Seismic identification of gas hydrates: a case study from Sakarya Canyon, western Black Sea

Abstract

Multichannel seismic, 3.5-kHz Chirp subbottom profiler and multibeam bathymetric data were collected along the westernBlack Sea margin, offshore Sakarya River, to investigate the bottom-simulating reflections (BSRs), free gas accumulations, and mudvolcanoes. Geometries from the seismic data indicate widespread BSRs along the continental rise between 750 and 1950 m water depths,70 to 350 ms below the seafloor. Seismic attribute analyses have been applied to the seismic data to reveal the acoustic properties ofthe gas hydrates. According to the results from such analyses, we conclude that there are acoustically transparent zones beneath mostof the BSRs in the area, which are interpreted as free gas accumulations, and the gas hydrate-bearing sediments are acting as seals forthe free gas in the underlying sediments. Stability analysis of the gas hydrates from different BSR zones in the area suggests that the gascomposition in the gas hydrates may change locally. As we do not have ground truth data from BSR zones, the exact composition ofthe gas forming the gas hydrates is unknown. However, hydrocarbon productivity of the area, chromatography results of the shallowsediment samples nearby, and stability analysis of the gas hydrates indicate the possible existence of a thermogenic gas component inthe gas hydrate composition, resulting in a mixture of gas hydrates.