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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
21
The most visible gas hydrates in nature are massive mounds
of solid hydrate, often many metres in diameter, exposed on
the sea floor and frequently covered with thin drapes of sedi-
ment (Fig. 1.8, bottom row). These mounds mark locations
where active fluid vents, or seeps, supply methane directly
to the sea floor. Seeps provide the methane for gas-hydrate
mounds to form and grow, but this growth must compete
not only with temperature changes that can destabilize gas
hydrate, but with erosion from the sea water itself, which is
undersaturated in methane and will therefore dissolve ex-
posed gas hydrate (Lapham
et al.
2010; Zhang
et al.
2011).
Gas hydrate mounds have been observed to decay, with
chunks of hydrate breaking away from mounds and float-
ing away (MacDonald
et al.
, 1994), but this is not a regular
occurrence (MacDonald
et al.
, 2005). Monitoring studies of
gas hydrate mounds in the Gulf of Mexico (MacDonald
et al.
,
2005) and offshore of Vancouver Island at the Barkley Can-
yon site (Lapham
et al.
2010) demonstrate that gas hydrate
mounds can persist for several years at least, in spite of being
continually dissolved by seawater and exposed to short-term
increases in bottom-water temperature.
The vast majority of gas hydrates, however, lay buried in
sediment. The sediment itself is 30 – 70 per cent pore space
(Santamarina
et al.
2001), and as shown in Figs. 1.8-1.10,
the manner in which gas hydrates fill or alter that space can
be quite different depending on the abundance of available
methane and whether the sediment is sandy or more fine-
grained (Fig. 1.9).
Hydrate in sands
The relatively high permeability of sands facilitates the flow
of water and methane needed for hydrate formation, and gas-
hydrates have been found filling more than 60 per cent of the
available pore space with saturations as high as 90 per cent
in some Arctic sands (Collett
et al.
2009) (Fig. 1.10, class F),
as high as 80 – 90 per cent in Gulf of Mexico sand bodies
(Boswell
et al.
2012) (Fig. 1.10 class C) and as high as 70 per
cent in the sandy sections of interbedded sands and muds
off Japan’s southeastern coast, on the margin of the Nankai
Trough (Tsuji
et al.
2004, 2009) (Fig. 1.10 class C). Though
only approximately 10 per cent of the world’s gas hydrates
likely occur in sands (Collett
et al.
2009), the high gas hy-
drate concentrations that can be found in sands have made
them research and development targets for potential gas hy-
drate exploration (see Volume 2).
Hydrate in fine-grained sediment
Marine drilling conducted initially on the Blake Ridge (off-
shore eastern United States) in 1995 (Paull
et al.
1998)
found gas hydrates occurring as microscopic pore-filling
grains in fine-grained sediments (clays and muds) (Fig.
1.10 Class E). These accumulations can cover large areas
and extend through thick vertical sequences. It is generally
believed the majority of Earth’s gas hydrates exist in this
dispersed form (Boswell 2009), even though the concentra-
tions are typically low, ranging from 1 or 2 per cent to as
high as 12 per cent of the pore volume. These low satura-
tions are probably due to the very small pore size and low
permeability of clay-rich sediments, which greatly hinder
the mobility of both gas and water. Gas hydrates likely form
more readily in zones within these fine-grained environ-
ments where porous microfossils or slightly coarser grains
provide a small increase in both porosity and permeability
(Kraemer
et al.
2000; Bahk
et al.
2011).
In areas where methane flux is particularly strong, it is pos-
sible for gas hydrates to accumulate to greater concentrations
within clay-rich sediments. In 2006, drilling off the coast of
eastern India revealed an approximately 150-metre-thick sec-
tion of fractured clay sediments with gas hydrate saturations
of 20 to 30 per cent or more (Collett
et al.
2008). An expedi-
1.4
WHAT FORMS DO
GAS HYDRATES TAKE INNATURE?