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OCR for page 3420
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3420-3426, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Potential effects of gas hydrate on human welfare
KEITH A. KVENVOLDEN*
U.S. Geological Survey, 345 Middlefield Road, MS999, Menlo Park, CA 94025
ABSTRACT For almost 30 years. serious interest has
been directed toward natural gas hydrate, a crystalline solid
composed of water and methane, as a potential (i) energy
resource, (ii) factor in global climate change, and (iii) sub-
marine geohazard. Although each of these issues can affect
human welfare, only (iii) is considered to be of immediate
importance. Assessments of gas hydrate as an energy resource
have often been overly optimistic, based in part on its very
high methane content and on its worldwide occurrence in
continental margins. Although these attributes are attractive,
geologic settings, reservoir properties, and phase-equilibria
considerations diminish the energy resource potential of
natural gas hydrate. The possible role of gas hydrate in global
climate change has been often overstated. Although methane
is a "greenhouse" gas in the atmosphere, much methane from
dissociated gas hydrate may never reach the atmosphere, but
rather may be converted to carbon dioxide and sequestered by
the hydrosphere/biosphere before reaching the atmosphere.
Thus, methane from gas hydrate may have little opportunity
to affect global climate change. However, submarine geohaz-
ards (such as sediment instabilities and slope failures on local
and regional scales, leading to debris flows, slumps, slides,
and possible tsunamis) caused by gas-hydrate dissociation are
of immediate and increasing importance as humankind moves
to exploit seabed resources in ever-deepening waters of coastal
oceans. The vulnerability of gas hydrate to temperature and
sea level changes enhances the instability of deep-water oce-
anic sediments, and thus human activities and installations in
this setting can be affected.
The potential effects of gas hydrate on human welfare are not
understood with certainty, but enough information has been
collected and enough knowledge gained over the past 30 years
to make preliminary assessments possible. To make these
assessments, however, some geoscience background is neces-
sary.
Definition. Naturally occurring gas hydrate is a solid, ice-like
substance, composed of rigid cages of water molecules that
enclose molecules of gas, mainly methane. Chemically, this
substance is a water clathrate of methane, where "water
clathrate" refers to the rigid cage structure of hydrogen-
bonded water molecules, but is commonly called "methane
hydrate" or, in general terms, "gas hydrate." The maximum
amount of methane is fixed by the geometry of the clathrate.
In an ideally saturated methane hydrate, the molar ratio of
methane to water is 1:5.75, that is equal to a volumetric ratio
at standard conditions of methane gas to water of 216:1 or a
volumetric ratio of methane gas to solid hydrate of 164:1 (1~.
Occurrence. Gas-hydrate deposits occur under specific con-
ditions of pressure and temperature, where the supply of
methane is sufficient to initiate the formation of, and to
stabilize, the hydrate (clathrate) structure (2~. These condi-
tions exist on earth in the upper 2,000 m of sediments in two
PNAS is available online at www.pnas.org.
regions: (i) continental, including continental shelves, at high
latitudes in polar regions, where surface temperatures are very
cold (<0°C) and (ii) submarine continental slopes and rises,
where not only is bottom water cold (ARC) but also pressures
are high (>3 MPa). Thus, in polar regions, gas hydrate is found
where temperatures are cold enough for onshore and offshore
permafrost to be present. In offshore sediment of outer
continental and insular oceanic margins, gas hydrate is found
at water depths >300-500 m, depending on bottom-water
temperatures.
The presence of gas-hydrate deposits in these oceanic
margins has been inferred mainly from the appearance on
marine seismic profiles of an anomalous reflection (Fig. 1) that
coincides with the predicted boundary (based on assumed
pressure/temperature considerations) of the base of the gas-
hydrate stability zone (Fig. 2~. This reflection is commonly
called a bottom-simulating reflection (BSR) because it ap-
proximately mimics sea floor topography. BSRs have been
mapped at depths below the sea floor, ranging from near the
sea floor to ~1,100 m (5~; the upper limit of the gas-hydrate
zone in outer continental margin sediment is ordinarily the sea
floor. Gas-hydrate samples have been recovered at 27 oceanic
continental margin locations (6), providing direct confirma-
tion of gas-hydrate occurrence. The worldwide locations of
known and inferred gas hydrate are shown in Fig. 3.
