Energy) and COP oil company (ConocoPhillips) completed the first methane-hydrate production test coupled with sequestration of CO 2 on a pilot site in Alaska. The primary objective was to demonstrate the feasibility of a CO 2 injection into methane hydrate-bearing sediments and to assess the CO 2 -CH 4 exchange, together with the storage integrity. The principle relies on the difference between the hydrate stability fields of the two hydrate formers methane and carbon dioxide in the low pressure range [START_REF] Kvamme | Effects of solid surfaces on hydrate kinetics and stability[END_REF]. One of the main advantages of this method consists on avoiding the geomechanical destabilization of the sedimentary layer as it may happen for the alternative methods (e.g. depressurization or thermal stimulation).

The purity of the injected fluid will depend on the emission source and the capture process (Table 1). The density of the injected fluid decreases when the percentage of common impurities in the injected fluid (e.g. N 2 , CH 4 , CO, H 2 S) increases. Moreover, the equilibrium temperature of a mixed gas hydrate, an ice-like solid phase which traps CO 2 and associated impurities deviates from the equilibrium temperature of pure CO 2 hydrate [START_REF] Sloan | Clathrate Hydrates of Natural Gases, Third Edition[END_REF][START_REF] Chapoy | Hydrate and Phase Behavior Modeling in CO2-Rich Pipelines[END_REF]. This work describes the influence of the CO 2 quality on the storage capacity of a deep offshore site.

Geological setting in the studied area and in the Paris Basin

Geological setting in the studied area

The sediments in the Biscay Abyssal Plain are composed of sediments with a variable origin (continental, marine), variable composition and grain size (carbonate, clastic, organic or mixed) and variable distribution (lenticular or stratified). These parameters depend on the distance from source of supply, the morphology of the deposits area, the geodynamic and climatic events. The post-rift sedimentary cover of Biscay Abyssal Plain (Upper Cretaceoustoday), are located below 4000 m water depth. Its thickness can reach more than 7 km. It covers a thinned continental crust with the pre-rift and syn-rift sediments, directly a transitional crust or an oceanic crust [START_REF] Thinon | Structure profonde de la Marge Nord Gascogne et du Bassin Armoricain[END_REF][START_REF] Thinon | Deep structure of the Armorican Basin (Bay of Biscay): a review of Norgasis seismic reflection and refraction data[END_REF]. The superficial part of the post-rift sedimentary cover, the most recent sediment, consists mainly of detrital deposits in deep-sea fans whose construction and spatiotemporal evolution are related to changes in sea level during the last climatic cycles. The deep-sea fans were deposited at the foot of the continental slope from the Early Miocene, and progressively extended toward the middle of the Bay of Biscay during the Plio-Quaternary [START_REF] Droz | L'éventail profond de la marge celtique: stratigraphie et évolution sédimentaire[END_REF][START_REF] Auffret | Premières observations sur la morphologie et les processus sédimentaires récents de l"Éventail celtique[END_REF]. Several deep fans like the Celtic and Armorican turbiditic systems are produced during the last glacial maximum and influenced by the melting of Britain and Ireland glaciers and dumps from the "Fleuve Manche" [START_REF] Zaragosi | Initiation of the European deglaciation as recorded in the northwestern Bay of Biscay slope environments (Meriadzek Terrace and Trevelyan Escarpment): a multi-proxy approach[END_REF][START_REF] Bourillet | The 'Fleuve Manche': the submarine sedimentary features from the outer shelf to the deep-sea fans[END_REF]. The distribution of the latest 30 ky sediments have been affected by the glacial oscillations of Britain and Ireland glaciers and climate changes [START_REF] Toucanne | Activity of the turbidite levees of the Celtic-Armorican margin (Bay of Biscay) during the last 30,000 years: Imprints of the last European deglaciation and Heinrich events[END_REF][START_REF] Toucanne | External controls on turbidite sedimentation on the glacially-influenced Armorican margin (Bay of Biscay, western European margin)[END_REF]. These fans consist of sedimentary lobes (lenticular deposits), channels, coarse and heterogeneous fills within the channels.

The Celtic Deep-Sea Fan located in the northwestern part of the Bay of Biscay has a surface area of more than 30,000 km 2 and is a mature mud/sand-rich submarine fan [START_REF] Zaragosi | Physiography and recent sediment distribution of the Celtic Deep-Sea Fan, Bay of Biscay[END_REF]. The knowledge of the heterogeneities in the Bay of Biscay is mainly supplied by the reflection-seismic data which show the architecture of sedimentary systems in depth and by geological sampling which reveal the sediment composition. If the existing seismic network can today offer a low resolution image of the sedimentary architecture, the existing geological data are insufficient to determine with a good accuracy the composition for each sedimentary layer in the Biscay Abyssal Plain. In the northern part of the abyssal plain, some samplings by sediment corer give the lithological composition of first 40 m deep below seabed [START_REF] Zaragosi | Physiography and recent sediment distribution of the Celtic Deep-Sea Fan, Bay of Biscay[END_REF][START_REF] Toucanne | Activity of the turbidite levees of the Celtic-Armorican margin (Bay of Biscay) during the last 30,000 years: Imprints of the last European deglaciation and Heinrich events[END_REF][START_REF] Toucanne | The first estimation of Fleuve Manche palaeoriver discharge during the last deglaciation: Evidence for Fennoscandian ice sheet meltwater flow in the English Channel ca 20-18 ka ago[END_REF][START_REF] Toucanne | External controls on turbidite sedimentation on the glacially-influenced Armorican margin (Bay of Biscay, western European margin)[END_REF]. The lithological knowledge of the entire post-rift sedimentary cover is based only on the Deep Sea Drilling Project (DSDP) boreholes. However, this information must be taken cautiously: DSDP boreholes, located on high reliefs, do not sample the first seventies meters of sediments and have several important hiatus. Between 70 and 300 m, there are calcareous muds and chalks with nano-fossils and in depth there are some limestones or calcareous mudstones [START_REF] Montadert | Sites 399, 400, and Hole 440A[END_REF] (see clay fraction in Supporting Information, Figure 3SI). Two adjacent zones in the studied area are investigated in this work: the French Exclusive Economic Zone (called EEZ) using the DSDP Leg 48 site 400 and a zone beyond 200 miles from the shores (called 200M) using the DSDP Leg 80 site 550 (Figure 1). The latter was adopted by the UN Commission on the Limits of the Continental Shelf (CLCS) on 24 March 2009 in regard to the joint submission made by France, Ireland, Spain and the United Kingdom of Great Britain and Northern Ireland.

