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7/30/2019 GlobalOccurrenceGasHydrate Kvenvolden&Lorenson,2001
http://slidepdf.com/reader/full/globaloccurrencegashydrate-kvenvoldenlorenson2001 1/6
Proceedings o f the Eleventh (2001) International O ffshore and Polar Engineering Conference
Stavanger, Norway, June 17-22, 2001
Copyright © 2001 by The International Society of O ffshore and Polar Engineers
ISB N 1-880653-51-6 (SeO; ISBN 1-880653-52-4 (Vo l. I ) ; ISS N 1098-6189 (SeO
G l o b a l O c c u r r e n ce s o f G a s H y d r a t e
K e i t h A . K v e n v o l d e n a n d T h o m a s D . L o r e n s o n
U . S . G e o l o g i c a l S u r v e y
M e n l o P a r k , C A , U S A
A B S T R A C T
N a t u r a l g a s h y d r a t e i s f o u n d w o r l d w i d e i n s e d i m e n t s o f o u t e r
c o n t i n e n t a l m a r g i n s o f al l o c e a n s a n d i n p o l a r a r e a s w i t hc o n t i n u o u s p e r m a f r o s t . T h e r e a r e c u r r e n t l y 7 7 l o c a li t ie s i d e n t if i e d
g l o b a l l y w h e r e g e o p h y s i c a l , g e o c h e m i c a l a n d / o r g e o l o g ic a l e v i d e n c e
i n d i c a t e s th e p r e s e n c e o f g a s h y d r a t e . D e t a i l s c o n c e r n i n g i n d i v i d u a l
g a s - h y d r a te o c c u r r e n c e s a r e c o m p i l e d a t a n e w w o r l d - w i d e - w e b
( w w w ) s i te ( h t t p : / / w a l r u s . w r . u s g s . g o v / g l o b a l h y d r a t e ) . T h i s si t e h a s
b e e n c r e a t e d t o f a c il i t at e g l o b a l g a s - h y d r a t e r e s e a r c h b y p r o v i d i n g
i n f o r m a t i o n o n e a c h o f t h e l o c a l i ti e s w h e r e t h e r e i s e v i d e n c e f o r g a s
h y d r a t e . A l s o c o n s i d e r e d a r e t h e i m p l i c a t i o n s o f ga s h y d r a t e a s a
p o t e n t i a l ( 1 ) e n e r g y r e s o u r c e , ( 2 ) f a c to r i n g l o b a l c l i m a t e c h a n g e ,
a n d ( 3 ) g e o h a z a r d .
KEY WORDS: gas hydra te , me thane , BSR, resource , c l ima te change ,
geohazard
I N T R O D U C T I O N
Interest in naturally occurring gas hydrate has increased steadily
s ince abou t 1969 whe n V as i l ' ev e t a l . (1970) recogn ized tha t na tu ra l
gas cou ld fo rm gas-hydra te depos i t s tha t shou ld occur g loba l ly
wherev er the p ressu re and tempera tu re cond i t ions a re favorab le .
Recogn i t ion o f the po ten t ia l fo r na tu ra l gas -hydra te occur rence w as
fo l lowed by i ts d iscovery , f i r s t in pe rmafros t reg ions o f no r the rn Russ ia
(M akogo n et al ., 1971, 1972). I t was then obs erved in sediment of the
B lack Sea (Yef rem ova and Zh izhchenko , 1974) and in fe r red by se ismic
surveys to be p resen t in sed imen t o f the B lake R idge , in the wes te rn
Atlantic O cean (Stoll e t a l ., 1971; Ewi ng and Hollister , 1972). By the
ea r ly 1980s, gas hydra te h ad been found in ou te r con t inen ta l marg in
sed imen t o f the Midd le Am er ica T rench o f fshore f rom Mexico
(Sh ip ley and Didyk , 1982) and Gua tem ala (Har r i son and Cur ia le,
1982) . S ince then , the ra te o f d iscovery o f ev idence fo r gas hydra te has
acce le ra ted . The ea r ly h is to ry o f d iscovery o f gas hydra te has been
summar ized by Kvenvo lden (2000) .
E V I D E N C E F O R G A S H Y D R A T E
Three k inds o f ev idence have been used to iden t ify the p resence o f
na tu ra l gas hydra te - -ge o log ica l , geochemica l , and geophys ica l .
