Formation on Hydrates

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Formation of submarine gas hydratesV. SOLOVIEV & G.D. GINSBURG

!^ Soloviev, V. & G insburg, G.D.: Formation of submarine gas hyd rates.Bulletin of the GeologicalSociety of Denmark, Vol. 41, pp. 86-94. Copenhagen, 1994-03-30.Submarine gas hydrates have been discovered in the course of deep-sea drilling (DSDP andes ODP) a n d bot tom sampl ing in many of fshore regions . This paper repor t s on expedi t ions car r iedout in the Black, Caspian a n d O khot sk Seas . G a s hydrate accumulat ions were di scovered a n dinves t igated in al l these areas . T h e data a n d a n analys i s of the results of the deep-sea dri l l ingprogramme sugges t tha t t h e infi l t rat ion of gas-bearing fluids i s a necessary condi t ion for gashydrate accumulat ion. This is conf i rmed b y geological observat ions a t three scale levels. First ly,hydrates i n cores a re usual ly associa ted wi th comparat ively coarse-gra ined, permeable sedimentsas wel l as voids a n d f rac tures . Secondly, hydrate accumulat ions a re cont rol led b y pe r meab l egeological s t ructures , i .e . faults, diapirs, m u d vo l canos as w el l a s l ayered sequences . Thi rdly , inthe wor ldwide scale , hydrate accumulat ions a re characterist ic of continental slopes a n d rises a n din t ra-cont inenta l seas where submar ine seepages a l so a re widespread. Both biogenic and ca t -agenic g a s m a y occur , and t he gas sources m a y b e located at various distances from t h eaccumul a t i on . G a s hydrates presumably or iginate f rom water -di ssolved gas . T h e poss ibi l i ty o f at ransi t ion from dissolved g a s in to hydrate is conf i rmed b y exper imenta l data . Shal low g a shydrate accumulat ion s associa ted wi th gas-bear ing f luid plumes are t h e most convenient fea turesfo r t h e s tudy of submar ine hydrate format ion in genera l . These accumulat ions a re know n f r omthe Black, Caspian a n d O khot sk Seas , t h e Gulf of M e x i c o and off northern California.V. Soloviev & G. D. Ginsburg, Research Institute for Geology and Mineral Resources of theOcean "VNIIOkean geologia". pr.Maklina 1, St. Petersburg 190121, Russia Augu st 23rd, 1992 .

IntroductionMore than forty regions are known today where gashydrates have been discovered or where they may beinferred from indirect evidence (Fig. 1) . Submarine gashydrates have been discovered in the course of deep-seadrilling and bottom sampling in the Caspian Sea (Efre-mova et al. 1979; Ginsburg et al. 1988), Black Sea (Efre-mova & Zhizhchenko 1974; Ginsburg et al. 1990), Okhotsk Sea (Zonenshain et al. 1987; Ginsburg et al. 1993)Japan Sea (Shipboard Scientific Party 1990), Gulf ofM exico (Brooks et al. 1984), Blake - Outer Ridge (Kvenvolden & Barnard 1983) and on the slopes of the following deep-sea trenches: Middle-American (Shipley & Di-dyk 1982; Harrison & Curiale 1982; Kvenvolden &McDonald 1985), Peruvian (Kvenvolden & Kastner1990) and Nankai (Shipboard Scientific Party 1991). Allgas hydrate regions are associated with sedimentary basins with young, thick, rapidly accumu lating sedimentarycover on continental slopes, rises and in intra-continentalseas. However, the origins of gas hydrates are poorlyunderstood.Gas hydrate formation is believed to be a commongeological phenomenon occurring under deep-water conditions, although only under certain geological conditions. The elucidation of the geological significance of

submarine gas hydrate formation is of undoubted interest,since it is associated with alteration of the overall state offluids in the sediments.The aims of this paper are to investigate the mostfavourable conditions for gas hydrate formation in marine sediments and to describe its common features basedon our cruise and experimental data and on deep-seadrilling studies.

