Water segregation in the course of gas hydrate formation and accumulation in submarine gas-seepage fields

MARINE

GEOLOGY INT~ANATIONAL

JOURNAL OF MARINE

GEOLOOY, GEOCHEMISTRY AND GEOPHYSICS

ELSEVIER

MarineGeology 137 (1997) 59-68

Water segregation in the course of gas hydrate formation and accumulation in submarine gas-seepage fields V.A. Soloviev, G.D. Ginsburg Research Institute for Geology and Mineral Resources of the Ocean, St. Petersburg, Russia

Received 22 December 1995; revision 12 May 1996; accepted 21 August 1996

Abstract Gas hydrate accumulations associated with submarine gas-seepage fields have been detected in the Okhotsk Sea and the Gulf of Mexico. It is possible that gas hydrates known from the continental margin offshore northern California, Oregon and Nigeria are also connected with gas-seepage fields. Gas-hydrated sediments in the accumulations (1) occur at very low subbottom depths, sometimes directly at the sea bottom; (2) have specific structures, resembling cryogenic structures, caused by hydrates (subhorizontal lenticular-bedded and porphyraceous structures); (3) the sediments can be distinguished from overlying non-hydrated sediments by a higher water content; (4) an association of gas hydrates and calcium carbonate concretions occurs. The behaviour of gas and water during gas hydrate formation under bottom gas-seepage conditions is considered. Gas hydrate formation occurs at the upper part of the sedimentary cover, within the limits of the diffusion halo around the up-going flow of gas which saturates the pore water. Water migrating to the gas hydrate accumulation sites from adjacent sediments is involved in hydrate formation as well. The mechanism of gas transportation to the gas hydrate accumulation sites is diffusion. The way of water transport during hydrate accumulation is similar to ice segregation during permafrost formation. The water segregation is responsible for the formation of specific structure caused by hydrates and for higher than usual water content of hydrate-bearing sediments in submarine gas-seepage fields. Keywords: gas hydrates; water segregation; formation; accumulation; gas seepage; Okhotsk Sea

1. Introduction

Submarine gas hydrate accumulations are known from a great number (16) of ocean regions (Sakai et al., 1990; Kvenvolden et al., 1993; Brooks et al., 1994). Gas hydrates have been discovered by deep-sea drilling at various subbottom depths (from 26 to 404 m) offshore Peru (Kvenvolden and Kastner, 1990), Costa-Rica (Kvenvolden and McDonald, 1985), Guatemala (Harrison and Curiale, 1982; Kvenvolden and McDonald, 1985) and Mexico (Shipley and Didyk, 1982), south-east

(Kvenvolden and Barnard, 1983) and west (Westbrook et al., 1994) of the United States, offshore Japan (Tamaki et al., 1990; Taira et al., 1991)and the Gulf of Mexico (Pflaum et al., 1986). By bottom sampling and submersible dives and sampling in another nine regions, the Black and Caspian seas (Yefremova and Zhizhchenko, 1974; Kremlev and Ginsburg, 1989; Yefremova and Gritchina, 1981; Ginsburg et al., 1992), offshore northern California (Brooks et al., 1991), north of the Gulf of Mexico (Brooks et al., 1984), offshore Nigeria (Brooks et al., 1994), the

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VA. Soloviev, G.D. Ginsburg / Marine Geology 137 (1997) 59-68

Mid-Okinawa Trough (Sakai et al., 1990) and in the Okhotsk Sea (Zonenshayn et al., 1987; Ginsburg et al., 1993; Soloviev et al., 1994). All known gas hydrate occurrences can be distinguished into two main groups: (1) gas hydrates associated in some or other way with fluid discharge areas at the sea bottom and (2) gas hydrates which do not have this connection. Gas hydrate formation and accumulation in deep water conditions occur mainly by filtration of gas-bearing fluid through thermobaric gas hydrate stability zone, and hydrates are formed predominantly by water-dissolved gas (Ginsburg, 1990; Soloviev and Ginsburg, 1994). Our previous data suggest that submarine gas hydrate accumulation occurs either as a result of precipitation from filtering gas-saturated water or through segregation of water by migrating gas. The latter takes place particularly in the course of hydrate formation in submarine gas-seepage fields. The main purpose of this paper is to consider the behaviour of gas and water during gas hydrate formation and to define the conditions for water segregation under gas hydrate formation with special reference to gas-seepage fields.

