Quantifying in-situ gas hydrates at active seep sites in the eastern Black Sea using pressure coring technique

Abstract

In the eastern Black Sea, we determined methane (CH₄) concentrations, gas hydrate volumes, and their vertical distribution from combined gas and chloride (Cl^−) measurements within pressurized sediment cores. The total gas volume collected from the cores corresponded to concentrations of 1.2–1.4 mol CH₄ kg⁻¹ porewater at in-situ pressure, which is equivalent to a gas hydrate saturation of 15–18% of pore volume and amongst the highest values detected in shallow seep sediments. At the central seep site, a high-resolution Cl^− profile resolved the upper boundary of gas hydrate occurrence and a continuous layer of hydrates in a sediment column of 120 cm thickness. Including this information, a more precise gas hydrate saturation of 22–24% pore volume could be calculated. This volume was higher in comparison to a saturation calculated from the Cl^− profile alone, resulting in only 14.4%. The likely explanation is an active gas hydrate formation from CH₄ gas ebullition. The hydrocarbons at Batumi Seep are of shallow biogenic origin (CH₄ > 99.6%), at Pechori Mound they originate from deeper thermocatalytic processes as indicated by the lower ratios of C₁ to C₂–C₃ and the presence of C₅.