Experimental dataThe possibility of gas hydrate formation from water-dissolved gas is confirmed by experimental data. Thesolubility of gas in water in equilibrium with hydrate islimited by the equilibrium pressure of hydrate formationat a given temperature rather than by hydrostatic pressure(Makogon & Davidson 1983). The lower the temperature, the lower is the equilibrium pressure and, correspondingly, the gas solubility. Formation of gas hydratesfrom dissolved gas under static conditions has been experimentally proved (Namiot & Gorodetskaya 1969). Forthis paper the mechanism of gas hydrate formation fromgas dissolved in flowing w ater has been tested (M ashirovet al. 1991). Freon-12 (CF2C1 2) was used as the hydrate-

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Fig. 1. Gas hydrate locationsin the Ocean. Gas hydrateobservation: 1 - deep seadrilling, 2 - bottomsampling; geochemicalindications: 3 - lowchlorinity of pore water, 4 -gassy cores; geophysicalindications: 5 - logging data;6 - seismic survey data.

forming gas. The low dissociation pressure of freon-12hydrate in comparison with hydrate of natural gas pro vided better conditions for the experiment. Moreover,like hydrocarbon gases, freon-12 hy drate is hydrophobic.Fig. 2 shows the arrangement of the experimentalequipment. The vessel for water saturation with gas (1)and the reaction vessel (2) are the main items. The experimental conditions were as follows: P = 200 kPa; T l =284.9 K; PI = 432 kPa; T2 = 274.3 K; P2 = 46.5 kPawhere P is the gas pressure in the plant during the time ofwater saturation with gas and the beginning of gas hydrate formation. T l is the temp erature in the saturationvessel; PI is the equilibrium pressure of freon-12 hydrateformation at T l; T 2 is the temperature in reaction vesseland P2 is the equilibrium pressure of freon-12 hydrateformation at T2.As a result of the experiment the freon-12 hydrate wasobtained in the reaction vessel directly from aqueoussolution. Gas hydrate formation was observed visuallyand pressure in the plant was decreased from 200 to 170kPa (Fig. 3) . Oversaturation of the water w ith gas in thereaction vessel was 3.4 at P = 200 kPa at the beginning ofgas hydrate formation and 2.9 at P = 170 pKa at the endof the experiment.

Fig. 2. Scheme of the experimental set. 1 - vessel for watersaturation with gas, 2 - buffer volume for gas separation, 3 -reaction vessel, 4 - refrigerator-thermostat, 5 - filter, 6 - glassfinger, 7 - gas-collector, 8 - magnetic pump, 9 - pressure gauge,10 - magnetic mixer, 11 - valves, 12 - resistance thermometer,13 - system for gas pumping, 14 - system for filling withsolution; A - liquid phase; B - gaseous phase.Soloviev & Ginsburg: Formation of submarine gas hydrates 87


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160 10 20 30 40 30 ?0 SO

fO 20 30 40 50 60 70 80

Fig. 3. Plot of pressure (P, kPa) and difference between temperature of thermostat and temperature behind reaction vessel (AT,C) vs. gas hydrate formation time (hours)

Observational dataThe Caspian SeaSpecific features of the southern Caspian Sea are thethick Cenozoic sedimentary cover, high oil and gas potential and the wide distribution of clay diapirism andmud volcan ism. M ore than fifty m ud volcanoes arethought to occur in deep-water areas of the sea (Fig. 4)

Fig. 4. Gas hydrate accumulations in the deep southern CaspianSea. 1 - detected gas hydrate accumulations at the mud volcanoes, 2 - submarine mud volcanoes, 3 - limit of the potentialgas hydrate prone area.and gas hydrate accumulations associated with these mudvolcanoes have been discovered (Efremova et al. 1979;Ginsburg et al. 1988). Diapiric structures and mud volcanoes are clearly displayed on medium frequency seismicrecords (Fig. 5).Gas hydrates were observed in two mud volcanic craterfields in a clay breccia, one occurrence being immediately on the sea bottom. The hydrate inclusions in thebreccia were up to 5 cm and of various shapes of whichthe most common were subisometric, though sometimesthin plates were observed. The hydrate content in the

Fig. 5. Seismic profile acrossthe Vezirov Rise (ShatskyRidge). Arrows indicatesampling stations; the leftscale is two-way traveltime(s).