Onshore CO 2 storage capacity estimate in the Paris Basin

The Paris Basin is the largest onshore sedimentary basin in France: it is a large, nearly circular sedimentary basin which occupies a vast part of Northern France (110 000 km 2 ) and extends northward below the English Channel [START_REF] Lopez | 40 years of Dogger aquifer management in Ile-de-France, Paris Basin, France[END_REF]. The central part of the Basin, where the subsidence was the greatest, is filled with about 3000 m of sediments. The geothermal potential of the reservoirs of the Paris Basin has been exploited for 40 years. This exploitation history, along with the data from various oil exploration wells, provided a better geological characterization of the lithostratigraphic units exhibiting aquifer properties. Among the four main aquifers in the Basin, the mid-Jurassic (Dogger) carbonate rocks and the underlying Triassic aquifers were identified as two promising storage targets in the Paris area where the major industrial CO 2 sources are located (Figure 1). The Dogger aquifer is recharged along the eastern border of the Basin where the formation outcrops and the major flow is directed toward the discharge area on the seafloor of the English Channel, in the north-western part of the Basin. Some of CO 2 storage potential targets correspond to the target layers for geothermal exploitation.

In the deep saline aquifers of the Paris Basin, the effective CO 2 storage capacity called 2 CO M is the product of the theoretical porous volume (V), the average density of supercritical CO 2 in each layer ( SC CO 2



) and the storage efficiency factor (E):

E V M SC CO CO    2 2  with c SC CO C S E   2 (1)
where SC CO S 2 is the supercritical CO 2 saturation level, C C the capacity coefficient. Cc is a sitespecific scaling factor and is determined by various methods including expert advice and analogy with oil exploration data [START_REF] Bachu | Estimation of CO 2 Storage Capacity in Geological Media[END_REF]. A CO 2 average density of 0.48 g/cm 3 and 0.7 g/cm 3 has been proposed by the French Geological Survey (BRGM) in the Dogger layer and in the Trias layer, respectively. Supposing a high value of the storage efficiency factor of 6%, the total CO 2 storage capacity in deep saline aquifers of the Paris Basin has been estimated to 27.1 Gt in the European FP6 EU GeoCapacity project [START_REF] Vangkilde-Pedersen | D16 WP2 Report -Storage capacity[END_REF]. Monte Carlo simulations of the USDOE Capacity and Fairways Subgroup give a value of E between 1 and 4% of the bulk volume of a deep saline aquifer for a 15 to 85% confidence range, with an average of 2.4% for 50% confidence [START_REF] Bachu | Comparison between Methodologies Recommended for Estimation of CO 2 Storage Capacity in Geological Media[END_REF]. Using this storage efficiency factor of 2.4%, the Paris Basin storage capacity has been estimated to 29.1 Gt in the METSOR II project of BRGM [START_REF] Leynet | Metstor-2 -Évolution technique de l'outil de sélection des sites de stockage de CO 2[END_REF]. A more conservative estimate of 7.9 Gt has been proposed in the GeoCapacity project using a storage efficiency factor of 2% for open aquifers, 1% for semi-open aquifer and 0.1% for closed aquifers [START_REF] Vangkilde-Pedersen | D16 WP2 Report -Storage capacity[END_REF]. This estimate probably gives the most realistic picture of the onshore storage capacity in the Paris Basin. The purpose is to compare this value to a conservative estimate of the deep offshore storage capacity in the French EEZ.

Input data and modeling tools

CO 2 stream composition (CO2-96, CO2-97, CO2-100)

The nature and the quantity of the impurities in the CO 2 stream depend on the emission source (e.g. gas/coal-fired power plant) and also on the capture process (e.g. pre/post/oxy-fuelcombustion). A high quantity of non-condensable gases (N 2 , CH 4, H 2 , O 2 , Ar, CO) may decrease the capture cost but may also increase the compression and transport costs. On the other hand, the presence of H 2 S may lead to the corrosion of the pipeline and increase the environmental risk. The Dynamis project (integrated FP6 European project) recommended therefore technical limits for the impurities in the CO 2 stream: the total volume fraction for all non-condensable gases together should be set to a maximum of 4% and H 2 S volume fraction to a maximum of 0.02% [START_REF] De Visser | Dynamis CO2 quality recommendations[END_REF].

It is assumed in this work that the three-phase equilibrium will not be significantly affected by very low concentrations of potential traces (such as SOx, NOx). Other impurities like H 2 and H 2 S are often associated in the CO 2 streams coming from coal-fired plants but they are not studied in this work because of the lack of data on H 2 S hydrates [START_REF] Ward | Phase Equilibrium Data and Model Comparisons for H2S Hydrates[END_REF]. Only two simplified stream compositions have been considered here and compared to the pure CO 2 case (called CO2-100): the first stream (called CO2-96) is simulated as a binary system CO 2 -N 2 and the second stream (called CO2-97) as a ternary system CO 2 -CH 4 -N 2 (Table 1). CO2-96 is representative of a medium purification level using the oxyfuel capture process in gas-fired plants and contains 4% (mole fraction) impurities with less than 0.5% of O 2 [START_REF] Yan | Impacts of non-condensable components on CO 2 processing, pipeline transport and geological storage[END_REF] (Table 1). All the oxygen should be consumed quickly after the injection due the anoxic conditions in deep-sea sediments and its relative high solubility: the CO2-96 case is therefore simplified and simulated only as a binary mixture CO 2 -N 2 . The second stream CO2-97 is representative of a fluid mixture coming from a gas-fired plant using the pre-combustion capture process and contains 3% of impurities with about 1% of N 2 and less than 2% of CH 4 (Table 1) [START_REF] Metz | IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change[END_REF]. Submarine venting of natural liquid CO 2 has been recently observed and it has been shown that CO 2 -saturated seawater should inhibit sulfate reduction and anaerobic methane oxidation (AOM) in deep-sea sediments [START_REF] De Beer | Saturated CO 2 inhibits microbial processes in CO 2 -vented deep-sea sediments[END_REF]. Therefore, it is supposed here that methane is not oxidized in the CO2-97 case.

