Minerals Report - International Seabed Authority
Minerals Report - International Seabed Authority Minerals Report - International Seabed Authority
It has been assumed that the structure of the gas hydrate layer is simple - that its thickness should gradually increase on moving to deeper water because gas hydrate becomes stable at higher pressure-temperature conditions. This assumes that (i) pressure is simply a function of total depth from the sea surface to a position in the sediments, and (ii) the chemistry of the pore waters and the thermal gradient are fairly uniform 29 . 3.3. Formation of Methane Hydrates After generation of methane, its transportation in sediment can be through various means such as movement of pore-water containing dissolved gas, free gas flow, and molecular diffusion. When the ascending methane molecules reach favourable subsurface-thermobaric conditions (i.e. hydrate stability zone) then, formation of hydrate takes place within the pore spaces of the sediments in the presence of water molecules. It can thus be seen that biogenic methane formation may take place both in situ within the hydrate stability zone (HSZ) and beneath it. Thermogenic methane on the other hand has to move upwards from depth into the HSZ. Scientists use various geochemical and isotopic techniques to identify the origin of methane in hydrate samples. After precipitation hydrate progressively fills the sediment pore-spaces and fractures, and eventually cements them to give rise to massive and vein type hydrate deposits 30 . It may be noted that the temperature and pressure conditions for hydrate stability depend on the composition of the gas and on the presence of salts and other components in seawater. It is generally believed that pore water has to be fully saturated with methane before natural hydrate can form. The condition of sufficiently high pore water methane concentration can 31 be met by i) supply of sufficiently large amounts of organic matter in the sediments to generate enhanced methanogenic decomposition; ii) large upward methane fluxes mostly related to fault zones, or other conduits such as diapirs, mud volcanism etc. 3.4. Free gas below hydrates In the context of methane hydrate deposits, often free gas is mentioned. This free gas refers to methane molecules that exist as gas, which are neither bound to other molecules (for instance, to form a complex hydrocarbon) nor trapped within a hydrate. More commonly, INTERNATIONAL SEABED AUTHORITY 526
free methane in a geologic formation exists within the pores of low-density rocks. Any hydrate layer may trap free methane as long as the layer forms a seal through which gas cannot migrate. The free methane may be thermogenic gas that has migrated upwards from the earth's crust, or it may be biogenic gas that was previously a hydrate layer but has now melted. Dillon et al. 4 suggested several different types of formations of gas hydrates that can trap free methane (Figure 5). GHL: Gas hydrate cemented layer; PGT: Possible Gas trap Figure 5: Schematic diagram of geological situations in which a gas hydrate cemented layer can act as a seal trapping free gas (adapted after Dillon et al. 4 ; Kvenvolden 2 ). 4. Indicators of Gas Hydrates in Sediments The presence of gas hydrates within sediments is manifested in various ways. The most common is through acoustically derived geophysical data. This remotely sensed data needs to be verified by ground-truth validation of actual occurrence. Similarly, non-geophysical proxies can also provide some indication of the occurrences of gas hydrate. 4.1. Geophysical indicators Although gas hydrates have been recognized in drilled cores, their presence over large areas can be detected much more efficiently by acoustical methods, using seismic reflection profiles (Dillon et al., 4 and references therein). The most commonly used acoustic signature for INTERNATIONAL SEABED AUTHORITY 527
- Page 484 and 485: dimensional seafloor massive sulphi
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- Page 500 and 501: Figure 4: Volume density of initial
- Page 502 and 503: ITIPRHC = Vnr φHC γ 103 (Mmtoe),
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- Page 516 and 517: REFERENCES 1. L.G. Weeks (1971), Ma
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- Page 522 and 523: In Southeast Asia, Dr. Vysotsky spo
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- Page 528 and 529: In the 1960's scientists discovered
- Page 530 and 531: Figure 2. Worldwide locations of kn
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- Page 552 and 553: at a catastrophic scale. Some of th
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- Page 560 and 561: NOTES AND REFERENCES 1. E.D. Sloan
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- Page 568 and 569: is that their exploitation might re
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free methane in a geologic formation exists within the pores of low-density<br />
rocks. Any hydrate layer may trap free methane as long as the layer forms<br />
a seal through which gas cannot migrate. The free methane may be<br />
thermogenic gas that has migrated upwards from the earth's crust, or it<br />
may be biogenic gas that was previously a hydrate layer but has now<br />
melted. Dillon et al. 4 suggested several different types of formations of gas<br />
hydrates that can trap free methane (Figure 5).<br />
GHL: Gas hydrate cemented layer; PGT: Possible Gas trap<br />
Figure 5: Schematic diagram of geological situations in which a gas hydrate cemented<br />
layer can act as a seal trapping free gas (adapted after Dillon et al. 4 ; Kvenvolden 2 ).<br />
4. Indicators of Gas Hydrates in Sediments<br />
The presence of gas hydrates within sediments is manifested in<br />
various ways. The most common is through acoustically derived<br />
geophysical data. This remotely sensed data needs to be verified by<br />
ground-truth validation of actual occurrence. Similarly, non-geophysical<br />
proxies can also provide some indication of the occurrences of gas hydrate.<br />
4.1. Geophysical indicators<br />
Although gas hydrates have been recognized in drilled cores, their<br />
presence over large areas can be detected much more efficiently by<br />
acoustical methods, using seismic reflection profiles (Dillon et al., 4 and<br />
references therein). The most commonly used acoustic signature for<br />
INTERNATIONAL SEABED AUTHORITY 527
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