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Table 1. Composition of gas from the gas hydrates.RegionCaspianSe a

BlackSeaOkhotskSe a

AreaBuzdag, mudvolcanoElm, mudvolcanooffshorethe Cr imeaoffshore Para-mushir islandoffshoreSakhalinisland

n.a. = not analyzedn.d. = not detected

SC+co2%99.093.499.543.098.299.274.097.996.2

100.0

Composition of gasC ,

74.776.095.381.499.199.199.999.999.397.0

c217.419.30.615.30.020.04


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canic brine and the accumulation of gas hydrates is associated with mud volcanoes and depends on the movements of fluids.The Black SeaT he gas hydrates in the Black Sea w ere discovered on thelower part of the Crimean continental slope at a depth of2050 m (Ginsburg et al. 1990)(Fig. 6) where gas hydrateaccumulation is associated with diapiric structures andmud volcanoe s (Fig. 7) . Th e hydrates w ere found bo th inclay breccia, where they formed inclusions ranging fromdispersed to massive, and in deformed silt-clay sedimentswhere the hydrates were observed as thin plates.Methane is the main component of the Black Seahydrates (see Table 1) . The water responsible for gashydrate formation is of a lower salinity than sea water. Itscomposition is similar to that of the mud volcanic watersin the Kerch-Taman region. The 8I3C values of methanefrom gas hydrates measured in two samples are -61.8%oand -63.4%o which is typical of mud volcanic gases fromthe same region. Hence, the hydrate-forming fluids off-shore from the Crimea must have come from the deepsedimentary cover and the gas hydrate formation is due toinfiltration.The Okhotsk SeaGas hydrate accumulations connected with submarinegas seepage plumes were discovered and investigatedover tw o areas of the Okhotsk Sea (Ginsburg et al. 1992)(Fig. 8). The locations of submarine seepages are easilydetected using echo sounding (Fig. 9) and coincide withfracture zones. Eleven gas plumes were identified in theOkhotsk Sea, one plume near Paramushir Island at adepth about 800 m and ten plumes on the continentalslope off Sak halin Island at depths of 620 to 1040 m. G ashydrate-bearing sediments were recovered in both regions at subbottom depths of 0.3-1.2 m.The hydrate cores obtained near Sakhalin Islandshowed subhorizontal lenticular-bedded structure causedby hydrates. The water content in the hydrate-bearingsediments ranged from 65 to 66% in contrast to 48% to60% in the overlying sediments. We consider that thesubhorizontal structure and specific water distributionnear the upper boundary of the hydrate-bearing sedimentsare brought about by the formation of hydrates fromupward diffusing methane whilst water moved in theopposite direction to the front of the reaction.All the gas hydrate occurrences in the Okhotsk Sea areassociated with carbonate cementation of the sediments.Many carbonate concretions were observed together withhydrates and allochthonous calcium carbonate depositswere noted on mollusc shells . These features are quitenormal under gas seepage conditions. T he carbonate concretions are the result of methane oxidation which saturates the pore water with carbon dioxide; calcium carbonate is then precipitated (Zonenshain et al. 1987).

Fig. 7. Seismic profile across diapirs in the Black Sea, Feodosiaregion. Dotted lines are diapir limits; figures on the verticalscales are two-way traveltime (s).According to the data from deep seismic and continuous seismic profiling, the area of fluid discharge nearParamushir Island coincides with a high of the acousticbasement. The loss of definition and the pull-down ofreflectors are presumably attributable to low seismic velocity in this part of the sedimentary sequence. This ismost probably associated with the occurrence of gas-charged sediments above the acoustic basement. The submarine fluid seepage fields off Sakhalin Island are in thezone in which there are numerous submeridianal faultsalong the west side of the Deriugin Basin near the oil andgas areas of Sakhalin Island and the adjacent shelf. T hese

faults are most likely conduits for migrating gas as is90 Bulletin of the Geological Society of Denmark


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Fig. 10. Lithological and hydrogeochemical control of gas hydrate locations, Site 491 DSDP, offshore Mexico (Initial Reports..., 1982). A - subbottom depth, m; B - geological age; C -particle size distribution, % (1 - sand, 2 - silt, 3 - clay); D - gashydrate locations; E - chlorinity of pore w ater, %o.indicated by the seismic evidence of gas-charged sediments .