Estimates of Methane Content. Chersky and Makogon (8)
proposed that the amount of methane in naturally occurring
gas hydrate is potentially "enormous," but the estimated
amounts were highly speculative because of incomplete knowl-
edge of gas-hydrate occurrence. The Potential Gas Committee
(9) summarized the early estimates for the world: methane in
gas-hydrate deposits ranging from 3.1 x 10~5 to 7,600 x 10~5
m3 for oceanic sediments and from 0.014 x 10~5 to 34 x 10~5
m3 for permafrost regions. Because oceanic gas hydrate ap-
parently contains significantly more methane, it is emphasized
in global estimations of the methane content of gas hydrate.
The upper limit estimates above are from Dobrynin et al. (10)
and appear to be overly optimistic. They are "rough estimates"
based on permafrost coverage and zones of gas-hydrate sta-
bility in oceanic sediments without apparent regard for distri-
butions of sedimentary basins or sources of methane.
Estimates made during the period from 1980 to 1990 of the
amounts of methane in oceanic sediments were summarized by
Kvenvolden (11~. During this decade, an increased under-
standing of gas-hydrate occurrence has generally resulted in
estimates within the lower ranges of previous ones (Fig. 4~. For
example, Kvenvolden (7) estimated the methane content of
global gas-hydrate occurrence at 21 x 10~5 m3. The calculated
amount of gas hydrate in the outer continental margin of the
Arctic Basin of 1.1 x 10~5 m3 (12) was extrapolated to outer
continental margins of the remainder of the world by multi
Abbreviations: GCM, general circulation model, BSR, bottom-
simulating reflection; LPTM, latest Paleocene thermal maximum.
*To whom reprint requests should be addressed. e-mail: kk@octopus.
wr.usgs.gov.
3420
OCR for page 3421
Colloquium Paper: Kvenvolden
sW - -
BASE OF
. _GAS HYDRATE .
-- NE
cq
4
2
CJ
CO
2
FIG. 1. A 12-fold multichannel seismic reflection profile from the
crest and eastern flank of the Blake Outer Ridge. The strong BSR is
inferred to represent the base of the gas hydrate stability zone.
Modified from ref. 3.
plying by 20 because the length of the Arctic margin is about
5% of the total length of continental margins worldwide.
By using a completely independent approach in which the
organic carbon content of sediment was used as one guide,
MacDonald (13) estimated the amount of methane in world-
wide gas-hydrate deposits at 21 x 1ois m3. This estimate
neglects any gas hydrate in sediment at water depth over 3,000
m. That the estimates of Kvenvolden (7) and MacDonald (13)
are equal at 21 x 10~5 m3 is a coincidence, but the convergence
of ideas has made this value the consensus estimate. This
consensus value is bounded at an upper limit of 40 x 1Ois m3
determined by Claypool (14) and a lower limit of 10 x 1ois m3
determined by Makogon (cited in ref. 2) in the same decade.
In the l990s, Gornitz and Fung (15) and Harvey and Huang
(16) used plausible ranges of relevant variables to provide
estimates of possible methane release under various general
circulation model (GCM) predictions of global climate change.
The issue here, however, is not global climate change, but
rather the amount of methane in gas hydrate, which serves as
the basis for considerations of global climate change. Both
approaches included 1° x 1° grid resolution of some of the
variables. In the case developed by Gornitz and Fung (15),
estimates of the amount of hydrate methane were made from
101 ~it ~1
so
1OO
. ~
-
~n
UJ
~ 500
sooo
co
is,
cr
cat
2
A, METHANE
~ GAS+ICE
:: '\
: \
\
; METHANE
: HYDRATE+ :~:
. . ~ICE+GAS ~
5
10
METHANE
GAS+WATER
:+ NaCi,. N: \ CO,, C2H..