Sediment temperature using a constant Heat Flux

The calculation of the vertical sediment temperature is based on the application of a simplified form of Fourier"s first law for the vertical heat transport:

  dz d z T TC   HF (2)
Where HF is the Heat Flux (W m -2 ), TC the thermal conductivity (W m -1 K -1 ), dT/dz the geothermal gradient (K m -1 ) and z is the depth below seafloor in m (z > 0). The complete physical description can be found in the recent review article of [START_REF] Fisher | Using seafloor heat flow as a tracer to map subseafloor fluid flow in the ocean crust[END_REF]. It is assumed here that the heat flux through the marine sediment in the studied area is purely conductive and occurs at steady state, i.e. the heat flux is constant along a vertical profile [START_REF] Pribnow | Thermal data collection and heat flow recalculations for ODP[END_REF]. Assuming a linear change of thermal conductivity TC with the depth z, the integration of Eq. ( 2) leads to the sediment temperature T (°C):

  1000 TC z z    β α (3)            α ) TC( ln β HF 1000 BWT T z z (4)
Where  is the surface thermal conductivity (W/m/K),  the variation of TC with depth (mW/m 2 /K) and BWT the bottom water temperature (°C). () is equal to (1.175 W/m/K, 1.36 mW/m 2 /K) in EEZ and (1.1 W/m/K, 1.81 mW/m 2 /K) in 200M, respectively. BWT along the continental rise and the abyssal plain is almost constant in the studied area and the mean value is about 2.6 °C [START_REF] Foucher | Thermal Regime of the Northern Bay of Biscay Continental Margin in the Vicinity of the D.S.D.P. Sites 400-402[END_REF][START_REF] Foucher | Geothermal Measurements during Deep-Sea Drilling Project Leg-80[END_REF] (Table 2). As given in the Global Heat Flow Database (http://www.heatflow.und.edu), the heat flux (HF) varies in the studied area between 33 mW/m 2 ("low HF case") and 60 mW/m 2 ("high HF case"), with a medium value fixed to 46.5 mW/m 2 ("medium HF case") (Table 2). The thermal conductivity (TC) at Leg 80 Site 550 increases linearly with depth (Figure 2). The linear relation between the porosity and the thermal conductivity at site 550 is used to calculate TC from the porosity at site 440.

Thickness of the Negative Buoyancy Zone (NBZ) using GERG-2008

The Negative Buoyancy Zone (NBZ) is determined by calculating the density difference between the seawater and the CO 2 -rich mixture (CO2-N with N=96, 97, 100):

  ) X 00 10.1325)/1 ) ( ( 273.15 DOTPX(T(z) ) ( ), ( , 80 RHO ) ( N N , z p , z p z T S z      (5)
Where RHO80 and DOTPX are the seawater and CO2-N stream density (kg/m 3 ), S the salinity, T the sediment temperature (°C), p the pressure relative to a standard atmosphere (10.1325 dbar), X N the total composition of the liquid mixture in mole fraction (Table 1).

The seawater density is calculated by the 1980 International Equation of State (EOS-80) [START_REF] Millero | Background papers and supporting data on the International Equation of State of Seawater[END_REF][START_REF] Fofonoff | Algorithms for computation of fundamental properties of seawater[END_REF]). The salinity is supposed almost constant along the vertical profile and fixed to S=34.92 in the studied zone. The relative pressure p is estimated from the depth z and the latitude using Saunder's formula [START_REF] Saunders | Practical Conversion of Pressure to Depth[END_REF].

The density DOTPX of the liquid mixture is calculated with the GERG-2008 equation of state (EoS) and depends on the temperature in Kelvin, the absolute pressure in MPa and the mole fractions X N in the liquid mixture. The standard mixing rules implemented in GERG EoS correspond to Lorentz-Berthelot mixing rules that have been modified with focus on natural gases and other mixtures [START_REF] Kunz | The GERG-2004 wide-range equation of state for natural gases and other mixtures[END_REF].

The bisection method is used to calculate the critical depth z of Eq. ( 5) for which   (z) kg/m 3 (Figure 3). The NBZ lies between the seabed and this critical depth. As shown in Figure 4, if the seafloor depth is higher than 4000 m, both liquid mixtures (CO2-96 and CO2-97) are gravitationally trapped in deep-sea sediments.

Thickness of the Gas Hydrate Stability Zone (GHSZ) using CSMGem

The Gas Hydrate Stability Zone (GHSZ) is determined by calculating the difference between the gas hydrate equilibrium pressure P CO2-N depending on the sediment temperature T (see section 3.2) and the porewater pressure p given by the Saunder's formula [START_REF] Saunders | Practical Conversion of Pressure to Depth[END_REF]:

  00 10.1325)/1 ( N      ) ( ) ( 2 z p z P z N CO (6)
For each type of mixture (N=96, 97, 100), the equilibrium points (T, P CO2-N ) in NaCl water (S=34.92) calculated by the CSMGem code Version 1.10 (January 1, 2007) [START_REF] Ballard | The next generation of hydrate prediction: I. Hydrate standard states and incorporation of spectroscopy[END_REF][START_REF] Sloan | Clathrate Hydrates of Natural Gases, Third Edition[END_REF] are fitted by a linear approximation in the high pressure range (40-60 MPa):

) ( b a ) ( N N N - CO2 z T z P    (7)
Where a N is a pressure (MPa) and b N is a variation of pressure with temperature (MPa K -1 ).

The parameters (a N , b N ) are (-137.99, 14.662), (-147.68, 14.69) and (-140.26, 15.945) for N=96, N=97 and N=100, respectively (Figure 5).

The bisection method is then used to calculate the critical depth z of Eq. ( 6) for which   (z) MPa (Figure 3). The GHSZ lies between the seabed and this critical depth. An example of GHSZ below a seafloor of 4500 m is given for the high and low Heat Flow in Figure 6. It is assumed in this work that the host-sediment physical properties do not affect significantly the equilibrium state of the bulk phase (see discussion in section 5.3).