American Trench is a typical example. On seismic profiles, the distinct landw ard-dipping reflectors (LDR) mayindicate the routes of squeezed gas-bearing fluids migrating to the gas hydrate formation zones (Fig. 12). Widespread bottom simulating reflectors (BSR) identified asthe base of gas hydrate-bearing sediments (Shipley et al.1979) are indicative of gas hydrate formation also involving upward migrating gas-bearing fluids.Discussion and conclusionsGenerally, the presented data support the assumption thatfluid infiltration is trie major hydrate-forming process inthe submarine environment. This may be observed atdifferent scales - from individual specimens to worldwide localities.1. Comparatively permeable lithological horizons andfractures are present in hydrate-bearing intervals of

ocean drilling holes. The structures of hydrate-containing sediments also suggest that hydrates shouldform by moving fluids and should be associated withfaults. Gas hydrates fill fractures and large voids andcement comparatively coarse-grained sediments.2. In regions with hydrates, it is possible to establish adirect relationship or spatial link betw een hyd rate occurrences and geological structures w hich may controlthe movements of fluids towards the sea bottom.These are faults, diapirs, mud volcanoes, crests ofanticlines, layered sequences and updipping reflectorsprobably caused by bedding or fracturing.3. Hydrate occu rrences are restricted to continental margins and to intra-continental and marginalseas. Submarine seepages (of various origins) and pockmarkareas coincide with the same worldwide structures.This is well il lustrated by the map of gas hydrate

Other locationsData on gas hydrates recovered by bottom sampling inthe Gulf of Mexico (Brooks et al. 1984) and offshoreNorth California (Brooks et al. 1991) also prove that gashydrate occurrences are associated with submarine fluidseepage areas and, in particular, with mud volcanoes.According to the deep-sea drilling data , the holes thatrecovered hydrate-containing sediments (Legs 66, 67, 76,84, 96, 112, 127, 131 and 146 DSDP-ODP) show acertain lithological control of gas hydrates. Normallytheir occurrences are associated with relatively porousash and silty sands (Fig. 10) as well as fracture zones(Fig. 11).All the areas w here gas hydrates have been discoveredin the course of deep-sea drilling are characterized byspecific geological and hydrogeological settings in whichfluids are squeezed from the sediments. This may beeither by mere simple consolidation (e.g. Blake OuterRidge area) or by tectonic overpressure in subductionzones (deep water trenches) . The slope of the Middle Fig. 11. Fractures in hydrate-containing site sections accordingto paleomagnetic data, offshore M exico (Initial Reports..,1982).92 Bulletin of the Geological Society of Denmark


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Fig. 12. Characteristicseismic reflectors which arepresumed to be fluid-conductors in hydrate-bearing region, MiddleAmerican Trench, offshoreM exico (Initial Reports...,1982, 1985). BSR - BottomSimulating Reflection; LDR- Landward DippingReflection; 486-492 - DSDPSites.

- 487 486

Crust o{ theocean

-^

1 0 - LKttl

. ^ . "" , I K X . X X

\ L D R / x x *

10 20Kmi .10Km

locations in the oceans (see Fig. 1) and the map ofpock mark and seepage distribution (Hovland and Judd1988: Fig. 4.73).Gas hydrates in the submarine environment may beformed both from water-dissolved gas and from free gas.The formation of gas hydrates in these contrasting situations is substantially different (Fig. 13). Water has animportant advantage over gas. Dissolved salts, concentrated as a result of the transformation of the water fraction into hydrate, are remo ved by filtering th e flow . In thecase of gas infiltration, salts remain in the reaction zoneand inhibit the process. Hydrate films forming at thegas-w ater interface are also inhibitors. Th us, the kineticsof hydrate formation from free gas are evidently characterized by the fact that diffusion plays a significant role.In the vicinity of an ascending flow of gas-saturatedwater, the reaction zone seems to be located where gashydrates are forming from diffusing gas.Natural gas hydrates derived from gas-saturated infiltrating w ater are likely to be the most common. T he m ostsignificant hydrate-forming infiltration takes place underthe conditions in which pore water is released from sediments by geostatic pressure and tectonic overpressure. Asimilar model has been recently developed by Hyndman(1990) on the basis of seismic survey data (BSR). In thismodel, gas hydrates form from pore fluids that are ex

pelled from an accretionary wedge. Another cause ofwater infiltration might be thermo-artesian pressureforming under local heating from a magma chamber or acooling intrusion. Sediments heated in this way maygenerate hydrocarbon gases.Thus , gas hydrates form mainly from fluids infiltratingtowards the sea floor through or from a thermobarichydrate stability zone. This zone appears to act as anocean-wide gas-geochemical barrier.