2. Data Gas hydrate accumulations associated with submarine gas-seepage fields have been detected in the Okhotsk Sea (Fig. I) and the Gulf of Mexico (Brooks et al., 1986). It is possible that the abovementioned gas hydrate accumulations on the continental margin offshore northern California, Oregon and Nigeria are connected with gas-seepage fields also. Gas hydrates in the Okhotsk Sea are known from offshore Paramushir Island and the continental slope off Sakhalin Island. Gas hydrate accumulations were discovered and investigated by expeditions in 1986 onboard R/V Akademik Mstislav Keldysh (Zonenshayn et al., 1987) and in 1991 onboard R/V Geolog Piotr Antropov (Ginsburg et al., 1993; Soloviev et al., 1994). Eleven submarine gas-seepage fields were discovered within the gas hydrate prone area offshore Paramushir and Sakhalin Islands. It is likely that

there are much more gas-seepage fields in these regions. At two fields gas hydrates were recovered by gravity corer, and at two other fields the presence of gas hydrates is inferred from pore water and gas composition data (Ginsburg et al., 1993; Soloviev et al., 1994). The submarine gas-discharge field associated with a gas plume in the water column offshore Paramushir Island is located in water depths of 790-800 m. About two dozens echo-sounder intersections show that the plume diameter is 350-400 m near the sea floor (Fig. 2). Ten gasseepage fields were revealed on the continental slope offshore Sakhalin Island in water depths of 620-1040 m. They occur in a comparatively narrow zone which is less than 20 km wide and about 130 km long. At one gas-seepage field estimated at about 250 m diameter gas hydrates were recovered at 5 sites (Fig. 3). Gas-hydrated sediments in the investigated accumulations are characterized by a number ofcharacteristic features. First, they occur at very low subbottom depth (Table I). In the Gulf of Mexico gas hydrates have been observed from submersibles directly at the sea floor (MacDonald et al., 1994); gas hydrates in the Okhotsk Sea have been recovered by bottom sampling at subbottom depth within 1 m of the seabed (Ginsburg et al., 1993; Soloviev et al., 1994). Even a short-period existence of gas hydrates near the seabed is only possible in the case of continuous gas saturation of water, co-existingwith gas hydrates, which is only feasible under the condition of continuous gas flow. Secondly, the specific structures caused by hydrate formation are typical. These structures resemble the cryogenic structures forming under water segregation during freezing of silt-sized sediments (Popov, 1967). Two types of hydrated structure have been observed in the Okhotsk Sea: subhorizontal lenticular-bedded, and porphyraceous structures. According to our observations in the Okhotsk Sea the former is presented by subparallellenticular streaks and by hydrate layers more or less equal in thickness included in the main body of diatomaceous sediments (Fig. 4). The thickness of hydrate layers ranges from a few fractions of a millimetre to several millimetres.

V.A. Soloviev, G.D. Ginsburg / Marine Geology 137 (1997) 59-68

,•

61

.•

Fig. 1. Overall outline of the Okhotsk Sea with the study areas where gas hydrates associated with gas-seepage fields have been observed.

The gas hydrate content within the hydrate-bearing intervals has reached up to 30-40% of sediment volume (Ginsburg et aI., 1993). No other layering except hydrated has been observed. The porphyraceous structure of the hydrated sediments is caused by the presence of more or less isometric inclusions of gas hydrates, which are relatively regularly distributed in the sediments. The size of the individual inclusions reaches a few centimetres. Thirdly, the anomalous water content distribution in the sedimentary section is of major importance. In the area offshore Sakhalin Island the natural water content of hydrate-containing sediments based on 3 sampling sites is 65-66%, including hydrate-forming water released by gas hydrate decomposition; the water content of the overlaying non-hydrate sediments does not reach 60% (Fig. 5). It is hard to assume that the water content naturally increases with increasing subbottom depth within uniform silty-clay sediments in the

upper part of subbottom sedimentary section. Therefore it is clear that not only the initial pore water, but also the water immigrating from outside is present. Examples of such water content distribution are entirely unknown. It should be emphasized that the water content is the total, including water from gas hydrates. But the residual water content, that is water of inter-hydrated intervals proved to be essentially lower. So, according to balance calculations, performed for one of the sites near Sakhalin Island, the residual non-hydrated water content of hydrate-bearing sediments does not even reach 45% (Table 2). Fourth, the association of gas hydrates and calcium carbonate secretions is generally a rule. Numerous calcium carbonate concretions and outgrowths on mollusc shells were observed in the cores containing gas hydrates. This is quite normal for sea bottom gas seepages. The calcium carbonate concretions may form as a result of methane

