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

Estimating the gas hydrate recovery prospects in the western Black Sea basin based on the 3D multiphase flow of fluid and gas components within highly permeable paleo-channel-levee systems

Gas hydrate deposits are abundant in the Black Sea region and confirmed by direct observations as well as geophysical evidence, such as continuous bottom simulating reflectors (BSRs). Although those gas hydrate accumulations have been well-studied for almost two decades, the migration pathways of methane that charge the gas hydrate stability zone (GHSZ) in the region are unknown. The aim of this study is to explore the most probable gas migration scenarios within a three-dimensional finite element grid based on seismic surveys and available basin cross-sections. We have used the commercial software PetroMod TM(Schlumberger) to perform a set of sensitivity studies that narrow the gap between the wide range of sediment properties affecting the multi-phase flow in porous media. The high-resolution model domain focuses on the Danube deep-sea fan and associated buried sandy channel-levee systems whereas the total extension of the model domain covers a larger area of the western Black Sea basin. Such a large model domain allows for investigating biogenic as well as thermogenic methane generation and a permeability driven migration of the free phase of methane on a basin scale to confirm the hypothesis of efficient methane migration into the gas hydrate reservoir layers by horizontal flow along the carrier beds

Potential impacts of gas hydrate exploitation on slope stability in the Danube deep-sea fan, Black Sea

Highlights • The Danube deep-sea fan offers best conditions for hydrate production. • Gas production out of a hypothetical methane hydrate reservoir was simulated. • Hazard assessment to investigate the hazard of production-induced slope failures. • Factor of Safety against slope failure is not affected by the production process. • Mobilized mass could hit the production site if landslide were to happen. Methane production from gas hydrate reservoirs is only economically viable for hydrate reservoirs in permeable sediments. The most suitable known prospect in European waters is the paleo Danube deep-sea fan in the Bulgarian exclusive economic zone in the Black Sea where a gas hydrate reservoir is found 60 m below the seafloor in water depths of about 1500 m. To investigate the hazards associated with gas production-induced slope failures we carried out a slope stability analysis for this area. Screening of the area based on multibeam bathymetry data shows that the area is overall stable with some critical slopes at the inner levees of the paleo channels. Hydrate production using the depressurization method will increase the effective stresses in the reservoir beyond pre-consolidation stress, which results in sediment compaction and seafloor subsidence. The modeling results show that subsidence would locally be in the order of up to 0.4 m, but it remains confined to the immediate vicinity above the production site. Our simulations show that the Factor of Safety against slope failure (1.27) is not affected by the production process, and it is more likely that a landslide is triggered by an earthquake than by production itself. If a landslide were to happen, the mobilized sediments on the most likely failure plane could generate a landslide that would hit the production site with velocities of up to 10 m s-1. This case study shows that even in the case of production from very shallow gas hydrate reservoirs the threat of naturally occurring slope failures may be greater than that of hydrate production itself and has to be considered carefully in hazard assessments

ADRIA LITHOSPHERE INVESTIGATION ALPHA - Cruise No. M86/3, January 20 - February 04, 2012, Brindisi (Italy) - Dubrovnik (Croatia)

The Adriatic Sea and underlying lithosphere remains the least investigated part of the Mediterranean Sea. To shed light on the plate tectonic setting in this central part of southern Europe, R/V METEOR cruise M86/3 set out to acquire deep penetrating seismic data in the Adriatic Sea. M86/3 formed the core of an amphibious investigation crossing Adria from the Italian Peninsula into Montenegro/Albania. A total of 111 OBS/OBH deployments were successfully carried out, in addition to 47 landstations both in Italy and Montenegro/Albania, which recorded the offshore airgun shots. In the scope of this shoreline-crossing study, the aim is to quantify the shallow geometry, deep boundaries and the architecture of the southern Adriatic crust and lithosphere and to provide insights on a possible decoupling zone between the northern and southern Adriatic domains. Investigating the structure of the Adriatic crust and lithospheric mantle and analyzing the tectonic activity are essential for understanding the mountain-building processes that underlie the neotectonics and earthquake hazard of the Periadriatic region, especially in the vicinity of local decoupling zones