Dansk sammendragDet foreliggende arbejde beskriver forekomsten af gashydrater og deres kemisk e samm nenstning fundet i Sortehavet, det Kaspiske Hav og i det Okhotske Hav.Dannelsen af gashydrater er et almindeligt forekommende geologisk fnomen i det submarine milj i forbindelse med relativt dybt vand (500-2000 meter) . Gashydrater findes som regel p kontinentalskrninger og imarginale eller intra-kontinentale have, ofte i forbindelsemed sedimentre bassiner med et relative tykt dkke afhurtigt akk umulerende unge sedimenter. rsagen til dannelsen af gashydrater er imidlertid kun drlig kendt.Gashydrater dannes almindeligvis udfra oplst gas iformationsvandet, men laboratorieforsg tyder p, at gashydrater ogs kan dannes i forbindelse med fri gas. Gas-

Fig. 13. Gas hydrateformation zones and fluidmigration in vicinity ofseabed seepages of gas-saturated water (A) and freegas (B). 1 - fluid-conductingzone, 2 - main direction offluid flow, 3 - zone ofhydrate formation byprecipitation from filteringw ater, 4 - zone in whichdiffusion is sufficient to leadto the formation ofconcretionary hydrates, 5 -zone of gas-undersaturatedwater, 6 - direction ofdiffusion flow of dissolvedgas, 7 - opposite directionof water flow into thehydrate formation zone.

. . . . -* / V \ V \ / it?'.--..y*'+ij-)(z A""*-,/

Sea Bottom**>> r A i t . . * \ -:i

o I + / V " I

\Z3 DQ EZX'

: : 5

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hydrater findes overvejende i porse og permeable over-fladenre sedimenter, og de geologiske forhold tyder p,at deres dannelse er betinget af infiltration af gasholdigtformationsvand fra dybere liggende lag. Forekomsten afgashydrater er sledes som regel forbundet med andregasrelaterede geologiske fnomener s som submarinegasudslip, mudder vulkaner, pockmarks og gasmttedesedimenter. Disse overfladenre fnomener kan ofte relateres til dybere liggende geologiske strukturer, somf.eks. forkastninger og diapirer, som kan vre den udlsende rsag til migration af fri gas eller gasmttetformationsvand til de overfladenre sedimenter og dermed dannelsen af gashydrat.

ReferencesBrooks, J. M ., Field, M . E. & Kennicutt, M . C. II 1991. Observations of gas hydrates in marine sediments, offshore Northern California. Marine Geology 96: 103-109.Brooks, J. M ., Kennicutt, M . C. II, Fay, R. R., McDonald, T. J.& Sassen, R. 1984. Thermogenic gas hydrates in the Gulf ofMexico. Science 225, 4660: 409^111.Efremova, A. G., Gritchina, N. D., Kulakova, L. S., Rateev, M .A. & Turovsky, D. S. 1979. About discovery of the gashydrates on the bottom of Southern Caspian Sea. EI VN IIE-Gasprom, seria: Geologia, burenie i razrabotka gasovykhmestorozhdeniy, 21, Moskwa: 12-13 (in Russian).Efremova, A. G. & Zhizhchenko, B. P. 1974. Discovery of gashydrates in sediments of recent aq uatoriums. Doklady Ak ade-mii Nauk SSSR 214, 5: 1129-1130 (in Russian).Ginsburg, G. D., Gramberg, I. S., Guliev, I. S. Guseinov, R. A.,Dadashev, A. A., Ivanov, V. L ., Krotov, A. G., M uradov, Ch.S., Soloviev, V. A. & Telepnev, E. V. 1988. Subaquaticmud-volcanic type of gas hydrate accumulations. DokladyAkademii Nauk SSSR 300, 2: 416-418 (in Russian).Ginsburg, G. D., Kremlev, A. N., Grigor'ev, M . N., Larkin, G.V., Pavlenkin, A . D. & Saltykova, N. A . 1990. Filtrogenic gashydrates in the Black Sea (twenty-first voyage of the researchvessel "Evpatoriya"). Soviet geology and geophysics (Geolo-giya i Geofizika) 31, 3, Allerton Press: 8-16.Ginsburg, G. D., Soloviev, V. A., Cranston, R. E., Lorenson, T.D. & K venvolden, K. A. 1993. Gas hydrates from the continental slope, offshore Sakhalin Island, Okhotsk Sea. Geo-Marine Letters 13: 41^48.Harrison, W. E. & Curiale, J. A. 1982. Gas hydrates in sedimentsof holes 497 and 498A, Deep Sea Drilling Project Leg 67.Initial Reports of the Deep Sea Drilling Project 67, Washington D.C., U.S. Government Printing Office: 591-594.Hovland, M . & Judd, A. G. 1988. Seabed pockm arks and seepages. Impact on geology, biology and the marine environ

