Gas hydrates and gas bubbles were collected during the MARNAUT cruise (May-June 2007) in the Sea of Marmara along the North Anatolian Fault system, Turkey. Gas hydrates were sampled in the western part of the Sea of Marmara (on the Western High), and three gas-bubble samples were recovered on the Western High, the Central High (center part of the Sea of Marmara) and in the Çinarcik Basin (eastern part of the Sea of Marmara). Methane is the major component of hydrates (66.1%), but heavier gases such as C 2 , C 3 , and i-C 4 are also present in relatively high concentration. The methane contained within gas hydrate is clearly thermogenic as evidenced by a low C 1 /C 2 + C 3 ratio of 3.3, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.1‰ PDB and δD CH4 of − 219‰ SMOW). A similar signature is found for the associated gas bubbles (C 1 /C 2 + C 3 ratio of 24.4, δ 13 C CH4 of − 44.4‰ PDB) which have the same composition as natural gas fromK. Marmara-af field. Gas bubbles from Central High show also a thermogenic origin as evidenced by a C 1 /C 2 + C 3 ratio of 137, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.4‰ PDB and δD CH4 of − 210‰ SMOW), whereas those from the Çinarcik Basin have a primarily microbial origin (C 1 /C 2 + C 3 ratio of 16,600, δ 13 C CH4 of − 64.1‰ PDB). UV-Raman spectroscopy reveals structure II for gas hydrates, with CH 4 trapped in the small (5 12) and large (5 12 6 4) cages, and with C 2 H 6 , C 3 H 8 and i-C 4 H 10 trapped in the large cages. Hydrate composition is in good agreement with equilibrium calculations, which confirm the genetic link between the gas hydrate and gas bubbles at Western High and the K.Marmara-af offshore gas field located north of the Western High. We calculate the characteristics of the hydrate stability zone at Western High and in the Çinarcik Basin using the CSM-GEM computer program. The base of the structure II hydrate stability field is at about 100 m depth below the seafloor at the Western High site, whereas in the Çinarcik Basin, P-T conditions at the seafloor correspond to the uppermost range for structure I hydrate formation from microbial gas.

Intact natural gas hydrates recovered on the West African margin in the South Atlantic Ocean (ZaiAngo and Neris II projects) and from the Norwegian Sea (Hakon Mosby Mud Volcano) are investigated by micro-Raman spectroscopy at ambient pressure and low temperature. The gas hydrates collected at different geological sites contain a high methane concentration relative to other minor components that are slightly dispersed in the samples. They crystallize in a type I cubic lattice structure as also confirmed by our preliminary synchrotron diffraction results obtained on the ZaiAngo specimen. However, detailed analysis of selected microscopic areas reveals a variation in the gas distribution among the different specimens. Trace amounts of CO 2 and H 2 S can be identified by their characteristic vibrational signatures in the 1000-3800 cm − 1 spectral range. They are found to be coclathrated with methane. Their presence produces a compositional effect on the relative cage occupancy of CH 4 , as determined from the integrated band intensity ratio corresponding to the molecular stretching modes of methane in the hydrate. The comparative Raman analysis of synthetic hydrates of H 2 S, CH 4 and CH 4-deuterohydrates allows the unambiguous assignment of weak band overtones of trapped methane and co-clathrated H 2 S molecular vibrations.

[1] Natural gas hydrates recovered from the Congo-Angola basin and Nigerian margins are analyzed by synchrotron X-ray powder diffraction. Biogenic methane is the most abundant gas trapped in the samples and others minor components (CO 2 , H 2 S) are co-clathrated in a type I cubic lattice structure. The refinement for the type I structure gives lattice parameters of a = 11.8646 (39) Å and a = 11.8619 (23) Å for specimens from Congo-Angola and Nigerian margins respectively at 90 K. These values, intermediate between the lattice constant of less pure methane specimens and pure artificial methane hydrates, indicate that lattice constants can be affected by the presence of encaged CO 2 , H 2 S and other gas molecules, even in small amounts. Thermal expansion is also presented for Congo-Angola hydrate in the temperature range 90-200 K. The coefficients are comparable with values reported for synthetic hydrates at low temperature and tend to approach thermal expansion of ice at higher temperature. Citation: Bourry, C., J.-L. Charlou, J.-P. Donval, M. Brunelli, C. Focsa, and B. Chazallon (2007), X-ray synchrotron diffraction study of natural gas hydrates from African margin,