Storage surfaces using three safety criteria and GMT

As the seafloor depth is less than 5000 m in the studied area, the bottom of the GHSZ is less than 600 mbsf for both cases N=96, 97 (Figure 7). In order to avoid the risk of formation of gas hydrate at the outlet of the injection pipe, the total sediment thickness should be larger than 600 m in the CO 2 storage zone. Three different safety criteria are therefore used to define the surface of the CO 2 storage in the studied area: (1) a seafloor depth higher than 4000 m to insure that the density of the injected fluid is higher than the seawater density (Figure 4); (2) a seafloor depth gradient lower than 0.01 to avoid the risk of slumps coming from the continental slope; (3) a sediment thickness higher than 600 m. GMT, the Generic Mapping Tool [START_REF] Wessel | New, improved version of generic mapping tools released[END_REF], has been used to apply these three criteria and to calculate the storage surfaces in both zones (EEZ and 200M). Further details about the method can be found in Appendix A.

Theoretical storage volumes using GASCO2 and GMT

A new fortran tool (called GASCO2) has been developed to calculate for each case the thickness of NBZ and GHSZ using GERG-2008 and CSMGem, respectively (Figure 3). If the thickness of NBZ is larger than the thickness of the GHSZ, the injected CO 2 (l) is expected to percolate downwards to the bottom of the NBZ and will accumulate on either side of the neutral buoyancy level, with the depth of CO 2 above the level matching the depth below it [START_REF] Levine | Gravitational trapping of carbon dioxide in deep sea sediments: Permeability, buoyancy, and geomechanical analysis[END_REF]. As shown in Figure 7, it is indeed the case for pure CO 2 but generally not the case for CO2-96 or CO2-97.

In the pure CO 2 case (N=100), we supposed here that the CO 2 accumulation cannot exceed twice the depth difference between the neutral buoyancy level and the hydrate formation level as supported by modeling calculations of [START_REF] Qanbari | CO2 disposal as hydrate in ocean sediments[END_REF]. This volume which may host only liquid CO 2 is a conservative estimate of the total available volume (GHSZ is not taken into account in our calculation).

In the case with impurities (N=96, 97), the injected stream will generally flow upwards into the GHSZ and will form mixed gas hydrates, except in the low HF case with the highest seafloor depth (Figure 7). The gas hydrate formation results in the decrease in porosity and permeability, hence in the limitation of the upward migration of residual flow (supposing no overpressure or fracturing). The result on the long time scale (e.g. 1000 years) will depend on the balance between two major processes: the hydrate formation kinetics and the decrease of permeability. The hydrate formation kinetics will tend to resist the buoyancy-driven migration. As this migration is much lower just below the neutral buoyancy level than at the hydrate formation level, the volume above the neutral buoyancy level is very difficult to access (only by diffusion in the very long term), as supported by modeling calculations of [START_REF] Qanbari | CO2 disposal as hydrate in ocean sediments[END_REF]. We supposed here that the theoretical storage volume is the porous volume in the sub-domain where NBZ is included in GHSZ and lies between the GHSZ"s lower limit (bottom boundary) and the NBZ"s lower limit (top boundary) (Figure 7). This volume which may host mainly gas hydrates is also a conservative estimate of the total available volume (NBZ and residual liquid along migration pathway beneath GHSZ are not taken into account in our calculation).

Both theoretical storage volumes that may host gas hydrates for N=96/97 or pure liquid CO 2 for N=100 are calculated by GMT. Further details about the method can be found in Appendix A.

Modeling results

Limits of the storage zones in the studied area

Three safety criteria described in the section 3.5 are applied to define the limits of the CO 2 storage potential in the French Exclusive Economic Zone (called EEZ-CO2) and in the zone beyond 200 miles from the shores (called 200M-CO2). In the French EEZ abyssal plain, the sediment thickness is always higher than 600 m (between ~3000 m and ~8000 m as shown in Figure 8). The inner limit of both storage zones lies within the continental rise between the 4000-mbsl and the 4500-mbsl isobaths (Figure 9). Due to a seafloor depth below 4000-mbsl, the Armoricain Seamount and the Trevelyan Escarpment are both excluded from the storage zone (Figure 9). Due to a seabed slope above 0.01, the Gascogne Knoll is also excluded. Both surfaces calculated by GMT are of the same order, 42 120 km 2 and 41 083 km 2 for EEZ-CO2 and 200M-CO2, respectively.

Theoretical storage volumes for CO 2 in the liquid state or as gas hydrates

By definition, the theoretical storage volume is the total physical porous volume that may host gas hydrates in both cases CO2-N (N=96, 97) or liquid CO 2 in the pure CO 2 case (section 3.6). It assumes that the entire volume is accessible and utilized to its full capacity to store gas hydrate in the pore space. It represents therefore a maximum upper limit to a storage volume estimate (and therefore unrealistic to calculate the storage capacity, see section 5.4).

In the case with impurities (N=96, 97), the storage surface is lower in the low HF case than in the high HF case because only the sub-domain where NBZ is included in GHSZ is taken into account (section 3.6 and Figure 7). Conversely, the mean thickness (i.e. the ratio of the calculated volume and the surface) is higher in the low HF case than in the high HF. As the sediment porosity is inversely proportional to the sediment depth (Figure 2), the mean porosity is lower in the low HF case than in the high HF case. The theoretical storage volume which is the product of the three parameters (surface, thickness and porosity) is therefore not a monotonic function of the heat flux and needs to be determined through calculations in both zones (EEZ and 200M). In the French EEZ, the estimate of the theoretical storage volume can vary as much as 50% (between 1600 km 3 and 2400 km 3 ) and the maximum is obtained with a medium HF (46.5 mW/m 2 ) (Table 3). In 200M, the theoretical storage volumes are almost identical to those calculated in EEZ in all sensitivity cases (Table 4).

In the pure CO 2 case (N=100), the theoretical storage volume that may host liquid CO 2 is one order of magnitude higher than the storage volume as gas hydrates in the case with impurities (22°627 km 3 instead of 1982-2400 km 3 ) (Table 3).

