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Minerals Report - International Seabed Authority

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methane by bacteria in an anoxic environment. This decomposition is<br />

considered 21 to take place either by acetate fermentation or by carbon<br />

dioxide reduction.<br />

i) Acetate fermentation:<br />

CH3COOH CH4 + CO2<br />

ii) Carbon dioxide reduction:<br />

CO2 + 4H2 CH4 + 2H2O<br />

Organic matter is composed of carbon, hydrogen and phosphorus<br />

in the ratio of 106:16:1. The summarized' decomposition of organic matter<br />

which results in production of methane is also given below:<br />

(CH2O)106 (NH3)16(H3PO4)1 53CO2 + 53CH4 + 16NH3 + H3PO4<br />

The resulting carbon dioxide may further be reduced 22 if there is<br />

sufficient supply of hydrogen:<br />

CO2 + 4H2 7 CH4 + 2H2O<br />

In the thermogenic process of methane generation, thermal<br />

cracking of organically derived materials takes place to form petroleum<br />

hydrocarbons (including methane). This generally occurs at considerable<br />

depth (>2km) in sedimentary basins where temperatures exceed 100°C.<br />

Thermogenic methane may also be derived by thermal degradation of oil<br />

at even greater depths, and by the maturation of coal 1, 22-25 .<br />

3.2. Gas Hydrate stability zone<br />

The occurrence of gas hydrates in nature is controlled by an<br />

interrelation among the factors of temperature, pressure and<br />

composition. The phase diagram (Figure 4) showing the boundary<br />

between free methane gas and methane hydrate for a pure water and<br />

pure methane system, provides a reasonable estimate of pressuretemperature<br />

conditions under which natural gas hydrates, composed<br />

mainly of methane, will be stable on the continental margins 5 . The<br />

INTERNATIONAL SEABED AUTHORITY 524

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