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

Chapter 5
accumulations. An acoustic wipe-out zone and an acoustically transparent zone in
conjunction with many faults above Anticline b indicate the possible migration zone for
free gas and fluids towards the Kerch seep. A local heat flow anomaly was also found in
the Kerch seep area. Therefore, the BGHSZ may be further elevated, and free gas can
migrate much further up along faults in the sediment column. Very shallow gas reservoirs
were observed beneath the Kerch seep area, which could provide the continuous gas
release found at the Kerch seep. The mechanism of gas migration for the area further
upslope and at Ridges R1 and R2 was studied. Two possible migration pathways for
gas/fluid are identified. The first migration path follows the deep faults beneath Anticline
b and reaches shallower hemipelagic fine-grained sediments interbedded with coarse
grained material. Here, gas hydrates may serve as an effective seal to prevent further
upward migration. The high permeability in coarser layers may in turn divert gas
migration upslope, while interbedded finer layers inhibit any further vertical migration.
The second path basically follows the coarser grained Unit 5, guided by fine-grained
distal Paleo-Don River fan deposits, diverting gas/fluid fluxes layer-parallel from
Anticline b laterally towards Anticline a. There, it connects to gas accumulations from
deeper sources at Anticline a. Subsequently, the gas/fluid can migrate vertically along
faults or fractures and/or coarser sediment, which are the mass transport deposits and
more proximal Paleo-Don River fan deposits. Being trapped by the hemipelagic finegrained
sediment layers on the top of the Paleo-Don River fan deposits, numerous
shallow gas reservoirs have formed. Finally, shallow faults, probably formed within the
uppermost units due to slope instability and/or earthquakes, can serve as vertical
migration pathways above these gas reservoirs for free gas to the seafloor and through the
observed gas flares into the water column.
In all, this study shows that fluid seepage within the GHSZ, e.g. mud volcanoes in the
central Black Sea and Kerch seep area in the Kerch Peninsula margin, are all probably
related to the deeper sources and long migration pathways, such as feeder channels and
steep upthrust faults. Free gas reservoirs could be observed beneath the fluid seepage in
the near-surface sediments. The free gas is trapped by the fine-grained sediments or gas
hydrate layers, and high overpressure is generated. Available pathways (feeder channels,
column zones, shallow faults or coarse grained sediments) along with the heat flow
anomaly would support the leakage of the free gas into the water column. For the gas
seeps above the GHSZ, shallow faults were observed between the shallow gas reservoirs
and the gas seeps. The shallow faults probably formed due to slope instability and/or
earthquakes. The sedimentary processes and tectonic movements, and the presence of gas
hydrates might play an important role in the formation of seabed fluid seepage.
5.2 Outlook
This thesis provides new insights into the controls of seabed fluid seepage, distribution
and types of related gas reservoirs, and migration pathways for gas and fluid by
sedimentary processes, structural characteristics and tectonic processes for two different
vent areas in the Black Sea. The study revealed that both mud volcanoes and gas seeps
must be fed from deep sources. However, 2D high resolution seismic data used here
could only image the shallow subsurface structures. For a full understanding of such
seabed fluid seepage systems, it would be desirable to incorporate deeper imaging of the
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