Discussion

Influence of heat flux and impurities on the theoretical storage volume

Using the heat flow (HF) of 60 mW m -2 , the model temperature from Eq. ( 3) fits quite well with the in situ measurements at site L80 550 (Figure 6). On the global scale of the Earth"s surface, there is a global trend in the data of decreasing mean heat flow with increasing age [START_REF] Davies | Earth's surface heat flux[END_REF]. The recently published Structural map of the Atlantic Ocean [START_REF] Miles | Structural Map of the Atlantic Ocean[END_REF] in this area highlights that the mean age is higher in EEZ than in 200M, the mean HF is therefore likely lower in EEZ than in 200M. A large uncertainty still remains on the local HF variations in the studied area: the observed values vary by a factor of two (between 33 and 60 mW m -2 ) (see Table 2). In the case with impurities (N=96, 97), the maximum storage volume found with a medium HF is still one order of magnitude lower than the storage volume in the pure CO 2 case (Table 3). This high volume difference is explained by the storage thickness difference (1343 m instead of ~100m) (section 3.6).

A comparison with the pure CO 2 case in NaCl water (Figure 5) shows that nitrogen and methane act as promoters in the high pressure range (between 40 MPa and 60 MPa), although both impurities act as inhibitors at lower pressure (below 5 MPa) as observed recently by [START_REF] Chapoy | Hydrate and Phase Behavior Modeling in CO2-Rich Pipelines[END_REF]. This comparison shows also that the mole fraction of CH 4 has a higher impact than the same mole fraction of N 2 on the mixed gas hydrate dissociation temperature, therefore on the storage volume (Table 3). The comparison between both cases (N=96, 97) highlights that the influence of impurities like nitrogen and methane on the theoretical storage volume is of the same order than that of local heat flux variation (ca. 20%, Table 3).

Although the French EEZ storage surface (42 120 km 2 ) is two times lower than the storage surface in Paris Basin (92 568 km 2 , Figure 1) [START_REF] Vangkilde-Pedersen | D16 WP2 Report -Storage capacity[END_REF], the French EEZ storage volume estimate (1600-2400 km 3 ) is two times higher than the Paris Basin storage volume estimate (722 km 3 ) (Table 4). This result is mainly explained by a porosity of sediments four times higher in the deep offshore case than in the onshore case (40-60% instead of 10-15%).

About the uncertainty on the NBZ thickness

The seawater density is calculated with the EOS-80 formulation [START_REF] Millero | Background papers and supporting data on the International Equation of State of Seawater[END_REF]. The EOS-80 formulation is easy to use because it does not depend on the concentrations of individual ions unlike the recent TEOS-10 formulation [START_REF] Ioc | The international thermodynamic equation of seawater -2010: Calculation and use of thermodynamic properties[END_REF], while providing fairly accurate results: the uncertainty on seawater amounts to 0.05 kg/m 3 over the oceanic pressure range [START_REF] Fofonoff | Physical properties of seawater: A new salinity scale and equation of state for seawater[END_REF]. The uncertainty of this first term of Eq. ( 4) is anyway lower than the uncertainty of the liquid mixture density DOTPX.

The GERG-2008 equation of state uncertainty amounts to (0.2 -0.5)% inside of the extended range of validity (P ≤ 70 MPa) for many multi-component mixtures [START_REF] Kunz | The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG-2004[END_REF]. A higher quality between (0.1 -0.3)% can be achieved for some binary mixtures, such as CO 2 -N 2 and CO 2 -CH 4, thanks to an additional departure function using extensive sets of available experimental data [START_REF] Kunz | The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG-2004[END_REF]. The difference between the experimental density and the predicted density with GERG-2008 is indeed lower than 0.1% for the binary mixture CO 2 -N 2 at a pressure up to 48 MPa [START_REF] Brugge | Densities of Carbon Dioxide + Nitrogen from 225 K to 450 K at Pressures up to 70 MPa[END_REF] and for CO 2 -CH 4 at a pressure up to 42 MPa [START_REF] Hwang | Densities of Carbon Dioxide + Methane Mixtures from 225 K to 350 K at Pressures up to 35 MPa[END_REF] (Supporting Information, Figure 1SI). In order to cross-validate the GERG-2008 density function, multi-component CO 2 -rich mixtures issued from the different capture processes should be further experimentally investigated at deep-offshore pressure conditions (40-60 MPa).

About the uncertainty on the GHSZ thickness

The accuracy of CSMGem code has been first verified for pure CO 2 , N 2 and CH 4 hydrates three-phase equilibrium in the deep offshore high pressure range (40-60 MPa). Pure CO 2 and N 2 models in pure water fit well the experimental data with an average deviation in temperature of 0.18 K and 0.1 K, respectively (Supporting Information, Figure 2SI). The gas hydrate equilibrium is at first order controlled by the dominating CO 2 component and secondly affected by both impurities, N 2 and CH 4 . The CO 2 -rich mixture model with 4.6 mol% N 2 in pure water fits also well recent experimental data of [START_REF] Chapoy | Hydrate and Phase Behavior Modeling in CO2-Rich Pipelines[END_REF] with an average deviation in temperature of 0.1 K (Figure 5 and Figure 2SI). NaCl water (S=34.92) has been chosen as a simplified seawater model (Figure 5). The pure CH 4 model with CSMGem in different NaCl water fits relatively well the experimental data with an average deviation in temperature of 0.28 K (Figure 2SI). All the equilibrium points of CO2-N hydrates are calculated by CSMGem with a solid structure of type I (SI) and without a gas phase for methane and nitrogen.

However, the CSMGem results are a priori only valid for the "bulk" equilibrium, i.e. without a solid matrix. The sediment properties can strongly affect key parameters like the hydrate saturation level, the distribution pattern, and possibly the hydrate stability field [START_REF] Waite | Physical properties of hydrate-bearing sediments[END_REF][START_REF] Santamarina | The impact of hydrate saturation on the mechanical, electrical, and thermal properties of hydrate-bearing sand, silts, and clay. Geophysical Characterization of Gas Hydrates[END_REF]. The effect of pore size and shape on the equilibrium temperature has been investigated in the literature, either experimentally [START_REF] Anderson | Experimental Measurement of Methane and Carbon Dioxide Clathrate Hydrate Equilibria in Mesoporous Silica[END_REF][START_REF] Uchida | Decomposition of methane hydrates in sand, sandstone, clays, and glass beads[END_REF][START_REF] Dai | Hydrate morphology: Physical properties of sands with patchy hydrate saturation[END_REF] or theoretically [START_REF] Clennell | Formation of natural gas hydrates in marine sediments: 1. Conceptual model of gas hydrate growth conditioned by host sediment properties[END_REF][START_REF] Henry | Formation of natural gas hydrates in marine sediments: 2. Thermodynamic calculations of stability conditions in porous sediments[END_REF][START_REF] Klauda | Modeling Gas Hydrate Phase Equilibria in Laboratory and Natural Porous Media[END_REF][START_REF] Mohammadi | Gas Hydrate Phase Equilibrium in Porous Media: Mathematical Modeling and Correlation[END_REF]. The capillary forces are significant only for very small pore diameters (i.e. below 30 nm) with a shift of the hydrate stability field to higher pressure with decreasing pore diameters [START_REF] Anderson | Experimental Measurement of Methane and Carbon Dioxide Clathrate Hydrate Equilibria in Mesoporous Silica[END_REF].