Geological evidence includes sediment properties, s tratigraphic
relationships, gas-migration pathways, and, most importantly, the
ac tual recovery and desc r ip t ion o f gas -hydra te samples . Pore f lu
chemis t ry and gas compo s i t ions (molecu la r and i so top ic ) a re im
aspec ts o f gas -hydra te geochemis t ry . F ina l ly , geophys ica l ev ide
includes data from seismic reflection profil ing, seismic refractioangle reflection studies, vertical seismic profil ing, and various k
well- logging . Seismic reflection profil ing is especially valuable
access ing the a rea l ex ten t o f gas -hydra te depos it s . Muc h o f the
geophys ica l ev idence fo r ocean ic gas hydra te i s based on obse rv
of Bo t tom-S imula t ing Ref lec to rs (BSRs) on m ar ine se ismic reco
These anom alous re f lec tions resu l t f rom the acous t ica l d i f fe renc
be tween hydra te -bear ing sed imen ts w i th in the hydra te -s tab i l i ty z
(HSZ) and non-hydra te bea r ing , gassy sed imen t be low the HS Z
(Shipley et al . , 1979).
O C C U R R E N C E S O F G A S H Y D R A T E
A compi la t ion o f g loba l gas -hydra te occur rences was f i r s t und
by Kvenvo lden and McM enamin (1980) . They showed 14 a reas
ocean ic and 5 con t inen ta l ) where geophys ica l and som e geochemevidence ind ica ted gas hydrate . Much o f the geophys ica l ev iden
ocean ic gas hydra te was based o n the obse rva t ions o f BSRs on m
se ismic records . By 1988 the numbe r o f iden t i fied gas -hydra te
had increased to 38 (30 oceanic including inland seas and 8
continental) (Kven volden, 1988). Indepen dently, Panayev (198
Ginsburg et al . (1990) identif ied 22 and 36 regions, respectively
subaquatic gas hydrate occurs. Kv envo lden et al . (1993) l is ted 4
loca tions wor ldwide w here sub-aqua t ic gas hydra te cou ld be exp
and gas -hydra te samples w ere recovered a t 14 o f these loca t ions
International interest in gas hydrate is increasing rapidly with
recogn i t ion tha t the seques te red methane m ay be use fu l a s an en
resource and may be a fac to r in g loba l c lima te change . With th i
increasing interest i t seems appropriate to review the inventory
known and in fe r red gas hydra te occur rences , bo th ocean ic and
continental . There are currently 77 localit ies identif ied globally
geophys ica l, geochem ica l and /o r geo log ica l ev idence ind icates t
presence o f gas hydrate (Fig. 1) . At 23 of these localities, gas-h
samples have been recovered and desc ribed . De ta i l s concern ing
ind iv idua l gas -hydra te occur rences a re compi led a t a new w or ld
web (www) s i te (h~: l lwa lrus .wr .usgs .gov lg loba lhs , d ra te ) . This
has been created to facil i ta te global gas-hydrate research by pro
in fo rmat ion on each loca l i ty where the re i s ev idence fo r gas hyd
Pre l iminary in fo rmat ion fo r th is s i te i s summar ized he re , w i th
4 6 2
7/30/2019 GlobalOccurrenceGasHydrate Kvenvolden&Lorenson,2001
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F ig .u re 1 . Map showing w or ldwide loca t ions o f known and in fe r red gas hyd ra te in aqua t ic (oceans , seas, and lakes ) sed imen t and in po la r con t
sed imen t . Open c i rc le s ind ica te loca t ions where samples o f na tu ra l gas hyd rate have been recove red . F i l l ed c i rc le s ind ica te loca t ions o f the in fe
p resence o f na tu ra l gas hyd ra te . Boxes marke d C5-C8 ind ica te a reas in wh ich gas hyd ra te i s in fe r red to occu r .
app rop r ia te re fe rences fo r each gas -hyd ra te occu r rence a t th i s webs i te
(Table 1) .To desc r ibe th is inven to ry , the w or ldwide d i s t r ibu t ion o f gas hyd ra te
has been o rgan ized in to seven reg ions , wi th bas ic in fo rma t ion tabu la ted
in Tab le 1 . The seven geog raph ic reg ions o f gas -hyd ra te occu r rence
a re Pac i fic Ocean , Ind ian Ocean , A t lan t ic Ocean , Nor th (Arc t ic ,
Ocean ic ) , Sou th (An ta rc t ica ) , Othe r ( In land Seas and Lakes ) , andCon t inen tal . At p re sen t the re a re 35 loca t ions in the Pac i f ic Ocean (P 1 -
P35), 3 in the Indian Ocean , (I 1-I3) , 19 in the Atlantic O cean (A 1-
A19) , 3 in the Nor th (Arc t ic Ocean ) (N1-N3) , 4 in the Sou th
(Antarctica) (S1-$ 4), 5 in Oth er (O 1-O5), and 8 in Continental (C1-
C8) . These g loba l loca t ions o f known and in fe r red na tu ra l gas hyd ra te
a re shown on F igu re 1 .
