Gas hydrates from the Sea of Marmara : Variability in composition and origin of the hydrate-forming gases L. Ruffine1*, J. P. Donval1, J. L. Charlou1, L. Geli1, P. Henry2, G. Etiope3 1 Département Géosciences Marines, IFREMER Centre de Brest, Plouzané, Cedex 29 280, France. 2 CEREGE / Collège de France, 13545 Aix en Provence, France 3 Istituto Nazionale di Geofisica e Vulcanologia Sezione Roma 2, * Corresponding author

During the MARNAUT Scientific Cruise (May-June 2007), gas hydrates have been discovered in the Sea of Marmara for the first time on the Western High. The recovered samples were massive with a yellow tint due to petroleum stains. This is not very surprising as their location is close to the Thrace basin, one of the more important gas-producing provinces in Turkey. From both scientific and petroleum-industry viewpoints, it is important to identify the gas supplier source, the migration pathways of the gases as well as the different processes leading to their formation. This will enable us to understand the behaviour, distribution and stability field of the gas hydrates in that location. The work presented here is devoted to the geo-physicochemical analysis of the gas hydrates in order to determine the origin of the hydrate-bound gases. It is based on the analysis of their molecular and isotopic compositions. The hydrate water has also been analysed to both determine its mineral composition and identify some characteristic petroleum-derived compounds. The hydrate composition can vary widely from one sample to another, being able to reach very high propane-content (up to 36.01 % in molar composition). The high and positive value of δ13C for carbon dioxide in the hydrate was similar to this of gas bubble seepages from the Western High. This work analyses the heterogeneity in composition of the hydrates. A discussion concerning the origin of the gases is also proposed.