ment. Graham & Trotman, London/Dordrecht/Boston.

Hyndman, R. D. 1990. A test of a model for the formation ofmethane hydrate and seafloor bottom simulating reflectors bydrilling on the northern Cascadia subduction zo ne. Proceedings of the second Canadian O DP Proposals Workshop, Feb.22-24, 1990: YII-2-YII-9.Initial Reports of the Deep Sea Drilling Project 1982 66, Washington D.C., U.S. Government Printing Office.Initial Reports of theDeep Sea Drilling Project 1985) 84, Washington D.C., U.S. Government Printing Office.Initial Reports of the Deep Sea Drilling Project 1986 96 Washington D.C., U.S. Government Printing Office.Kvenvolden, K. A. & B arnard, L. A . 1983. Gas hydrates of theBlake Outer Ridge Site 533, DSDP/IPOD Leg 76. InitialReports of the Deep Sea Drilling Project 76, WashingtonD.C., U.S.Government Printing Office: 353-3 65.Kvenvolden, K. A. & Kastner, M. 1990. Gas hydrates of thePeruvian outer continental margin. In Suess E., von Huene R.,et al.1990. Proc. ODP, Sci.Results, 112, College Station, TX(Ocean Drilling Program): 517-526.Kvenvolden, K. A. & M cDonald, T. J. 1985. Gas hydrates of theM iddle America Trench - Deep Sea Drilling Project Leg 84.Initial Reports of the Deep Sea D rilling Project 84, W ashington D.C., U.S. Government Printing Office: 667-682.M akogon, Yu. F. & Davidson, D. V. 1983. Affect of ex cessivepressure on the stability of methane hy drate. Gazovaya Pro-myshlennost' 4: 37^40 (in Russian).M ashirov, Yu. G., Stupin, D. Yu., Ginsburg, G. D. & Soloviev,V. A. 1991. Experience of hydrate formation by dissolvedgas. Doklady Akademii Nauk SSSR 316, 1: 205-207 (inRussian).Namiot, A. Yu. & Gorodetskaya, A . I. 1969. Alteration of gassaturation of w ater under the cooling. Gazovaya Promyshlen-nost' 11: 20-22 (in Russian).Shipboard Scientific Party 1990. Site 796. In Tamaki, K., Pis-ciotto, K., Allan, J. et al., Proc. ODP, Init.Repts. 127, CollegeStation, TX (Ocean Drilling Program): 247-322.Shipboard Scientific Party 1991. Site 808. In Taira, A., Hill, I.,Firth, J. V., et al., Proc. ODP, Init.Repts. 131, College Station,TX (Ocean Drilling Program): 71-269.Shipley, T. H. & Didyk, V. M. 1982. Occurrence of methanehydrates offshore Southern Mexico. Initial Reports of theDeep Sea Drilling Project 66. Washington D.C., U.S.Government Printing Office: 547-555.Shipley, T. H., Houston, M. H., Buffler, R. T., Shaub, F. J.,M cM illen, K. J., Ladd, J. W & Worzel, J. L. 1979. Seismicreflection evidence for widespread possible gas hydrate horizons on continental slopes and rises. AAPG Bull. 63 : 2204 -2213.Zonenshain, L. P., Murdmaa, I. O., Baranov, B . V.; Kuznetsov,A. P., Kuzin, V. S., Kuz'min, M. I., Avdeiko, G. P., Stunzhas,P. A., Lukashin, V. N., Barash, M. S., Valjashko, G. M. &Demina, L. L. 1987. Subaqueous gas seepage in the OkhotskSea off w est Paramushir Island. Okeanologia 27, 5: 795 -800(in Russian).

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Formation on Hydrates

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