~ .~.:.-~:: i-; H2S, C,H.
aft:.-: 'I . :~ :'~ \
~ i. . METHANE -I \
: .:: HYDRATE+. ::: ~ \t
~ ~:WATER+GAS::
I: ~ .: : ' ~ 'I: \
-
-
a'
a'
50 cc
1 0000 -1 0
0 1 0 20 30 40
TEMPERATURE (a C)
100
. ALA
FIG. 2. Phase diagram showing boundary between free methane
gas (no pattern) and methane hydrate (pattern) for a pure water and
pure methane system. The addition of salts, such as NaCl, to water
shifts the curve to the left. Adding CO2, H2S, C:H6, or C3Hs to
methane (CH4) shifts the boundary to the right, thus reducing the
pressure for gas-hydrate stability at a given temperature. Depth scale
assumes lithostatic and hydrostatic pressure gradients of 10.1 kPa m-~.
Redrawn after Katz et al. (~4~.
Proc. Natl. Acad. Scz. USA 96 /61999) 3421
pressure-temperature phase relations and a plausible range of
thermal gradients, sediment porosities, and pore fillings, based
on two theories of gas-hydrate formation: (i) in situ microbial
gas generation and (ii) pore fluid expulsion models. A proxy
for organic carbon (the ultimate source of methane) was
cleverly obtained by using data from a coastal zone color
scanner, which measures oceanic photosynthetic pigments and
is indirectly related to primary production. The calculated
ranges of amounts of methane in oceanic gas hydrate are
between 26 x 1Oi: and 139 x 1ois m3, with the most likely
values at the lower end of the range, i.e., 26 x 1ois m3 for the
in situ microbial gas generation model and 115 x 1Ois m3 for
the pore fluid expulsion model. These estimates lie within the
range of early values but are higher than the consensus
estimate of 21 x 1Oii m3 (7, 13~.
Harvey and Huang (16) used data on ocean depths, tem-
perature at the sediment-water interface, ocean sediment
thermal conductivity, and the geothermal heat flux on a 1° x
1° global grid to estimate the volume of the zone of gas-hydrate
stability in oceanic sediments. They calculated the total meth-
ane content in gas hydrate in this volume of sediment to be
23 x lOi5, 46 x lOi5, and 91 x 1ois m3, depending on the
assumptions regarding the pore fractions occupied by gas
hydrate. They selected the intermediate value (46 x dots m3)
as the best estimate.
Other global estimates of hydrate methane were made
during this same time period and resulted in values smaller
than the consensus estimate of 21 x 10~5 m3. Holbrook et al.
(17), using mainly seismic information from the Blake Ridge
(Ocean Drilling Program Leg 164), concluded that global
estimates of methane in gas hydrate are too high by as much
as a factor of 3, which calculates to about 7 x 10is m3. This
conclusion was discussed by Dickens et al. (18), who used
mainly direct measurements of in situ methane abundances
stored in gas hydrate and free gas in sediment from the same
area, i.e., the Blake Ridge. They concluded that global esti-
mates of methane in gas hydrate in the range from ~2 x 10~5
to 20 x 10ii m3 are acceptable, based on current knowledge.
Other low estimates of the methane content of worldwide
gas hydrate include 10 x 10~5 m3 by Makogon (cited in ref. 2),
a value which has now been revised to 15 x 10~5 m3 (19) and
1 x 10~5 m3 by Ginsburg and Soloviev (20), who challenge all
larger estimates. Table 1 summarizes the critical worldwide
estimates of hydrate methane, and Fig. 4 shows how the
magnitude of these estimates has changed over about two
decades. It is evident that by 1990, the range of estimates had
been greatly constrained from estimates available in 1980.
However, since 1990, the range of estimates has expanded
slightly, but the consensus value of 21 x 10~5 m3 remains about
midway between the extremes. It is quite likely that the global
amount of hydrate methane is considerably less than 10~7 m3
but probably is greater than 10~5 m3, with the actual value in
the lower or intermediate part of the range.
Gas Hydrate and Human Welfare. Chemists have known
about gas hydrate since the early part of the 19th century (2~.
The petroleum industry became aware of this substance in the
1930s when gas-hydrate formation was discovered to be the
cause of pipeline blockage during transmission of natural gas
(23~. In the 1960s, naturally occurring gas hydrate was found
in the Siberian Messoyakha gas field (19), and in the 1970s it
was recognized that gas hydrate occurs naturally not only in
polar continental regions but also in shallow sediment under
deep water of the oceanic outer continental margins (24, 25~.
