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FROZEN HEAT
18
10 000
1 000
100
10
Approximate depth,
metres
Increasing
temperature
and pressure
Methane depleted zone.
Hydrate dissolves without active replenishment
Organic material
buried
by sedimentation
Geologic processes
Biologic processes
Burial
Thermogenic break-down
of organic matter
Excess methane forms:
Gas bubbles
Excess methane forms:
Gas hydrate
Oil and other
hydrocarbons
Organic material
settling to the sea oor
Sea oor hydrate
outcrop above active
methane seep
BGHS
Fate of buried organic matter
along faults or other
permeable paths
Biogenic methane
generated by microbes
from organic matter
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
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).