Therefore, the pore size distribution of the sediment has to be estimated in order to fairly predict the hydrate stability field. Besides the pore diameter, the size and nature of the particles constituting the geologic matrix play also an important role in both the hydrate saturation level and the hydrate habit [START_REF] Lu | Particle size effect on the saturation of methane hydrate in sediments -Constrained from experimental results[END_REF][START_REF] Prasad | Effect of silica particles on the stability of methane hydrates[END_REF]. High hydrate saturation level (up to 100%) is found in coarse-grain sediment like sandy sediment. Those are the main type of reservoirs targeted for a hydrate accumulation. Lower hydrate saturation levels were obtained with particles smaller than 50 μm [START_REF] Lu | Particle size effect on the saturation of methane hydrate in sediments -Constrained from experimental results[END_REF]. In rich-clay sediment, the hydrate saturation level is even lower, e.g. 10% in a nannofossil-rich claystone [START_REF] Lu | Preliminary experimental results of the stable P-T conditions of methane hydrate in a nannofossil-rich claystone column[END_REF]. The hydrate formation involves the creation of cracks and faults within the matrix where they can accumulate as vein-filling hydrate or nodules. Therefore, large uncertainties remain with regard to the effective storage capacity due to the lack of knowledge on deep-sea sediment composition and heterogeneities in the studied zone.

The phase diagram of multi-component CO 2 -rich mixtures issued from the different capture processes should be further experimentally investigated with different types of marine sediments and at deep-offshore pressure and temperature conditions (40-60 MPa, 2-20 °C).

Conservative estimates of the CO 2 storage capacity in the studied area and comparison to the onshore case (Paris Basin)

In the deep offshore case, most of the CO 2 can be stored either as a liquid in the pure CO 2 case or as gas hydrates in the case with impurities (N=96, 97) (section 3.6). In a similar way to the Eq. ( 1) for the onshore case, the effective CO 2 storage capacity N -CO2

M

is the product of the theoretical storage volume (V), the CO 2 density and the storage efficiency factor (E): Cc is a site-specific fraction of deep-sea sediments that have suitable permeability for CO 2 injection and storage. The major influent parameter on the permeability in those marine sediments is the clay content and the available storage volume is likely to contain a low fraction of clay. As described in [START_REF] Levine | Gravitational trapping of carbon dioxide in deep sea sediments: Permeability, buoyancy, and geomechanical analysis[END_REF], only a limited number of offshore sites may contain deep sediments with sufficient permeability for injection without hydraulic fracturing. Using the permeability-porosity relationship as a function of clay content for marine mudstones [START_REF] Yang | A permeability-porosity relationship for mudstones[END_REF], we found that 10% and 2% are conservative estimates in the studied area of Cc and E, respectively (Supporting Information, Figure 3SI).

E V M L CO CO     2 100 100 2  with c L CO C S E   2 (8) E V M N CO N    2 N - CO2  with c H C S E   (9) Where L CO 2  is the liquid CO 2 density, N CO 2  the CO 2 density in CO2-N hydrate (N=96, 97),
In the pure CO 2 case, the liquid CO 2 density at the neutral buoyancy level is equal by definition to the seawater density which is almost constant (~1.045 g cm -3 ). As the liquid CO 2 will accumulate on either side of the neutral buoyancy level, this value is supposed to be the mean value in the available storage volume. The CO 2 storage capacity in the liquid state is calculated using the Eq. ( 8) and is about 473 Gt in the French EEZ, more than one order of magnitude higher than the results in the case with impurities (Table 4).

In the case with impurities (N=96, 97), the density of CO2-97 hydrate (1.096 g cm -3 ) and the density of CO2-96 hydrate (1.108 g cm -3 ) are very closed and lower than the pure CO 2 hydrate density (1.15 g cm -3 ) [START_REF] Takenouchi | Dissociation Pressures of the Phase CO 2 .5.75 H 2 O[END_REF]. Using the cage occupancy for each component given by CSMGem, we found that the CO 2 density is 0.3 g/cm 3 and 0.29 g/cm 3 in CO2-96 and CO2-97 hydrate, respectively (Supporting Information, Eq. 1SI). The CO 2 storage capacity as gas hydrates is calculated using the Eq. ( 9) and lies between 10 Gt and 14 Gt in the French EEZ (Table 4). Our most conservative estimate of the storage capacity in the French EEZ (10 Gt) is therefore higher than the conservative estimate (8 Gt) of the Paris Basin storage capacity. Moreover, the storage capacity beyond 200 miles from the shores is of the same order of magnitude than in French EEZ (Table 4).

Conclusions

This work has provided evidences in support of the innovative concept of a deep offshore CO 2 storage in deep-sea sediments. Based on available data from DSDP boreholes and from cruises on the R/V Le Suroît and Jean Charcot, the CO 2 storage volume has been estimated in the area of the Celtic Sea and the Bay of Biscay. The storage volume depends essentially on the geothermal gradient and on the quality of the injected CO 2 .

The available volume is one order of magnitude lower if the injected CO 2 contains some impurities like N 2 or CH 4 . A sensitivity analysis has been carried out and has showed that the influence of the variation of the impurities (N 2 or CH 4 ) on the storage volume is of the same order than that of local heat flux variations. Using the amount of French CO 2 emissions from large point sources emitting more than 0.1 Mt/year (116 Mt in 2012 from http://www.pollutionsindustrielles.ecologie.gouv.fr), we found that the mean estimate of the storage capacity in French EEZ (12 Gt) exceeds 100 years of storage of the current French CO 2 emissions from large point sources. Moreover, the most conservative estimate in the French EEZ is higher than the conservative estimate of the storage capacity in the deep saline aquifers in the Paris Basin. Furthermore, this storage capacity is doubled when considering the zone beyond 200 miles adopted on 24 March 2009 by the UN Commission on the Limits of the Continental Shelf (CLCS) in the studied area.

