Minerals Report - International Seabed Authority

freezing and cement loose sediments into a hard layer several hundred metres
thick before natural processes provide that rigidity.
Gas hydrate is a crystalline compound composed of gas molecules,
normally methane (Kvenvolden, 1993), en-caged within water molecules to
form a solid similar to ice. It forms within the sediment spaces cementing the
grains together. This dramatically changes the physical properties of the
sediment. One volume of hydrate also stores 164 volumes of un-pressurised
methane – a measure of the value of the resource.
The methane in gas hydrate forms naturally by organic decay in the
thick sediments normally found in the deep water adjacent to continental
margins. The deep-sea pressures (> 500 m depth) and intra sediment
temperatures (increasing with depth) determine the gas hydrate stability. At
depth in the sediment the temperature becomes too high for hydrate to be
stable and the abrupt change of physical properties inherent with free gas in
the sediment pore spaces generates a seismic reflect – the Bottom Simulating
Reflector (BSR).
BSRs have been used to identify gas hydrate on seismic reflection
profiles for some time. They are a useful way to identify the presence of
hydrate, although other seismic characteristics also occur. The BSR at the base
of the hydrate stability zone is to date is poorly mapped world-wide (Max,
1990; Lee et al, 1994; Miles, 1995).
Gas hydrates are currently considered as hydrocarbon resources. They
are also being exploited as methane reserves by Japan. Whether the methane
in the hydrate or the free gas beneath the hydrate is targeted, they represent
up to 10 times the fuel value of current conventional hydrocarbon reserves.
(Gornirz & Fung, 1994). The distribution of potential gas hydrate resources
continues to drive national research strategies as evidenced by the new
US$27M USA initiative, and current research programmes in India and
Japan.
IINTERNATIONAL SEABED AUTHORITY 727