V.A. Soloviev, G.D. Ginsburg! Marine Geology 137 (1997) 59-68

62

18.5' 155°17.7' 18.4' 18.6' 18.7' 155°18.8' 17.8' 18.0' 18.4' 18.2' 18.3' 17.9' 50"31.2' rrT7-r7.....,--r~~-rr-IT:rrfh--::~----:::==::::----'~--:7//7-A 50"31.2'

31.1'

31.1'

50"31.0'

5O~l.O'

30.9'

30.9'

30.8'

30.8'

50"30.7' LL-~-4-..r.....I'-L....t,-_.l-.I..-'-.I...I......l.,--~--....-""""----.I,rI-'-..I..---r-"""~~~-'-'i~u....I.~ 50"30.7' 18.4' 18.5' 18.6' 155°17.7' 17.8' 17.9' 18.1' 18.3' 18.7' 155°18.8' 18.0' 18.2' 500m Fig. 2. Echo-sounder bathymetric map with location of the gas plume no. I (dashed line) offshore Paramushir Island. Dots are sampling sites with gas hydrates (big solid circles, sites no. 5 and 7) and indirect gas hydrate indications (open circles).

oxidation causing the saturation of pore water with carbon dioxide, which precipitates in the form of carbonates (Brooks et al., 1986; Zonenshayn et al., 1987).

3. Discussion

A higher water content and specific structures of hydrate-containing sediments associated with gas-seepage sites suggest that gas hydrate formation and accumulation are an analogue to the formation of segregated ice. Ground ice forms by water migration to the freezing front. In this case, the presence of bound water, a temperature gradient, and a permeability of the sediments sufficient for water transport are necessary and sufficient conditions for migration of water in freezing

porous sediments (Ananian, 1964). These conditions are all met during gas hydrate formation near the sea bottom in gas-seepage areas. If the process of gas hydrate accumulation is like ice formation by freezing of fine-grained sediments, it is needed to assume also that water migration takes place during accumulation of the gas hydrates, forming lenticular-bedded and porphyraceous structures. Let us consider a case of gas hydrate formation under a free gas flow through a thermobaric gas hydrate stability zone, since this is the more probable in geology. Let us suppose also that gas hydrates were not formed here in the past because of insufficient saturation of pore water with gas. As follows from Figs. 6 and 7, the more favourable kinetical conditions of gas hydrate formation occur in the upper part of the thermobaric gas hydrate

V.A. Soloviev, G.D. Ginsburg / Marine Geology 137 (1997) 59-68

144°3.6' 54°27.2'

3.8'

4.0'

4.2'

4.4'

4.6'

4.8'

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63

5.6'

5.8'

144°6.0' 54°27.2'

27.0'

27.0'

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26.6'

26.4'

26.4'

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SOOm Fig. 3. Echo-sounder bathymetric map with locations of gas plumes N1, N2, and N3 (dashed lines) offshore Sakhalin Island. Dots within plume N1 are sampling sites where gas hydrates were recovered.

Table 1 Subbottom depth of some gas hydrate accumulations Okhotsk Seal

Gulf of Mexico-

Site number depth(m)

Subbottom depth(m)

Area

Subbottom

91-01-05 91-01-07 91-02-39 91-02-40 91-02-41 91-02-42 91-02-44 1395-23 14233

0.10 0.30 0.30 0.95 1.20 1.00 0.70 1.85 2.20

GC-234 GC-204 MC GC-320 GB-388 GC-257

1.20; 2.80 1.40 3.80 3.20 2.80 4.20

'Ginsburg et al., 1993; Soloviev et al., 1994. 2Brooks et al., 1986. 3Zonenshayn et al., 1987.