The implications are therefore potentially important for the long-term deployment of the CO 2 storage as well in France as in Western Europe. However, large uncertainties remain with regard to the effective storage capacity due to the lack of knowledge of the influence of the sediment composition on CO 2 hydrate formation and saturation level. Those preliminary results need therefore further refinements. This will mainly imply permeability measurements of deep-sea sediments samplings and building up a consistent database from experiments performed at in situ conditions, i.e. in the high pressure range (40-60 MPa) and with different representative solid matrices.

The grd2xyz tool is used to write the ascii file bathy3.data: grd2xyz bathy3.grd -S > bathy3.dat

The ascii file bathy3.dat is read as input file by the code GASCO2 to calculate both output files nbz.dat and ghsz.dat which contains the NBZ"s thickness (in mbsl) and the GHSZ"s thickness (in mbsl), respectively (Figure 3).

In both CO2-N cases (N=96, 97), the volume which may host gas hydrates lies between the GHSZ and NBZ"s lower limits, but only in the sub-domain where NBZ is included in GHSZ (see section 3.6). The grdmath tool is used to extract this sub-domain: grdmath nbz.grd ghsz.grd LT nbz.grd MUL 0 NAN = nbz2.grd grdmath nbz.grd ghsz.grd LT ghsz.grd MUL 0 NAN = ghsz2.grd The thickness.grd grid contains the total thickness of the storage zone (in m): grdmath ghsz2.grd nbz2.grd SUB = thickness.grd A mean vertical porosity is calculated using the linear approximation from site 400 in EEZ and site 550 in 200M: The grdvolume tool is used to calculate the sediment volume and the theoretical porous volume (m 3 ) that may host gas hydrate: grdvolume -S thickness.grd > volume.txt grdvolume -S storage.grd > porous_volume.txt

In the pure CO 2 case, the volume which may host liquid CO 2 cannot exceed twice the depth difference between the neutral buoyancy level and the hydrate formation level (section 3.6):

As the thickness of NBZ is between 500m and 1200 m (Figure 7), the linear relation for the porosity given in Figure 2 is no more valid. Therefore, 40% has been chosen as a conservative estimate of the porosity: grdmath nbz.grd ghsz.grd SUB 2 MUL 0.4 MUL = storage2.grd

The grdvolume tool is used to calculate the sediment volume and the theoretical porous volume (m 3 ) that may host pure CO 2 as a liquid: grdvolume -S thickness2.grd > volume2.txt grdvolume -S storage2.grd > porous_volume2.txt

In the 200M zone, a bathymetry has been extracted from the 1 arc-minute global grid ETOPO1 (http://maps.ngdc.noaa.gov/viewers/wcs-client) directly in the GMT NetCDF format [START_REF] Amante | ETOPO1 1 arc-minute global relief model: procedures, data sources and analysis[END_REF]. a In the pure CO 2 case (N=100), the theoretical storage volume is calculated in the medium HF case. In the other cases (N=96, 97), the minimum volume is the low HF CO2-96 case and the maximum volume is the medium HF CO2-97 case (see Table 3) b 2% has been considered as a conservative estimate of the storage efficiency factor in the studied area (see Supporting Information). The same value has been used in the conservative estimate of the storage capacity in Paris Basin [START_REF] Vangkilde-Pedersen | D16 WP2 Report -Storage capacity[END_REF] c In the pure CO 2 case, a mean liquid CO 2 density of 1.045 g/cm 3 has been used. In the cases with impurities (N=96, 97), a CO 2 density of 0.3 g/cm 3 in CO2-96 hydrate and 0.29 g/cm 3 in CO2-97 hydrate has been calculated (see Supporting Information).

S

  the liquid CO 2 and hydrate saturation level, C C the capacity coefficient.

  the porosity at seafloor and b the variation of porosity with depth (in km). The parameters (a,b) are (0.6366,-0.45) and (0.6605,-0.596) in EEZ and 200M, respectively (Figure 2). This linear relation is used to calculate the mean vertical porosity at each node: grdmath nbz2.grd ghsz2.grd ADD 2 DIV bathy3.grd SUB b MUL a ADD = porosity.grd The storage potential as gas hydrates is calculated using the thickness multiplied by the mean porosity in each domain (EEZ or 200M): grdmath ghsz2.grd nbz2.grd SUB porosity.grd MUL = storage.grd

Figure 1 .

 1 Figure 1. Location map in the area of the Celtic Sea and the Bay of Biscay showing surface heat flux observations by circles and DSDP boreholes by white triangles (L48 400 in French EEZ and L80 550 in 200M zone). Also shown are the Paris Basin potential CO 2 storage zone in pink color and the main onshore CO 2 sources in 2010 by white squares (Source: EC-JRC/PBL. EDGAR version 4.0. http://edgar.jrc.ec.europa.eu).

Figure 2 .Figure 3 .

 23 Figure 2. Porosity and Thermal Conductivity (TC) vertical profiles at DSDP Leg 80 Site 550 and Leg 48 Site 440. TC profile from Site 440 is deduced from the porosity profile using the linear relation given by the Site 550.

Figure 4 .

 4 Figure 4. Variation with the depth of the liquid density of pure CO 2 , CO2-96, CO2-97 calculated by GERG-2008 and the seawater density supposing a constant temperature of T=2.6°C (mean Bottom Water Temperature).

Figure 5 .Figure 6 .

 56 Figure 5. Pure CO 2 , CO2-96 and CO2-97 hydrate equilibrium curves calculated by CSMGem in NaCl water (S=34.92) and in high pressure range (between 40 and 60 MPa). Also shown are some experimental results in pure water for CO 2 hydrate (Takenouchi, 1965) and for mixed CO 2 -N 2 hydrate (Chapoy, 2015) compared to the equilibrium curves calculated by CSMGem.

Figure 7 .

