Free gas and gas hydrates from the Sea of Marmara, Turkey. Chemical and structural characterization

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 C₂, C₃, and i-C₄ are also present in relatively high concentration. The methane contained within gas hydrate is clearly thermogenic as evidenced by a low C₁/C₂ + C₃ ratio of 3.3, and carbon and hydrogen isotopic data (δ¹³C_CH4 of - 44.1‰ PDB and δD_CH4 of - 219‰ SMOW). A similar signature is found for the associated gas bubbles (C₁/C₂ + C₃ ratio of 24.4, δ¹³C_CH4 of - 44.4‰ PDB) which have the same composition as natural gas from K. Marmara-af field. Gas bubbles from Central High show also a thermogenic origin as evidenced by a C₁/C₂ + C₃ ratio of 137, and carbon and hydrogen isotopic data (δ¹³C_CH4 of - 44.4‰ PDB and δD_CH4 of - 210‰ SMOW), whereas those from the Çinarcik Basin have a primarily microbial origin (C₁/C₂ + C₃ ratio of 16,600, δ¹³C_CH4 of - 64.1‰ PDB). UV-Raman spectroscopy reveals structure II for gas hydrates, with CH₄ trapped in the small (5¹²) and large (5¹²6⁴) cages, and with C₂H₆, C₃H₈ and i-C₄H₁₀ 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.