stability zone beneath the sea floor. This is the place where the probability of overcooling (A'I') and oversaturation (ilP) is highest. Up-going gas flow saturates pore water with gas and at a certain instant a secretion of gas hydrates occurs. This is due to the fact that pore water proves to be oversaturated with respect to gas hydrate. Further gas hydrate accumulation in this environment is supported by both a continuous increase of additional portions of gas-saturated pore water within a diffusion halo and water migration to gas hydrate formation sites. In this case, the entering gas plays the role of an agent extracting water from sediments and causing hereby the possibility of water migration. It is obvious that diffusion is the mechanism of gas transfer to gas hydrate accumulation sites. A diffusion halo is formed around ascending gas flows in the gas-seepage areas caused by a horizontal gradient of fugacity. The gradient is defined

64

V.A. Soloviev, G.D. Ginsburg / Marine Geology 137 (1997) 59-68

(a)

(b)

Fig. 4. The lenticular-bedded structure caused by gas hydrates, site 91-02-44, Okhotsk Sea, offshore Sakhalin Island: (a) general view of gas-hydrated sediments; (b) part of the core broken along vertical axis-vsubhorizontal layers and lenticules of gas hydrates are seen.

by the difference in pressure AP (see Fig. 7) between the fugacity of free gas, close to Ph' and that of dissolved gas, close to Peq • Moreover, the ascending diffusion gas flow following the pattern of "directional diffusion recondensation" (Geodekian et aI., 1984) also takes place in the vicinity of gas-seepage areas. This is a process of diffusive transfer of dissolved matter in the matter saturated medium in the gradient field of parameter affecting the matter solubility. Recondensation

means the matter dissolving in one part of the system where the solubility is higher, and condensation in another part. In our case a parameter affecting the solubility, is a temperature which decreases towards the sea floor. As a result of this process gas hydrate accumulation may occur in the upper part of the subbottom. Near the sea floor the subvertical diffusion flow of dissolved methane is defined also by the vertical gradient of the methane concentration, which is the most

65

V.A. Soloviev, G.D. Ginsburg / Marine Geology 137 ( 1997) 59-68 Water content, % I

59 63 II I

I

so~-, mM

cr, gil

I

8

14

I

6

I

18 II

-300

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TEMPERATURE (T),°C

Eh, mV

I

5

10

0

I I I I

3 4

<, 5 6

10

\

\--Teq \ \

.............................

11

BASEOF METHANE HYDRATE STABlUTYZONE

P, MPa

120 160

em

SEA BOTTOM

9

91-02-44

40 80

-, ~

8

em

o

-,

" sr

91-02-41

1~~ l~~LIJ

-,

01

Fig. 5. Water content and chlorinity of both hydrate-bearing and non-hydrated sediments, sites 91-02-40, 41 and 44 from the Okhotsk Sea, offshore Sakhalin Island (Ginsburg et aI., 1993). 1 = non-hydrated sediments, 2 = hydrated sediments, 3 = sea water. Table 2 Water content in sediments on the site 91-02-44, the Okhotsk Sea, offshore Sakhalin Island Water content(% wet mass) Non-hydrated sediments

60

Hydrate-bearing sediments

total gas hydrate water liquid water

65 20 45

significant for the lower part of the sulphatereduction zone, since the methane concentration is very low because of methane oxidation.

Fig. 6. Thermobaric conditions of gas hydrate formation in the vicinity of hydrate accumulations near Sakhalin Island. Th ~ the actual distribution of temperature vs. depth; Teq ~ equilibrium temperature curve of methane hydrate formation under a pore water salinity equal to the salinity of sea water-the methane hydrate is stable in the area on the left of this curve.

The mechanism of water transfer during gas hydrate accumulation is apparently the same as under segregated ice formation. At the same time, osmosis should play an important role for the water transfer as far as gas hydrate formation itself causes the occurrence of essential salinity gradients near a zone of gas hydrate accumulation. As has been already noted, the more favourable kinetical conditions of gas hydrate formation occur in the uppermost sediments. Upon the saturation of pore water with migrating gas, the initial gas hydrate formation is essentially accidental. Gas hydrate formation and accumulation may start at the same time in various places at some different subbottom depths. In the future, the features of gas hydrate accumulation will depend on the degree of spatially arranged order of geophysical

V.A. Soloviev, G.D. Ginsburg j Marine Geology 137 (1997) 59-68

66

PRESSURE (P), MPa

20

10

30

Top of gas hydrate stability zone ---r-- - o r - - - r - - o r -

sea .i P

~

---r--

---,...

water

==", • Peq gas seepages

tt t rt

2

I

I

I

I

I

I

:

:

I

I

I

I

I

I

I

I

:

I

I

sea bottom J_..... 1-=::::-:==:-:::::-:1:!::=...............)....J~,...., v 'v sea bottom

-_.~::.~~~ ...: :.. -

3 H,km Fig. 7. Pressure distribution (P) vs. depth (H) within the limits of the submarine gas hydrate stability zone. H = total depth; Ph= conditional hydrostatic pressure; P eq = equilibrium pressure of methane hydrate formation in water depth of I Ian (A) and 2 Ian (B); p.= saturation pressure of dissolved gas in the sulphate-reduction zone. Assumptions: the pore water is fresh; the bottom temperature is 2°C; the geothermal gradient is 30°Cjlan and the hydrobaric gradient is 10 MPajlan.

,=---==--- ....:..: ".' . ;\ \ - - - -'7";.. • . • . . . \

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and geochemical parameters. By analogy with segregated-ice formation the pore water from adjacent sites with already existing gas hydrates must be involved into gas hydrate formation also. The amount of formed hydrates, the size of inclusions, and the thickness of hydrated lenses and layers will depend on the presence of the reacting gas and water, and their capacity to move to sites of gas hydrate accumulation. It is clear that the upper limit of gas hydrate formation in the subbottom and in particular the position of the gas hydrate accumulation front are controlled by surfaces of equal gas concentrations which are defined by the depth to the seabed, at least in the immediate area of the upward gas flow. Hence, the formation of the subhorizontallenticular-bedded hydrated structure near the flat sea bottom (for example site 91-02-44) is quite regular (Fig. 8). With increasing subbottom depth or with clearly irregular seabed morphology, where geochemical parameters are forced to become irregular

AcA_

:-:.: • ~ ...- ...,' -r-;»:..,

-'-- -'-- -'--

A '" ,

~ ~

I

'

15§!1 ~

~2 -'--

--'-

Base of gas hydrate : : stability zone

L.J

up-going gas now Fig. 8. Scheme of the structural formation of hydrate-bearing sediments in submarine gas-seepage fields. The shape of the diffusion halo is inferred from data in Fig. 7. 8P is decreased with subbottom depth. 1 = lenticular-bedded structure; 2 = porphyraceous structure.

too, the formation of this structure is less probable (see Fig. 8). The porphyraceous structures have been observed in the Okhotsk Sea (for example site 1423, Table 1) in addition to subhorizontal lenticular-bedded structures. Offshore Paramushir Island porphyraceous structures caused by gas hydrates are associated with the anomalous relief characterized by the presence at the sea floor of carbonate crusts, craters and gaps (Zonenshayn et al., 1987).

V.A. Soloviev, G.D. Ginsburg / Marine Geology 137 (1997) 59-68

4. Conclusions Water segregation during gas hydrate formation and accumulation in submarine gas-seepage fields is responsible for the formation of specific structures of sediments caused by hydrates and for the increased water content in hydrate-bearing sediments. The principal significance of this mechanism is the existence of a gas diffusion halo; gas hydrate accumulation takes place within the halo only. It is evident that water segregation in gas hydrate accumulation occurs in diffusion haloes also associated with the filtration flows of gassaturated water through the gas hydrate stability zone.

Acknowledgements This study was supported and sponsored by the Russian Foundation of Fundamental Research (Project 93-05-8815) and International Science Foundation (Grant R13000).

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Yefremova, A.G. and Gritchina, N.D., 1981. Gazogidraty v morskikh osadkakh i problema ikh prakticheskogo ispol'zovania (Gas hydrates in sediments beneath seas and the problem of their exploitation). GeoI. Nefti Gaza, 2: 32-35 (in Russian). Yefremova, AG. and Zhizhchenko, B.P., 1974. Obnaruzhenie kristallgidratov gazov v osadkakh sovremennykh akvatoriy (Occurrence of crystal hydrates of gas in sediments of modem marine basins). Dokl. AN SSSR, 214: 1179-1181 (in Russian). Zonenshayn, L.P., Murdmaa, 1.0., Baranov, B.V., Kuznetsov, AP., Kuzin, V.S., Kuzmin, M.I., Avdeiko, G.P., Stunzhas, PA, Lukashin, V.N., Barash, M.S., Valyashko, G.M. and Dyomina, L.L., 1987. An underwater gas source in the Sea of Okhotsk west of Paramushir Island. Oceanology, 27(5): 598-602.