Since a review by Kvenvolden and McMenamin (5) of the
geological occurrences of natural gas hydrate, it has become
increasingly evident that naturally occurring gas hydrate is a
significant component of the shallow geosphere and has been
postulated by Kvenvolden (26) to be of societal relevance in a
least three ways: resource, climate, and hazard. Now the author
OCR for page 3422
3422 Colloquium Paper: Kvenvolden
.
Proc. Natl. Acad. Sci. USA 96 (1999J
ARCTIC OCEAN
;,_
i, ~ ,,~ PACIFIC
INDIA New `, I.- ~
~ .~
OCEAN
.~
~ ~OCEAN OCEAN
% ~ ~ ~
OCEAN
FIG. 3. The Earth showing locations of known and inferred gas-hydrate deposits in oceanic sediment of outer continental margins and in
permafrost (continental) regions. Modified from ref. 7.
believes that only the hazard aspect is of immediate impor-
tance in considerations of human welfare.
Potential Energy Resource. Two factors make gas hydrate
attractive as a potential energy resource: (i) the enormous
amount of methane that is apparently sequestered at shallow
sediment depths within 2,000 m of the surface of the Earth and
(ii) the wide geographical distribution of gas hydrate (Fig. 4~.
According to MacDonald (27), the energy density (volume of
methane at standard conditions per volume of sediment) of
methane hydrate is 10-fold greater than the energy density of
other unconventional sources of gas, such as coal beds, tight
sands, black shales, and deep aquifers, and 2- to 5-fold greater
than the energy density of conventional natural gas.
10000 -
1 000
1nO
90
a)
~ 80
a)
I
to
x
70 _
60 _
50 _
40 _
30 _
20
10
~ \
$\ 1 i-;7
1 [~1- ......... ~
.....
L5 to 25 EM EZ}
_ a_
. ~^ ~
)
1 980
1990 1 998
Year
Given these attractive factors, it is reasonable to conclude
that natural gas hydrate could serve as a future energy
resource, as suggested recently by Collett and Kuuskraa (28~.
However, there are some negative factors that suggest that
overly optimistic assessments are being made of gas hydrate as
a future energy resource. One such factor is the overall
petroleum geology of the gas-hydrate deposits. As Levorsen
(29) pointed out years ago, an essential element in any oil or
gas reservoir is permeability. Without permeability, there can
be little gas accumulation, nor can the accumulated gas be
produced by drilling, for it cannot move into production wells
quickly enough. Low-permeability sediments (mainly clay and
claystone) are the principal lithologies in gas hydrate-bearing
sections of the Blake Ridge, offshore from the southeastern
United States (30~. Dillon et al. (31) recognized the low
permeability of the sediments in this region and suggested that
faults act both as permeability barriers to gas and as conduits
for gas. The idea of trying to produce gas from such a system
Table 1. Summary of world estimates of methane content X1015
m3 of oceanic gas hydrate
Before 1988
After 1988
Methane estimate
x 1O~5 m3
Best High, Low
estimate Value
3.1
7,600
40
21
26 26-140
46 23-91
7
FIG. 4. Changing magnitude of estimates of the methane content 2-20
(X1015 m3) of worldwide gas-hydrate deposits from 1980 to 1998.
Estimates are rounded to two significant figures.
Reference
McIver (21)
Trofimuk et al. (22)
Dobrynin et al. (10)
Kvenvolden and Claypool (14)
Makogon (cited in ref. 2)
Kvenvolden (7)
MacDonald (13)
Gornitz & Fung (15)
Harvey & Huang (16)
Ginsburg & Soloviev (20)
Holbrook et al. (17)
Makogon (19)
Dickens et al. (18)
OCR for page 3423
Colloquium Paper: Kvenvolden
of clay and claystone does not appear to be particularly
attractive, especially with current technology. This one exam-
ple certainly does not apply to all regions of oceanic gas-
hydrate occurrence, but it is the best known example discov-
ered and described thus far and warns of the potential petro-
leum geologic problems that may be found elsewhere.
