Gas Hydrate Forming Fluids

Analysis of chemical and isotopic composition of water and gas from gas hydrates deposits formed in the fluid discharge areas, allows us to estimate the composition and the genesis of the original gas hydrate-forming fluids. For example, studies of oxygen and hydrogen isotopes from water produced by hydrate dissociation sampled from mud volcano sediments (Buzdag in Caspian Sea, Haakon Mosby in Norwegian Sea, Ginsburg in the Gulf of Cadiz, Amsterdam and Kula in the Eastern Mediterranean) have revealed anomalous trends between &180, 5D and chloride ion concentrations.

Three types of relationships were distinguished (Fig. 5) with decreasing chloride, (i.e., with an increase of gas hydrate content) (1) where hydrogen isotopes become heavier, and where oxygen isotopes become lighter (Caspian Sea); (2) where both hydrogen and oxygen gets heavier (Norwegian Sea); (3) and where hydrogen becomes lighter, but oxygen becomes heavier (Gulf of Cadiz and the Mediterranean Sea). It is known that the water included in the hydrate structure, may also become heavier in oxygen and hydrogen as a result of isotopic fractionating during gas hydrate formation. According to experimental data (Maekawa & Imai, 2000), the fractionation coefficients of protium-deuterium and S180/6160 during the reaction of gas hydrate formation from salt ocean-type water are 1,016-1,020 and 1,0028-1,0032, respectively.

Only the Haakon Mosby mud volcano (Fig. 5B) yielded data that correspond to the isotopic fractionation expected during gas hydrate formation (Ginsburg et al, 1999). Isotopic measurements on natural gas hydrate samples gave fractionation coefficients of 1,024 for hydrogen and 1,0029 for oxygen, which is close to the experimental values. Anomalous relationships between 6180, 8D and chloride of water in other mud volcano areas (Fig. 5, A & C) can be best explained by special attributes of the isotopic composition of the original mud volcano fluid. The chloride ion concentration of the mud volcano fluids in the Gulf of Cadiz and Mediterranean may correspond to oceanic values. The isotopic composition of the original mud volcano fluids in these regions varied, but often are rich in oxygen and poor in hydrogen (Table 1). The mud volcano fluid of the Caspian Sea, which has chloride values that exceed seawater values by more than 2x, on the contrary, is characterized by lighter oxygen.

6"0

Figure 5. Water, oxygen, and hydrogen isotopic composition vs. water chlorinity diagram from different regions of mud volcanism: A, Gulf of Cadiz and Mediterranean Sea; B, Haakon Mosby mud volcano; C, Caspian Sea.

study area (mud volcano)

coring station

water depth cm

subbottom depth, m

%

C2, %

c3, %

c2i

Ô"C of CHj, %o, PDP

SMOW

ÔD, %0, SMOW

References

Gulf of Mexico, Green canyon

320

800

3.2-3.6

99.90

0.080

N/A

1300

-66.50

N/A

N/A

N/A

Brooks et al., 1984, 1986

Gulf of Mexico, Missisipi canyon

MC

1300

3.8

97.40

1.20

1.30

37.40

-48.20

N/A

N/A

N/A

Brooks et al., 1984, 1986

Gulf of Mexico, Garden Banks

388

850

2.5-3.8

99.90

0.120

N/A

830

-70.40

N/A

N/A

N/A

Brooks et al., 1984, 1986

Off California, Eil River

105

567

0-0.2

99.90

0.010

0010

11000

-57.60

N/A

N/A

N/A

Brooks et al., 1991

Black Sea, Sorokin Trough

57

2050

0.7

99.90

0.045

0.0004

2200

-61.80

300

N/A

N/A

Ginsburg et al., 1990;

Central Black Sea (Kovalevsky

319

2169

1.2-2.2

99.90

0.033

0.0030

11000

N/A

120

-0.8

-28

Mazurenko & Soloviev, 2002

Caspian Sea (Buzdag)

7c+7b

475

0-1.2

59.10

19.40

15.80

1.70

-44.80

1200

-0.8

-23

Ginsburg & Soloviev, 1994

Caspian Sea (Elm)

17

600

0-0.5

96.20

0.60

1.50

45

-56.50

900

N/A

N/A

Ginsburg & Soloviev, 1994

Sea of Okhotsk

N/A

710

0.3-1.2

99.9

0.003

0.0018

22000

-64.30

140

+4.2

N/A

Soloviev & Ginsburg, 1994

Gulf of Cadiz (Ginsburg)

238

910

1.5-1.7

81.20

9.510

6.160

4.10

N/A

60

+8.9

-11

Mazurenko et al., 2002

Norwegian Sea (Haakon Mosby)

N/A

1250

0.1-2.0

99.9

0.002

0.0004

12000

N/A

200

+2.5

-6

Ginsburg et al., 1999

lake Baikal

5A

1380

0.3-0.4

99.0

0.114

N/A

860

N/A

N/A

-15.6

-122

Matveeva et al., 2000

Table 1. Examples of gas hydrate water and gas compositions. *-chloride anomalies in water produced by dissociation of hydrate.

Table 1. Examples of gas hydrate water and gas compositions. *-chloride anomalies in water produced by dissociation of hydrate.

We propose that isotopic fractionation of gas and gas component mixtures takes place during hydrate formation and that under certain conditions hydrate may be considerably enriched in heavy hydrocarbons. Gas obtained from hydrates of the Ginsburg MV, contained only 81% methane (Table 1). In this three-phase equilibrium condition (water-gas-hydrate), the source of gas is enriched in C2-C6 by up to 5%.

These recently available data make it possible to draw some inferences about the genesis of the hydrate-forming gas. The gas in hydrate, which is predominantly methane (frequently < 99 %), can be biogenic, thermogenic or mixed in origin. There must be gas migration since thermogenesis is normally impossible within the HSZ. Even where pure biogenic methane is the hydrate forming gas both near the sea bottom (the Sea of Okhotsk), and at significant sub-bottom depths (the Blake Outer Ridge, Paull et al, 2000), it is not generated in situ. These gases have migrated into the HSZ, presumably from below.

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