The g loba l gas hyd ra te ww w s i te i s a work in p rog ress . Bes ides
p rov id ing re fe rences to a l l o f the know n gas -hyd ra te occu r rences , the
s i te a lso g ives o the r k inds o f in fo rma t ion such a s abs t rac ts f rom the
re fe rences and techn ica l commen ts . Add i t iona l in fo rma t ion , such a s
4 6 3
se i smic l ines , we l l - logs , geochem ica l measu rem en ts , sample
photographs, e tc . , wil l be displayed at the s i te when available .
A M O U N T O F M E T H A N E
All e s t ima tes o f the me thane con ten t o f me thane hyd ra te a re h
specu la t ive , bu t they sugges t tha t me thane quan t i t ie s a re ve ry la r(Kvenvo lden , 1999). Che rsk iy and Makogo n (1970) p roposed t
amoun t o f m e thane in na tu ra l gas hyd ra te i s po ten t ia l ly "eno rmo
Ear ly e s t ima tes ranged f rom 1 .7 to 4 ,100 exag rams (exa g ram =
o f me thane ca rbon fo r ocean ic sed imen t and f rom 0 .0075 to 18
exag rams o f me thane ca rbon fo r po la r reg ions ( summ ar ized by t
Po ten t ia l Gas Comm it tee , 1981). The S I un i t 'exag ram ' i s u sed
and in l a te r d i scuss ion because i t min imizes the need fo r expone
Because ocean ic gas hyd ra te depos i t s a re much mo re vo luminou
po la r - reg ion gas hyd ra te accumula t ions , the ocean ic gas hyd ra te
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Tab le 1. Sum mary of know n and inferred subaquatic gas-hydrateoccurrences in the Pacific, Indian Oceans, and Atlantic Oceans; inpolar oceans and continents; and in inland seas and lakes. [BSR ,
bottom-simu lating reflection; < CI' , low chloride content of pore water;
CH4, high methane content; V AM Ps, velocity amplitude pulldowns;Logs, well-log response; PCS , Pressure core sample; Geophysical,seismic evidence of past occurrence o f gas hydrate]
D e s i g n a t i o n I L o c a t i o n O f f s h o r e i E v i d e n c e
P A C I F I C O C E A NP1 , PanamaP2 Costa Rica (Middle Am erica
Trench)!
P2 [ (DSDP Leg 84) ,,
P2 I (ODp Leg 770)P3 ' Nicarag ua (Middle Am erica
Trench)
P4 Guatemala (Middle Am erica
Trench)!
P4 I (DSDP Leg 67)
(DSDP Leg 84)4
P5
P5
P6
P7
P8
P8
, P8
P 9
~P10
' P l l
I
P12I
, P1 3
P14i
P I5
!
P16i
P1 7
P1 8I P19
P2 0
P2 0
P2 1
P22
P2 3
P24
P24
i P25
i P26
Mexico (Middle America
Trench(DSDP Leg 66)
Mex ico (G ulf of California,Guaym as Basin)
California, USA (Eel River
Basin)
Oregon, USA (Cascadia
Basin)
(ODP Leg 146)
(Hydrate Ridge)
Canada (Cascadia Basin)
(Fjords of Brit ish Columbia)
Alaska, USA (EasternAleutian Trench)
I
Alaska, USA (MiddleAleutian Trench),
Bering,,Sea (Alaska, USA),
(USA, Bering Sea Shelf) .....
(USA/Russia, Navarin
Margin)
(Russia, Shirshov Ridge)
Okhotsk Sea (Paramushir
Island, Russia)
(Sahkalin Island, R ussia)
(off Abashiri , Kitam i-Yam ato Mount, Japan) ,,
Japan Sea (DSDP Leg 57)
(Okushiri Ridge, ODP Leg
127)(W estern Tsu garu Basin ) ..... I
(Tatar Trough)
Japan, Nankai Trough (off
eastern Miyazaki)
(off southern Shikoku)
(ODP Leg 131)
(Muroto Trough)
(Kumano-nada thru of f
Omaezaki Cape)
B SR
B SR
Samples
Samples
BSR
B SR
Samples, < C l "
S a m p l e s , Logs,
<CI"
B SR
Samples
B SR
BSR, Samples
B SR
Samples
Samples
B SR
Slumps
B SR
BSR, <C1-
V A M P s
B SR
B SR
B SR
Samples
Samples
B SR
<C1-
BSR, Sample
B SR
B SR
B SR
B SR
Sample
B SR
BSR, CH4, <C1-
4 6 4
T a b l e 1 c o n t .