Although there is a large amount of methane in naturally
occurring gas hydrate, there is a constraint on this amount that
is often not appreciated. Hunt (32) pointed out that the
gas-hydrate enrichment factor decreases with depth. That is,
the gas-hydrate reservoir can hold, ideally at standard condi-
tion, "about six times as much gas as free gas held in the same
space." However, this enrichment factor decreases with depth
because free gas compresses considerably with depth and gas
hydrate does not. In the example given by Hunt (32), the
enrichment factor decreased from 6 to 1.25 in the depth range
from 274 to 1,219 m. Thus, with increasing depth, the presence
of gas hydrate can be a disadvantage in that a given volume of
gas hydrate will contain less gas than could be present if the gas
were In a tree state.
Interest in gas hydrates by the gas industry has waxed and
waned over the past 30 years. At present, however, interest
appears to be increasing, and certainly nations such as Japan
and India, with immediate energy needs, have undertaken
major efforts to investigate gas hydrate as an energy source
(28~. Successful recovery of methane from gas hydrate will
require the attention and infrastructure of the gas industry. In
fact, the location of this infrastructure is likely to dictate where,
if ever, gas hydrate is to be first produced commercially.
Although naturally occurring gas hydrate was recognized in
the 1960s, the gas industry has been slow to develop method-
ologies to recover methane from this substance. This slowness
is due, in part, to generally abundant gas supplies and a lack
of economic incentives leading to recent assessments such as
those of Rogner (33), who wrote that "in the foreseeable
future, there will be little need for the development of gas
hydrates." Various schemes have been considered at an aca-
demic level (34), but application of these schemes and suc-
cessful results have not been documented. Development of the
Messoyakha gas field in Western Siberia during the past 30
years (19) is often cited as an example of successful gas-hydrate
exploitation (35), but even this single example is now ques-
tioned (36) because the observed gas hydrate may be second-
ary, a result of production of conventional gas from this field.
Therefore, all of these discouraging factors, such as low
permeability sediments, decreasing enrichment factor with
depth, lack of sustained gas-industry interest, current limited
gas-industry infrastructure at most gas-hydrate locations, and
no good field example yet of successful methane production
from gas hydrate, diminish the potential for gas hydrate
becoming a significant energy resource. As far as human
welfare is concerned, methane hydrate as an energy resource
is not of immediate interest. Any attempts at full scale pro-
duction will probably not happen until well into the 21st
century (2~.
Global Climate Change. Methane is an important trace
component of the atmosphere, having a concentration of about
6.9 x 10~2 m3. This concentration was increasing at a rate of
0.9% yew, (37-39) until a few years ago, when this rate
decreased (40~. Because methane is radiatively active, it is a
"greenhouse" gas that has a global warming potential 20 times
greater than an equivalent weight of carbon dioxide when
integrated over 100 years (414. The earth's atmosphere has a
wide variety of sources and sinks for methane (42), including
methane hydrate. Methane hydrate exists in metastable equi-
librium and is affected by changes in pressure and temperature
that occur mainly with changes in sea level. The amount of
methane that is present in gas hydrate onshore and offshore is
perhaps 3,000 times the amount in the present atmosphere; an
instantaneous release of methane from this source could have
Proc. Natl. Acad. Sci. USA 96 (1999J 3423
an impact on atmospheric composition and thus on the radi-
ative properties of the atmosphere that affect global climate
(~13~.
There are obstacles, however, to methane from gas hydrate
ever reaching the atmosphere. Instead of instantaneous re-
lease, much methane associated with gas hydrate could vent
slowly over geologic time, providing opportunities for its
oxidation to carbon dioxide by microbial and chemical pro-
cesses; the oceans could then act as a sink for the produced
carbon dioxide. If any methane did reach the atmosphere, it
should react with hydroxyl radicals (42) in about 10 years,
unless the supply of radicals is overwhelmed. Thus, the role for
methane hydrate in global climate change undoubtedly de-
pends on the rate of methane release, and that rate is largely
unknown. Evidence for slow gas release from oceanic sedi-
ments is provided by the widespread occurrence of pockmarks
on the ocean floor (43) and direct observations of slow release
of methane from sea-floor gas hydrate and associated vents in
the Gulf of Mexico (44~. In contrast, a possible example of
rapid release is given by a 700-km2 collapse depression on the
crest of the Blake Ridge (45~. The amount of methane released
from this depression during its formation might have been
limited, however, by rehydration of escaping methane caused
by a combination of hydrostatic pressure and cold bottom
waters reaching the floor of the newly formed feature.