 7 Figure 7. Influence of the seafloor depth on the thickness of NBZ and GHSZ in medium HF case (46.5 mW/m 2 ) (A) with pure CO 2 , (B) with CO2-96/97 and in low HF case (33 mW/m 2 ) (C) with CO2-96/97. Both black vertical lines represent the depth boundaries of the CO 2 storage zone in French EEZ and the pale green colour area represents the available storage volume as gas hydrates (see text). The black arrow indicates whether the buoyancy-driven migration is up or down (from a supposed depth injection of 600m).

Figure 8 .

 8 Figure 8. Post-rift sediment thickness (color lines=isopachs in meter) in the Biscay abyssal plain. The thickness values are derived from interpreted reflection seismic data in Thinon (1999) from Thinon et al. (2003). Also shown are the French EEZ borders with the black line.

Figure 9 .

 9 Figure 9. Locations of both CO 2 storage zones in the French EEZ (in green) and in the 200M zone (in orange) after applying the three criteria (see text). The black line is the shelf-break of 200-mbsl and the red lines are the isocontours of 4000-mbsl and 4500-mbsl. Also shown are some physiographic features of interest in the studied area: (1) Armoricain Seamount, (2) Trevelyan Escarpment, (3) Gascogne Knoll.

Table 1 . CO2-96, CO2-97 and CO2-100 stream compositions Concentrations in dried CO 2 (mole fraction %) CO2-96 Oxy-fuel combustion in gas-fired plants CO2-97 Pre-combustion in gas-fired plants CO2-100

 1

	Pure CO 2

b It is here supposed in the CO2-96 case that all oxygen is quickly consumed after the injection in deep-sea sediments due to an anoxic environment

Table 2 . Bottom Water Temperature, Surface Thermal Conductivity and Heat Flux in the studied zone (see locations in Figure 1)

 2

	Global Heat
	Flow Database GHFD09 http://www.hea	Leg 80 550 a	CH5	CH9	CH13	CH15	SU2 b	BD17	BD9 c BD10 c	BU5
	tflow.und.edu
	Depth (m)	4420	4732	4081	4378	4052	4485	4844	4592	4413	4710
	Bottom Water
	Temperature	2.65	2.54	2.53	2.52	2.52	2.69	-	-	-	-
	(°C)
	Surface
	Thermal Conductivity	1.48 (1.1)	1.17	1.05	1.06	1.14	0.86	0.91	0.95	0.91	1.11
	(W m -1 K -1 )
	Heat Flux (mW m -2 )	59 (60)	33	40	50	46	44.9	58.2	31.4 (33)	45.6 (50)	50.2

a Data from DSDP Leg 80 site 550 have been recalculated (values in brackets) from the original values

[START_REF] Foucher | Geothermal Measurements during Deep-Sea Drilling Project Leg-80[END_REF]

using a linearly increasing conductivity with depth b The SU2 observation acquired during the cruise on Le Suroit in 1984 is reported in

[START_REF] Louden | Variations in heat flow across the Goban Spur and Galicia Bank continental margins[END_REF]

c Data of BD9, BD10 within shallow sediments (<10 m) have been recalculated (values in brackets) from the original values

[START_REF] Bullard | The Flow of Heat through the Floor of the Atlantic Ocean[END_REF]

using only the short junction in the lower part of the tube in order to avoid the heat transport by water in the superficial part of the sedimentary cover

Table 4 . Conservative estimates of the storage capacity in the studied area and comparison with the onshore storage capacity in the Paris Basin Theoretical storage volume a (km 3 )

 4

			Storage efficiency factor b	Density c	Storage capacity
			(%)	(g CO 2 /cm 3 )	(Gt CO 2 )
	EEZ (N=100)	22 627	2	1.045	472.9
	EEZ (N=96, 97)	1602 -2400	2	0.3 -0.29	9.6 -13.9
	200M (N=96, 97)	1623 -2412	2	0.3 -0.29	9.7 -14.0
	Paris Basin
	(N=100)
	(Vangkilde-	722	2	0.48 -0.7	7.9
	Pedersen et al.,
	2009)

Acknowledgements

This work is a part of a Development Project funded by the Research Division of the Bureau de Recherches Géologiques et Minières (BRGM, i.e. the French Geological Survey).One of the authors (AB) thanks Aurélien Leynet and Delphine Allier for the assistance brought in the retrieval of the Paris Basin data. Dr. J. Levine and two anonymous reviewers are thanked for their helpful comments to the manuscript and their constructive suggestions.

Appendix A. Computing method using GMT and GASCO2 to calculate the CO 2 storage volume in the French EEZ and in 200M

In the French EEZ, the bathymetric data at a 1 km grid spacing have been extracted from the file bathy_1000m.grd (source: IFREMER, http://sextant.ifremer.fr/record/7462a470-b290-11de-b102-000086f6a603/) [START_REF] Loubrieu | Bathy-morphologie régionale du Golfe de Gascogne et de la Manche (édition 2008, résolution 1000m)[END_REF]. This file has been transformed using GMT V4.5.9. The public domain computational package GMT is a collection of publicdomain Unix tools that allows to manipulate geographic and Cartesian x,y,z data sets and produce geo-referenced illustrations [START_REF] Wessel | New, improved version of generic mapping tools released[END_REF].

The xyz2grd tool is used to convert the ascii ESRI format to the binary GMT netCDF format:

The grdproject tool is used to transform the gridded data set from a rectangular coordinate system (extended Lambert II conic projection) onto a geographical system: grdproject bathy_1000m. The grdvolume tool is used to calculate the surface (km 2 ) of the storage potential: grdvolume -Sk bathy3.grd > surface.txt a Surface is by definition the EEZ-CO2 area in the pure CO 2 case (N=100) and the subdomain where NBZ is included in GHSZ in the case with impurities (N=96, 97).

b Sediment Volume is defined as the product of the surface and the thickness. In the pure CO 2 case (N=100), the thickness is twice the depth difference between the neutral buoyancy level and the hydrate formation level. In the case with impurities (N=96, 97), the thickness is the difference between the hydrate formation level and the neutral buoyancy level.

c Theoretical Storage Volume is the porous volume that may host liquid CO 2 in the pure CO 2 case (N=100) and gas hydrates in the case with impurities (N=96, 97) (section 3.6).