D e s i g n a t i o n L o c a t i o n O f f s h o r e E v i d e n c e
P27 Japan, Offshore Chiba BSRBasin
P28 i Japan, Kuril Trench (off BS R
P29
P30
P3 1
P3 1P32
P3 3
Tokachi/Hidaka)
New Zealand (HikurangiTrough)
Chile (Peru-Chile Trench)
Peru (Peru-Chile Trench)
(ODP Leg 1 7)Taiwan (South China Sea)
Australia (Tasman Sea,Lord Howe Rise)
P34 (Timor Trough)
P35 Indonesia (Celebes Sea)
I N D I A N O C E A N
I 1 Oman (Gulf of Oman,Makran Margin)
I2 India (Arabian Sea)
I3 . (Bay ofBen gal )
A T L A N T I C O C E A N
A1 Argentina (Central
A2
A3
A4
A5
A6
A7
A8
A 9
A9
A9
A10
A l l
A12
A13
Argentine Basin)
Brazi l (Amazon Fan)
(Pelotas Basin)
Barbados (Barbados Ridge)
Southern Caribbean Sea
Panama and Colombia(Colombia Basin)
Gulf of Mexico (of fMexico, western area)
(offUSA, northern area)
Southeastern USA (BlakeRidge)
(DSDP Leg 76)
(ODP Leg 164)
Eastern USA (Carolina
Trough),Eastern USA (ContinentalRise)
Newfoundland, Canada(Laborador Shelf)
Norway (Storegga Slide)
A13 , (ODP Leg 104) r
A 14 (Barents Sea) !
A15 (off Svalbard)
A16 (Haakon-Mosby mu d vol. ) .
A17 Ireland (Porcupine Basin) .
A18 Africa (S.W. Africa)
A1 9 , (Nig eria) .,, j
.... NO RTH ( A r c t i c )
N 1 Alaska (Beaufort Sea)
N2 Canada (Beaufort Sea)
N3 ~ Canada (Sverdrup Basin) .
S O U T H ( A n t a r c t i c )
S 1 Antarctica (Wilkes LandMargin)
$2 (Ross Sea)
$3 (Weddell Sea)
$4 (South Shetland margin)
B SR
BSR , CH 4, <C
B SR
Samples
B SR
B S R
CH 4
B S R
B SR
B SR
B SR
B SR
B SR
B S R
B S R
B S R
B SR
B SR
Samples
BSR, Slumps
Samples, <C1
BSR, SampleLogs
B SR
B S R
B SR
BSR, Slumps
<CI"
CH4, < C 1 -
B SR
B SR
Samples
Geophysical
Slumps
Samples
BSR, Slump
Logs
Logs
B SR
CH4, <C1-
B SR
B SR
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Tab le 1 con t .
D e s i g n a t io n I L o c a t i o n O f f s h o r e I E v i d e n c e
O T H E R ( IN L A N D S E A S A N D L A K E S )
O 1 Black Sea, Russia
02 Caspian Sea , Russ ia
03 Lake Baika l , Russ ia
• Samples
Samples
BSR, Samples
04 Medi te r ranean Sea (ODP
Leg 160)
05 Turkey (Kula mud volcano)
C O N T I N E N T A L
<CI-, CH4
Sample
C1C2
C3
C4
C5
C6
,
C7
C8
Alaska (North Slope)Canada (Mackenz ie Del ta )
(Arctic Islands)
Russ ia (M essoyakah Fie ld)
(Yiman-Pech ora Province) .
(Western Siberian
Platform)
(Eastern Siberian craton)
(Northeast Siberia)
Logs, PCSSamples, Logs
Logs
CH 4
Interpretat ion
Interpretat ion
Interpretat ion
Interpretat ion
empha sized in global est imates of the methan e content of gas hydrate.
During the late 1980s, est imates of methane in gas hydrate were
greatly constrained when K venvolde n (1988) and MacD onald (1990a),
working independently, est imated the global methane-carbon content
of gas hydrate to be 11 and 12 exagram s, respectively. These nearly
equal est imates are coincidental , but the convergence of independent
ideas made this value, expressed to one significant f igure as 10
exagrams, the consensus est imate.