Two ideas have been proposed to explain the possible role
of methane hydrate in global climate change, but neither of
these ideas considered the powerful effect of oxidation on the
methane released from gas hydrate. Nisbet (46) suggested that
methane from continental gas hydrates contributed to the
rapid rise in atmospheric methane at the end of the last major
glaciation about 13,500 years ago. In this scenario, polar
continental gas-hydrate deposits are destabilized by pressure
reduction of melting ice sheets, causing temperature increases
because of the released methane. The resulting warming
provides a strong positive feedback that amplifies methane
emissions and ultimately helps to end the ice age.
A different scenario was proposed by Paull et al. (47~. They
suggested that outer continental margin gas-hydrate deposits
release methane during a falling sea level, that is, during global
cooling. The resulting decrease in pressure causes this gas
hydrate to dissociate. The released methane enhances global
warming and triggers deglaciation. Thus, methane derived
from outer continental margin gas-hydrate deposits in this
scenario is believed to be an important factor in limiting the
extent of glaciation during a glacial cycle.
As interesting as both of these scenarios are, they are both
quite speculative, because it is not even known how gas
hydrates behave in the present climate regime. Kvenvolden
(48) suggested that gas-hydrate deposits of the polar conti-
nental shelves are presently most vulnerable to climate change.
These areally extensive shelves, formerly exposed to very cold
surface temperatures (-10° to -20°C) have been and are
being transgressed by a much warmer polar ocean (ARC). The
polar shelf surface, therefore, has experienced a +10°C or
more change in temperature over at least the past 10,000 years.
If this suggestion is correct, then escape of methane from
assumed gas-hydrate deposits of polar continental shelves
should be observable.
To test this idea, methane concentrations in water overlying
the Beaufort Sea continental shelf of Alaska have been
measured when ice is present and absent during 1990-1995
(Fig. 5~. Preliminary results (49) showed that methane con-
centrations in the Beaufort Sea under the winter ice canopy
were greater than when the ice is absent. Continued studies
(50, 51) confirmed these observations and showed that the
average difference in methane concentrations between ice-
covered and ice-free conditions is ~15 nM; however, carbon
isotopic determinations indicated that most of the methane
came from coastal microbial processes and not from gas
.
1 1
OCR for page 3424
3424 Colloquium Paper: Kvenvolden
,
f
W: : : ~ :
1 _ ~
: i:: :~50:m_
Proc. Natl. Acad. Sci. USA 96 (1999 J
Me~ane Concentration Difference (nM):
r >_
: J Aft. by:
: ~ ~ PEA
Camden: -
It
FIG. 5. Map of a portion of the Beaufort Sea continental shelf offshore from Alaska showing station locations and concentrations of methane
(in nM) in water samples collected when ice was present and absent from 1993 through 1995. Based on data from refs. 50 and 51.
hydrate. Nevertheless, the Beaufort Sea continental shelf of
Alaska appears to be a seasonal source of a very minor
contribution of methane to the atmosphere. Although this test
failed to show significant release of methane from assumed
gas-hydrate occurrence, the test remains inconclusive until
similar research is carried out on the broad continental shelves
of the Siberian Arctic Ocean.
Large excursions in the carbon isotopic record of carbonate
in oceanic sediments from the latest Paleocene thermal max-
imum (LPTM) have been attributed to massive dissociation of
gas hydrate (52, 534. Although this idea is attractive in ex-
plaining the isotopic record, the processes and consequences
of this idea need further examination. With an increase in
temperature or a decrease in pressure, gas hydrate dissociates.
Slow dissociation of massive amounts of gas hydrate could take
place, with the methane becoming trapped. If this trap is
breached then the methane release could be very rapid,
causing a "blast of gas" as suggested by Dickens et al. (~53~.
Evidence for trapping of gas beneath gas hydrate is provided
by the worldwide occurrence of BSRs marking the interface
between gas hydrate above and free gas below. As discussed
previously, it is the rate of release of trapped methane or the
size of the "blast" that is unknown. Also unknown is the
effectiveness of the water column in oxidizing the methane as
it passes through toward the atmosphere. The amount of
methane that can actually reach the atmosphere to affect
global climate change is uncertain; however, the oxidized
methane, now present as carbon dioxide in the water of the
ocean, may cause change in the carbon isotopic record of
sediments as observed during the LPTM (52, 53~.
