Seismoacoustic Study of the Shallow Gas Transport and ... - E-LIB

Chapter 4
The narrow-beam Parasound data (4° opening angle) (Fig.s 3 and 4) show numerous
gas seeps located exactly on the seismic lines in less than 700 m water depth, which
complement the overall ~600 gas seeps reported by Römer et al. (2012) from water
column imaging with a multibeam system. Except for the Kerch seep, however, no other
gas emissions had been detected deeper than 700 m water depth in the area, which is the
presumed boundary of the GHSZ. This observation confirms that the distribution of gas
seeps is primarily controlled by the presence or absence of gas hydrates, which act as an
effective seal. A similar gas seep distribution was reported from the continental margin of
the northwestern Black Sea, west of the Crimea Peninsula (Naudts et al., 2006).
In general, on the Kerch margin gas/fluids are likely to migrate upwards along fault
planes, fractures as well as more permeable horizons in upslope direction, as e.g. towards
the top of anticline structures where in turn gas accumulations can form in reservoirs.
Faults must have developed in Pliocene and Quaternary times at the top and the flanks of
the anticlines due to the intense tectonic movements in conjunction with the Crimean
Mountain uplift and Black Sea basin subsidence (Chapter 3), and further fracturing may
have been supported through earthquake-triggered slope destabilization.
For the second scenario, the seepage on the upper slope, a geological model of the
upper slope area of the Ridge R1 is presented in Figure 15. In this geological model, two
potential major pathways for gas/fluid upward migration to the upper slope are shown
(Fig. 15). The first migration path follows the deep faults from Anticline b across Unit 4,
reaching the sediments of Units 1 to 3. Here, gas hydrates may serve as an effective seal
to prevent further upward migration. Units 1-3 (Chapter 3.5.2.3) are characterized by
hemipelagic fine-grained sediments interbedded with coarse grained sediments. The high
permeability in coarser layers may support gas migration which diverts the gas flux in
upslope direction, while interbedded finer layers inhibit further vertical migration. The
second path is guided by fine-grained distal Paleo-Don River fan deposits of Unit 4,
diverting gas/fluid fluxes layer-parallel within Unit 5 from Anticline b laterally towards
Anticline a, complementing the gas accumulations from deeper sources at Anticline a.
Subsequently, the gas/fluid can migrate vertically along faults or fractured and/or coarser
sediment, which are the mass transport deposits and Paleo-Don River fan deposits (Units
4 and 3, Chapter 3.5.2.3). Also there the gas is likely trapped by the hemipelagic
fine-grained sediment layers on the top of the Paleo-Don River fan deposits. This way
shallow gas reservoirs can form on the upper slope. Shallow faults are observed above
these gas reservoirs, which have formed within the uppermost units, probably due to
slope instability and/or earthquakes, finally serving as vertical migration pathways to the
seafloor and through the observed gas flares into the water column.
4.6. Conclusions
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