In the 1990s some new es t imates were made us ing the power of
general circulation mode ls (GCM s) in which p ert inent variables, such
as pressure-temperature phase relat ions, geotherm al gradients, and
sediment porosit ies, are entered into a 1o x 1° global grid (Gornitz and
Fung, 19 94; Harvey and Huang , 19 95). Other global est imates of
methane-ca rbon in gas hydrate were m ade during the same period,
extrapolating from m easurem ents obtained at the Blake Ridge
(Holbrook et al ., 1996; Dickens et al. , 1 997a). In addit ion, e st imates by
Makog on, (1997) and Ginsburg and Soloviev (1998) were based on
theoretical and emp irical information. The est imates published in the
1990s are summ arized in Table 2. The best est imates range from a low
of 0.5 to a high of 24 exagrams. The consensus value of 10 exagram sof methane carbon in gas hydrate worldwide remains about midway
between these extremes.
T a b l e 2. Es t imates made dur ing the 1990s of the amount of methane
carbon [exagrams (10 TMg)] in natural gas hydrate.
E s t i m a t e ( e x a g r a m s ) M e t h o d
R a n g e B e s t
14 to 75 14 GC M 1
15 to 49 24 GCM ~
m
1to 10
- - -3 Ext rapola t ion2
Extrapolation2
8 Theoretical
0.5 Empirical
R e f e r e n c e
Gomitz & Fung (1994)
Harvey & Huang
(1995)
Holbrook et al . (1996)
Dickens et al . (1997)
Makogon (1997)
Ginsburg & Soloviev
(1995)
1GCM, General Circulation M odels
2Extrapolation from one geo graphic area to worldwide est imate
I M P L I C A T I O N S
The global gas hydrate w ww site will also provide discussion with
references to the impo rtant implications of gas hydrate. Gas hydrate
has global importance as a potential (1) energy resource, (2) factor in
46 5
globa l c l imate change , and (3) geohazard . Wi th the inc
wor ldwide in te res t in gas hydra te , ready access to back
informat ion for fu ture research on the g loba l d i s t r ibu t i
significance of naturally occurring gas hydrate is important to
hydrate community.
R e s o u r c e
Methane is the most important energy com ponent in natural ga
a significant port ion of world industry depen ds on m ethane for fu
With i ts large methane content, gas hydrate is a very at tractive po
energy source. Gas hydrate is located within 2000 m of the solid
surface, making this methane tech nologically within reach of the
bit. Although restr icted to polar regions and o uter continental m
gas hydrate is geographically distr ibuted worldwide, mak ing the
methane a possible international comm odity. Meth ane-hy drate 's
energy density (volume of methane at standard condit ions per vo
of sediment) is ten-t imes greater than that o f other unconventiona
sources, such as coal beds, t ight sands, black shales, and deep aqu
it is two- to f ive-t imes greater than the energy density of convent
natural gas, depending on the depth of gas-hydrate occurrence
(MacDonald, 1990b).
Interest in gas hydrate by the energy industr ies has vacil lated o
past 30 years, but interest is presently increasing, especially in th
nations that face or wil l face an energy sho rtage in the near futur
Two countries, Japan and India, have und ertaken major efforts toinvestigate gas hydrate as a potential energy resource. The succe
discovery in 1999 of gas hydrate offshore from Japan, as part of
year study by a Japanese industr ial consortium, is a step forward
direct evaluation of gas hydrate as a p otential energy resource (JN
press release, 2000). I t should be remembe red, however, that dif
scientif ic and technological problems, such as ge nerally poor res
characteristics and d iff icult geographic locations for exploitat ion
be addressed and solved before methane hydrate can ever be used
econom ical source of fossi l fuel (Kvenvolden, 1999).
C l i m a t e
Methane in natural gas not only serves as a com bustible fuel fr
which energy is derived, i t also acts as a greenhouse gas w hen pr
in the atmosphere. Although methan e has a global warming-pote20-times greater than the equivalent weight of carbon dioxide, w
integrated over 100 years, (Shine et al., 1990), its half-life in the
atmosphere is very short, about 10 years. I ts concentration is als
lower than carbon dioxide by a factor of about 160 (Cicerone and
Oremland, 1988). The greenhou se effects of methane in the pres
atmosphere are, therefore, much less than those caused by carbon
dioxide and water vapor. On the other hand, the amo unt of meth
gas hydrate is est imated to be abou t 3000 t imes that in the atmosp
If an appreciable proportion of that methane entered the atm osph
the effect on global temperatures could be significant (MacDo nal1990a).
There has been speculation that methane released from destabil
gas hydrate could affect global cl imate. For example, Nisbe t (19
suggested that methane released from co ntinental gas hydrate cau
warm ing that helped to end the last ice age, and Pau ll et al . (1991
speculated that methan e released from oceanic g as hydrate l imite
extent of glaciat ion during glacial cycles. These scenarios have been proven.