Excursions in the atmospheric concentration of methane
during the Holocene have been noted in ice cores from
Antarctica (54~. Consideration has been given to the role that
gas hydrate might have played in producing these excursions
(55), and the conclusion was reached, based on ice cores from
both Antarctica and Greenland, that gas hydrate was a rela-
tively small source of methane during at least the later
Holocene.
The potential impact of methane hydrate on future global
warming has been evaluated in a GCM study (16~. This study
concluded that even for worst case scenarios, the impact on
future global warming caused by gas hydrate will be small; the
uncertainty in future global warming due to gas-hydrate
destabiliztion is smaller than the uncertainty due to warming
caused by fossil fuel use.
Therefore, the potential role for gas hydrate in global
climate change is diminished because of the possibility of rapid
oxidation of the released methane to carbon dioxide, thus
enhancing the solubility of the methane carbon in the ocean
water. Scenarios of past global climate change caused by
methane released from gas hydrate are all very speculative;
one test in the Arctic of methane release during the current
climate cycle failed to show much methane from gas hydrate;
one GCM study demonstrated little impact of gas hydrate on
future global warming; and records of excursions of methane
concentrations during the Paleocene and Holocene, although
possibly caused by gas-hydrate dissociation, do not provide
compelling evidence that this methane actually affected global
climate. With respect to human welfare, it is not immediately
important whether gas hydrate has a role in global climate
change. To be on the safe side, however, it would be best in the
future not to perturb natural gas-hydrate stability by continu-
ing societal practices that enhance global warming.
Geologic Hazard. Gas hydrate as a geohazard has been
considered in detail previously (56~. Before gas hydrate forms
in usual geologic settings, vast quantities of methane and water
are free to migrate within the interstitial pore spaces of
consolidating sediment. During gas-hydrate formation, meth-
ane and water become immobilized as a solid, restricting pore
space and retarding the migration of fluids. Solid water (rather
than liquid water) occupies the pore spaces, and the sedimen-
tological processes of consolidation and mineral cementation
are greatly inhibited, although gas hydrate itself can act as
metastable cementation (bonding) agent. The permeability of
the sediment to gases and liquids decreases as more gas hydrate
forms. Eventually, gas hydrate may occupy much of the pore
space within the zone of gas-hydrate stability. Continued
sedimentation leads to deeper burial of the gas hydrate.
Finally, the gas hydrate will be buried so deeply that temper-
atures at the base of the stability zone will be reached at which
the gas hydrate is no longer stable. The solid gas/water mixture
(i.e., the gas hydrate) will become a liquid gas/water mixture.
Thus, the basal zone of the gas hydrate becomes undercon-
solidated and possibly overpressured because of the newly
OCR for page 3425
Colloquium Paper: Kvenvolden
. . . .. . . .
Gas O Original Slope
Large block of hydrated
sediment breaking off and
sliding down slope
MOO
~Lower Boundary of Hydrate at Low Sea Stand
Debris
Flow
/C0Yo,°' ~ Dissociated (gas-fluidized)
monsoon Gas Hydrate
' Lower Boundary of Hydrate at
High Sea Stand
FIG. 6. Diagram showing the effects of changes in sea level on
submarine gas hydrate and the resulting failures and gas release.
Adapted from McIver (57~.
released gas, leading to a zone of weakness (low shear strength,
where failure could be triggered by gravitational loading or
seismic disturbances), and submarine landslides result (57~.
The same conditions that cause gas-hydrate dissociation
during continued sedimentation can also be brought about by
the lowering of sea level or by an increase in bottom-water
temperatures. These processes change the in situ pressure or
temperature regime. In adjusting to the new pressure/
temperature conditions, the gas hydrates dissociate, producing
an enhanced fluidized layer at the base of the gas-hydrate zone.
Submarine slope failure can follow, giving rise to debris flows,
slumps, slides, and collapse depressions such as described by
Dillon et al. (45~. Failure would be accompanied by the release
of methane gas at least into the water column, but much of the
methane is likely to be oxidized unless the gas release is
catastrophic. A scenario illustrating submarine slope failure is
shown in Fig. 6.
