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 western Black Sea margin, offshore Sakarya River, to investigate the bottom-simulating reflections (BSRs), free gas accumulations, and mud volcanoes. 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 of the gas hydrates. According to the results from such analyses, we conclude that there are acoustically transparent zones beneath most of the BSRs in the area, which are interpreted as free gas accumulations, and the gas hydrate-bearing sediments are acting as seals for the free gas in the underlying sediments. Stability analysis of the gas hydrates from different BSR zones in the area suggests that the gas composition in the gas hydrates may change locally. As we do not have ground truth data from BSR zones, the exact composition of the gas forming the gas hydrates is unknown. However, hydrocarbon productivity of the area, chromatography results of the shallow sediment samples nearby, and stability analysis of the gas hydrates indicate the possible existence of a thermogenic gas component in the gas hydrate composition, resulting in a mixture of gas hydrates.

Seismic Characterisation of Multiple Bsrs in the Eastern Black Sea Basin

“…BSR-like reflections associated with the top of the GHOZ are rarely observed (e.g., South China Sea; Wang et al, 2018). This absence of reflection has been explained in different ways: (i) the gas hydrate concentrations decrease gradually towards the seafloor (Andreassen et al, 1997), (ii) the limited resolution of seismic data preventing proper imaging (Hyndman and Spence, 1992) or (iii) the presence of disseminated gas hydrate present as small-scale nodules that has limited impact on seismic velocity (Nasif et al, 2020). Point (iii) has been used to explain the absence of a visible top of the GHOZ in the south-western Black Sea, based on evidence from sampled gas hydrates offshore Crimea, Georgia and Sorokin Trough which showed the presence of small gas hydrate crystals or finely disseminated gas hydrates dispersed in the mud within the uppermost sediments (Limonov et al, 1997;Woodside et al, 2003;Bohrmann et al, 2003;Klaucke et al, 2006;Sahling et al, 2009).…”

“…Numbered black circles show the DSDP Holes 379, 380 and 381 locations (Ross, 1978). Boxes overlaid on map represent the locations of studies previously identifying single BSRs (blue boxes) and multiple BSRs (orange boxes) across the whole Black Sea region ( 1 Zander et al, 2017, Popescu et al, 2006, Gassner et al, 20192 Riedel et al, 2020, Ker et al, 2019, Hillman et al, 20183 Zillmer et al, 2005, Lüdmann et al, 20044 Xing and Spiess, 2015; 5 Merey and Sinayuc, 2016;6 Dondurur et al, 2013, Menlikli et al, 20097 Vassilev and Dimitrov, 2002;8 Minshull and Keddie, 2010; 9 Cifci et al, 2012; 10 Dondurur and Çifçi, 2009; 11 Nasif et al, 2020).…”

Late Pleistocene-Holocene characteristics of the North Anatolian Fault at Adapazarı Basin: evidence from the age and geometry of the fluvial terrace staircases

“…Passing through the Mağara Gorge for some 50 km, the river meets the Black Sea, forming the Karasu Delta (Görür et al, 2001;Figure 1B). Here, the continental shelf is very narrow and the effect of the sea level changes are reflected in the high relief of the Sakarya Canyon (Algan et al, 2002;Nasif et al, 2020). The age of the establishment of the river has been estimated as Early-Middle Quaternary (Emre et al, 1998) based on fluvial facies within the Karapürçek Formation.…”

Research into Dissociation Zones of Gas Hydrate Deposits with a Heterogeneous Structure in the Black Sea

“…Despite the significant potential of the main types of renewable energy sources, their practical use today contributes only a small share in the fuel and energy balance of Ukraine [38]. However, interest in the development of technologies for producing new types of energy raw materials is significantly increasing, the most promising of which, according to the authors, are gas hydrates of the Black Sea sedimentary stratum [39][40][41].…”