Determining the possible role of gas hy drate in cl imate change
difficult . In fact, just d emonstrat ing the fate of methane from
destabil ized gas hydrate has proven to be elusive. A study test in
hypothesis that gas hydrate in p olar continental shelves is current
decom posing and releasing methan e to the ocean fai led to provid
convincing evidence (Kvenvolden, 19 99). Large excursions in th
carbon isotopic record of carbonate in oceanic sedim ent during th
7/30/2019 GlobalOccurrenceGasHydrate Kvenvolden&Lorenson,2001
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la test Paleocene (Dickens et al . , 1995; 1997b) provided compelling
ev idence tha t me thane f rom des tab i l ized gas hydra te has a f fec ted the
g loba l ocean ic ca rbona te record . Because i t is uncer ta in i f the methane
and i t s ox id ized p roduc t , ca rbon d iox ide , eve r reached the a tmosphere
to affect radiative properties, i .e. , act as greenho use gases, a l ink to
g loba l c l ima te change has no t been demons t ra ted . Thus , the case
relating gas hydrate and global climate change is st i l l open.
Haza rd
A poss ib le connec t ion be tw een gas hydra te and submar ine s lope
fa i lu re was f i r s t ar t icu la ted by McIve r (1977) . Al though no one has
obse rved these fa i lu res tak ing p lace , the ev idence i s ve ry conv inc ing .
For example , a g ian t co l lapse s t ruc tu re on the B lake R idge (Di l lon e t
a l ., 1998) may have resu l ted f rom the ca tas t roph ic re lease o f gas f rom
destabilized gas hydrate. That the giant Storrega slide offshore from
Norw ay was t r iggered by gas re leased f rom gas hydra te has a t t rac ted a
num ber of adherents (Jan sen et al. , 1987; Bugg e et al. , 1987; Mienert et
al . , 1998). These exa mp les and others strongly support the idea that
gas hydra te i s an impor tan t agen t in modify ing the mo rpho logy o f the
sea f loor and cons t i tu tes a s ign i f ican t submar ine geohazard .
Of the th ree g loba l i s sues posed by gas hydra te - - resource , c l ima te,
and hazard - - the la t ter appears to be the mos t re levan t to hum an
welfa re in the nea r te rm (K venvo lden , 1999) . Th is re levance comes
f rom the fac t tha t humank ind i s exp lo i ting the sea f loor a t eve r -
inc reas ing wa te r dep ths in i t s sea rch fo r new sources o f pe t ro leum-based energy . The p resence o f gas hydra te a t o r nea r the sea f loor
ma kes this interval potentia lly unstable. Such instabili ty can affect
eng ineer ing s t ructu res , such as pe t ro leum produc t ion p la t fo rms and
pipeline installations that are located on the seafloor. Thus, the search
fo r pe t ro leum-based energy sources in the deep ocean i s confounded by
the presence of gas hydrate, but the possibil i ty exists that the
confound ing gas hydra te may la te r p rove to be an energy resource
itsellq
CONC LU S I ON
Natura l gas hydra te i s a g loba l phenom enon now rece iv ing
international attention. I t occurs wor ldw ide but is restr icted to two
env i ron men ts - - in po la r con t inen ta l and deep wa te r (mos t ly on ou te r
con t inen ta l and insu la r marg ins ) sed imen t o f the sha l low geosphere .An inven to ry o f g loba l occur rences show s 77 p laces in which the
presence o f gas hydra te i s in fe r red by geophys ica l , geochemica l , and
geo log ica l me thods . Th is inven to ry inc ludes 23 p laces where samples
o f the subs tance have ac tua l ly been recovered . De ta i l s concern ing
ind iv idua l gas -hydra te occur rences and the g loba l impl ica t ions o f gas
hydra te a re d iscussed a t a new wor ld -wide-w eb (ww w) s i te
(_http:/ /walrus.wr.usgs.gov/globalhydrate) . The potential amo unt of
methane in g loba l gas -hydra te occur rences i s ve ry la rge , wi th cu r ren t
es t ima tes converg ing a t abou t 10 exagrams o f methane ca rbon . In te res t
in gas hydrate is increasing because of i ts potential as (1) an energy
source, (2) a factor in climate change, and (3) a submarine geohazard.
Th is new web s i te has been c rea ted to he lp fac i l ita te g loba l gas -hydra te
research.
R E F ER ENC E S
Bugge , T , Bef r ing , S , Be lde rson , RH, E idv in , T , Jansen , E , Kenyon , NH,
Dickens , GR, Cas t i llo , MM, and Walker , JCG (1997b). "A b las t o
in the la tes t Pa leocene : s im ula t ing f i r s t o rde r e f fec ts o f mass ive
d issoc ia t ion o f ocean ic m ethane hyd ra te , " Geology, Vol 25 , pp 25
262.