The possible connection between gas-hydrate boundaries
and submarine slide and slump surfaces was first recognized by
McIver (58), and several possible examples were described
later. These examples include surficial slides and slumps on the
continental slope and rise of West Africa (59), slumps on the
U.S. Atlantic continental slope (60), large submarine slides on
the Norwegian continental margin (61, 62), sediment blocks on
the sea floor in fjords of British Columbia (63), and massive
bedding-plane slides and rotational slumps on the Alaskan
Beaufort Sea continental margin (64~.
Periodic Pleistocene eustatic sea level transgressions and
regressions provide mechanisms to account for the waxing and
waning of submarine gas hydrate (64~. For example, during the
last Pleistocene regression, sea level lowered approximately
100 m between about 28,000 and 17,000 years before present,
resulting in a reduction of total stress acting on the seafloor.
The reduction in the total pressure initiates dissociation at the
base of the gas hydrate, releasing excess methane and water.
Failure follows on moderate slopes unless the increased fluid
pressures can be adequately vented. On the Beaufort Sea
continental slope is a zone of massive slides and slumps that
coincides with a region of sediment inferred, from seismic
reflection studies, to contain gas hydrate. Fluctuations in
global climate, reflected in Pleistocene sea-level lowerings,
likely caused these submarine slides and perhaps caused other
slides on other continental margins where gas hydrate is
present (64~.
These submarine disruptions of the seafloor, caused by
gas-hydrate dissociation, impact human welfare if human
Proc. Natl. Acad. Sci. USA 96 (1999' 3425
made structures are located in regions of potential failure. As
humankind expands its interest in the seafloor at increasing
water depth, such as in the petroleum industry's search for oil
and gas, stability of the seafloor becomes increasingly impor
nume Am t ant for any engineering structures. The potential vulnerability
Ooze , ~of engineering structures to gas-hydrate dissociation in oceanic
O -A sediments has been recently recognized and described (65, 66~.
Risks to drilling and production through gas hydrate-bearing
sediment have been addressed by Yakushev and Collett (67)
for gas hydrate in Arctic regions, and these same concerns,
such as casing collapse, gas leakage outside the conductor
casing, and gas blowouts, will be applicable to gas hydrate of
deep oceanic regions, only the problems will likely be more
severe, but not necessarily as severe as suggested by Bagirov
and Lerche (68), who discuss possible gas-hydrate hazards in
the Caspian Sea. The geohazard aspects of gas hydrate provide
an additional constraint on exploiting oceanic gas hydrate as
a future energy resource.
CONCLUSIONS
The amount of methane sequestered in gas hydrate is un-
doubtedly very large, probably >10~5 m3, but considerable
<10~7 m3. How this methane can or will affect human welfare
is not yet defined. There is much current interest in gas hydrate
as a potential (i) energy resource, (ii) factor in global climate
change, and (`iii) submarine geohazard. Of these three issues,
only the third is considered here to be important at the present
time for human welfare. It is argued that gas hydrate as a future
energy resource has received overly optimistic assessments
because of inadequate evaluation of the reservoir qualities of
the geologic settings in which oceanic gas hydrate is found.
Vacillating interest by the gas industry and the absence of good
industrial examples of successful methane production from gas
hydrate diminish enthusiasm for gas hydrate as a potential
energy resource. It is also argued that gas hydrate is likely not
a major factor in global climate change in that much of the
methane that could be released from the dissociation of gas
hydrate is probably oxidized to carbon dioxide, which dissolves
in the water; most hydrate methane never reaches the atmo-
sphere where it could function as a powerful "greenhouse" gas.
Scenarios of climate change caused by methane release from
gas hydrate in the geologic past are all speculative, and one test
of methane release from gas hydrate in the current climate
cycle failed. Gas hydrate may be an agent in global change (for
example, altering the isotopic record of oceanic sediments),
but not necessarily in global climate change. The evidence
seems clear that gas hydrate is a geohazard, particularly in the
oceans. As a geohazard, gas hydrate will affect human welfare
as humankind moves to exploit the seafloor at ever increasing
water depths. Human activities and installations in regions of
gas-hydrate occurrence must take into account the presence of
gas hydrate and deal with the consequences of gas-hydrate
dissociation.
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Representative terms from entire chapter:
energy resource
