Minerals Report - International Seabed Authority

Figure 8. Development of hydrate related unstable layer beneath continental slope after
lowering of sea level (modified after Dillon et al. 4 ;Mclver 65 ).
In the continental slope area, continued sedimentation may lead to
burial of hydrated sediment to such a depth, that the hydrates are no
longer stable and dissociate into liquid-gas-water mixtures. The
dissociated hydrates create a zone of weakness 2 in the sediment where
slope failure could be triggered by further gravitational loading or
earthquakes.
Similar slope failures may also occur because of changes of
pressure and temperature resulting from fluctuations in sea level (Figure
8) or bottom water temperatures, such as those that took place during the
Pleistocene epoch. These changes, though of long duration, may have
caused 5 solid sediment to become gas-cut mud and resulted in mud
diapirs, mud volcanoes, mud slides or turbidite flows, depending on
sediment composition and bottom topography.
7.3. Hazards related to drilling into gas hydrate zones
Drilling for recovery of methane from the hydrate is a challenging
task because of the characteristics of the hydrates, especially its unstable
nature with change in pressure - temperature conditions. Hydrates may
dissociate 2 during the process of drilling and initiate a process of
uncontrolled gas release and site subsidence.
INTERNATIONAL SEABED AUTHORITY 546