Methane hydrates in Black Sea deep-sea fans: Characteristics, implications, and related geohazards

This thesis focusses on two gas-hydrate-bearing study sites in the Black Sea. These sites are located in the vicinity of the Danube and Don-Kuban deep-sea fan complexes. By using a variety of geophysical methods, including 2D and 3D seismics, sidescan sonar, and sediment echosounder methods, as well as geochemical analysis and geotechnical modeling, three case studies were investigated. In the first case study, anomalous multiple bottom simulating reflectors (BSRs) were investigated, which occur in the levees of a buried channel-levee system of the Danube deep-sea fan. The BSRs are likely paleo-BSRs caused by a change in pressure and temperature conditions during different limnic phases of the Black Sea. The BSRs remain visible in seismic data because free gas is likely still present beneath them. As the free gas can only be transported by diffusion, it largely remains trapped within the fine-grained levee deposits, and therefore only small amounts of gas are released after gas hydrate dissociation. The second case study focusses on a potential shallow gas hydrate reservoir that is of interest in terms of exploitation of gas hydrates as an energy resource. To determine whether a hypothetical gas production out of the hydrate reservoir induces slope failures along the seabed slopes near the production area, a slope stability analysis was carried out. Seafloor subsidence due to reservoir compaction likely does not reduce the stability of the nearby slope. Consequently, naturally occurring slope failures are more likely to occur than landslides triggered by gas hydrate production. The third case study focuses on the Kerch seep site located in the Don Kuban deep-sea fan in the northeastern Black Sea. The seep site is located within the GHSZ in about 900 m water depth. Each of the three seeps hosts its own gas pocket underneath the seep domes. The transport of biogenic methane predominantly occurs in the form of gas bubbles along narrow pipes through the GHSZ