FROZEN HEAT | Volume 1
dition to appropriate pressure and temperature conditions, gas hydrate formation requires adequate supplies of water and hydrate-forming guest molecules (Fig. 1.4). The inter- val in which gas hydrates actually occur within the GHSZ is designated as the gas hydrate occurrence zone or GHOZ. As discussed in Volume 1 Chapter 2, the methane incorporated into gas hydrates comes from organic carbon. In shallow
sediments, the organic carbon is broken down by microbes, with methane being one of the by-products. At significant depths, it is broken down by thermal processes in which heat cracks the organic matter into smaller molecules, such as methane (Fig. 1.5). Organic carbon itself is not uniformly distributed, nor has it always been distributed in the same locations. In modern times, for example, approximately 90
Fate of buried organic matter
Organic material settling to the sea oor
Sea oor hydrate outcrop above active methane seep
Approximate depth, metres
10
Methane depleted zone. Hydrate dissolves without active replenishment
Biogenic methane generated by microbes from organic matter
Excess methane forms: Gas bubbles Excess methane forms: Gas hydrate
100
Organic material buried by sedimentation
BGHS
Biologic processes
along faults or other permeable paths
1 000
Geologic processes
10 000
Oil and other hydrocarbons
Increasing temperature and pressure
Burial
Thermogenic break-down of organic matter
Figure 1.5: Fate of buried organicmatter. Buried organicmaterial is degraded bymicrobes, thermogenically altered by heat and pressure, or buried more deeply and lost to the surface carbon cycle. Methane produced during microbial (also called “biogenic”) and thermogenic decomposition can slowly migrate through overlying sediment with fluids or rise rapidly along faults or other permeable paths. As methane-saturated fluids rise and cool, excess methane forms gas bubbles below the base of gas hydrate stability, BGHS. Above the BGHS, excess methane generally forms methane hydrate, but can also form bubbles (Suess et al. , 1999, Liu and Flemings 2006) (Figure modified from Pohlman et al. 2009).
FROZEN HEAT 18
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