Dickens , GR, O 'Ne i l , JR , Rea , DK, and Owen , RM (1995) .
"Dissoc ia t ion o f ocean ic methane hydra te as a cause o f the ca rbon
iso tope excurs ion a t the end o f the Pa leocene , " Paleoceanography10, pp 9 65-971.
Dickens , GR, Pau l l , CK, W al lace , P , and ODP Leg 164 Sc ien ti f ic(1997a). "D irec t measurem ent o f in s i tu methane quan t i t ie s in a la
gas-hydrate reservoir ," Nature (London), Vol 385 , No 6615, pp 4
428.
Di l lon , WP, Danfor th , WW , Hutch inson , DR, Drury , RM, Tay lo r
and Booth, JS (1998). "Evidence for faulting related to dissociatio
gas hydra te and re lease o f me thane o f f the sou theas te rn Uni ted S t
in Gas Hydrates--Relevance to Worm Margin Stabi l i ty and ClimaChange, ed i ted by J . -P H enr ie t and J . Miener t , The G eo log ica l So
London , Spec ia l Pub l ica t ion 137 , pp 293-302 .
Ewing , JI , and Hollister , CH (1972). "R egion al aspects o f deep se
wes te rn Nor th At lan t ic ," Ini t ial Reports, Deep Sea D ri l l ing Proje
973.
Ginsburg , GD and So lov iev , VA (1998) . "Subm ar ine Gas Hydra t
T rans la ted f rom Russ ian , Nor ma P ub l i she rs, S t . Pe te rsburg , Russ
216 pp.
Ginsburg , GD, Gram berg , IS , and So lov iev , VA (1990) . "Gas hyd
of continental s lopes," Geologiya N eft i i Gaza, Vol 11, pp 43-45.
Gornitz , V, and Fung, I (1994). "Poten tial distr ibution of methan e
hydra tes in the wor ld ' s ocean , " Global Biogeochemieal Cycles , V
pp 335-347 .
Harr i son , WE, and C ur ia le , JA (1982) . "Ga s hydra tes in sed imen t
ho les 497 and 498A, D eep Sea Dr i l l ing P ro jec t Leg 67 , " Ini t ial
Reports , Deep Sea D ri l l ing Project, 67, pp 591-594 .
Harvey , LDD , and Huang , Z (1995). "Eva lua t ion o f the po ten tia l
impac t o f me thane c la thra te des tab i l iza t ion on fu tu re g loba l warm
Journal o f Geophysical Research, Vol 100 , No D 20 , pp 2905-292
Holbrook , WS, Hosk ins , H, Wood , WT, S tephen , RA, L iza r ralde
and Leg 164 Sc ience Par ty (1996). "M ethane hydra te and f ree ga
the B lake R idge f rom ver t ica l se ismic p ro f i l ing ," Science, Vo127
1840-1843.
Jansen, E, Befring, S, Bugge, T, Eidvin, T, Hotedahl, H, and Sejr
HP (1987). "Large submar ine s l ides on the Norw eg ian con t inen ta
marg in : sed imen ts , t ranspor t and t iming , " Marine Geology, Vol 7
77-107.
JNOC press re lease (2000) . "P roo f o f the methane-hydra te -con ta i
layer in the na tiona l wi ldcat , 'N anka i T rough ' , based on the Na t io
Hol tedah l , H, and Se j rup , HP (1987) . "A g ian t th ree -s tage submar ine s lide o f f Dom es t ic Pe t ro leum Exp lo ra t ion P rogram," Japan Na t iona l Oi l
N o r w a y , " Geo-M arine Let ters , Vol 7, pp 191-198. Corporation , January 20, 2000.
Chersk iy , N, and Makogon , YuF (1970) . "So l id gas - -Wor ld rese rves
a re enormous , " Oil and Gas Internat ional , Vol 10, No 8, pp 82-84.
C ice rone , RJ , and Oremland , R S (1988) . "B iogeochem ica l a spec ts o f
a tmospher ic methane , " Global Biogeochem. Cycles , Vol 2 , pp 299-327
Kvenvo lden , KA (1988) . "Methane hydra temA majo r rese rvo i r
ca rbon in the sha l low geosphere? , " Chemica l Geo logy , Vol 71, p
51 .
466
7/30/2019 GlobalOccurrenceGasHydrate Kvenvolden&Lorenson,2001
http://slidepdf.com/reader/full/globaloccurrencegashydrate-kvenvoldenlorenson2001 6/6
Kvenvolden, K A (199 9). "Potential effects of gas hydrate on hum an
welfare," Proceedings o f the National Academy o f Sciences (USA), Vol96, pp 3420-3426.
Kvenvolden, K A (2 000). "Natural gas hydrate: introduction and history
of discovery," in Natural Gas H ydrate in Oceanic and PolarEnvironments, M. Max, ed., Kluwer Academic Publishers.
Kvenvolden, KA , and M cMenamin, M A (1980). "Hydrates of natural
gas: a review o f their geologic occurrence," U.S. G eological SurveyCircular 825, 11 pp.
Kvenvolden, KA, Ginsburg, GD, and Soloviev, VA (1993).
"Worldw ide distribution of subaquatic gas hydrates," Geo-MarineLetters, Vol 13, pp 32-40.
MacDonald, GT (1990a). "Ro le of methane clathrates in past and
future climates," Climate Change, Vol 16, pp 247-281.
MacDonald, GT (1990b). "The future of methane as an energy
resource," Annual Reviews o f Energy, Vol 15, pp 53-83.
Makogon, YuF (1997) "Hydrates of Hydrocarbons," PennW ell, Tulsa,
Oklahoma, 482 pp.
Makogon, YuF., Trebin, FA, Trofimuk, AA, Tsarev, VP, and Cherskiy,NV (1971). "Detection o f a pool of natural gas in a solid (hydrated gas)
state," Doklady Akademii Nauk SSSR, Vol 196, pp 203-206
[Translation in Doklady-Earth Science Section,Vol 196, pp 197-200,
1972].
Makogon, YuF, AA Tsarev, VP, and Cherskiy, NV(1972). "Formation
of large natural gas fields in zones o f permanently low temperatures,"
Doklady Academii Nauk SSS R, Vo1205, pp 700 -703 [Translation in
Doklady-Earth Science Section, Vo1205, pp 215-218, 1973].
McIver, RD (1997). "Role of natural gas--important agent in
geological processes," Geol. Soc. Am. A bst. Programs, Vol 9, pp 1089-
1090.
Mienert, J, Posewang, J, and Baum ann, M (1998). "Gas hydrates alongthe northeastem Atlantic margin: possible hydrate-bound m argin
instabilities and possible release of methan e," in Ga sHydrates--Relevance to W orm Marg in Stability and Climatic Change,edited by J.-P Henriet and J. Mienert, The Geological Society, London,
Special Publication 137, pp 275-291.
Nisbet, EG (199 0). "The end of the ice age," Canadian Journal o fEarth Science, Vol 27, pp 148-157.
Panayev, VA (1987). "Gas hydrates in the oceans," InternationalGeology Review,V ol 29, pp 569-602.
Paull, CK, Ussler III, W, an d Dillon, WP (1991). "Is the extent of
glaciation limited by m arine gas-hydrates?," Geophysical Research
Letters, Vol 18, pp 432-434.
Potential G as Comm ittee (1981). "Potential Supply of Natural G as in
the United States (as of Decem ber 31, 19 80)," Potential Gas Agency,Colorado School of Mines, Golden, Colorad o, 119 pp.
Shine, EG, Derwent, RG , Wuebbles, D J, and Morcrette, J-J (1990). "Radiative
forcing of climate," in Climate Change, The IPCC Scientific Assessment, editedby JT Houghton, G J Jenkins, and JJ Ephraums, Cam bridge University Press,New Y ork, pp 41-68.
Shipley, TH, and Didyk, BM (1982). "Occurrence o f methane hoffshore southern M exico," Initial Reports, Deep Sea Drilling P66, pp 547-555.
Shipley, TH, Houston, MH , Buffier, RT, Shaub , FJ, McMillen,
Ladd, JW, and Worzel, JL (197 9). "Seismic reflection evidence
widespread occurrence o f possible gas-hydrate horizons on cont
slopes and rises," American Association o f Petroleum G eologisBulletin, Vol 63, pp 2204-2213.
Stoll, RD, Ewing, JI, and Bryan, G M (197 1). "Ano malou s wave
velocities in sediments containing gas hydrates," Journal ofGeophysical Research, Vol 76, pp 2090-2094.
Vasil 'ev, VG, Makogon, YuF, Trebin, FA, Trofimuk, AA, and
Cherskiy, NV (19 70). "The property o f natural gases to occur in
Earth crust in a solid state and to form gas hydrate deposits," Otv SSSR, 1968-1969, pp 15-17.
Yefremova, AG , and Zhizhchenko, BP (19 74). "Occurrence of c
hydrates of gas in sediments of mod em m arine basins," Doklady
Akademii Nauk SSSR,Vol 2 14, pp 1179- ! 181.
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