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THERMOGENICGAS HYDRATES, GULFOF MEXICO CONTINENTALSLOPE - page 2 / 3

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Table 1. Normalized C1-C5 hydrocarbon compositions and methane 813C of vent gases and thermogenic gas hydrates (Structure I1 and Structure H) at Bush Hill. Number in superscript indicates the citation to the data.

Vent Gas 10

93.2

-43.3

4.3

1.5

0.3

0.6

0.3

Vent Gas 10

93.5

-42.5

4.3

1.4

0.2

0.4

0.2

Vent Gas 10

94.7

-45.6

3.9

0.7

0.1

0.5

0.2

Vent Gas 10

94.6

-43.8

3.8

0.7

0.1

0.5

0.3

Vent Gas 10

91.1

-42.4

4.8

1.8

0.4

1.2

0.8

i;

Sample

C1

813~

C2

C3

i-C4

n-C4

i-C5

Hydrate (1I)'o

71.7

-36.3

10.6

12.6

2.6

1.7

0.8

Hydrate(n)lo

80.2

-38.5

9.4

7.3

1.6

1.2

0.3

Hydrate(I1)IO

72.1

-39.9

12.4

11.4

2.3

1.6

0.3

t

Hydrate(H)6

9.5

7.5

2.5

17.5

41.1

21.2

  • -

    29.3

Structure I1 hydrate Hydrocarbon compositions of massive hydrate lenses of Bush Hill are shown in Table 1. The C1-C5 hydrocarbons of the hydrate gases are dominated by C1 (71.7-

80.2%). The 813C values of C1 are in the range of -36.3 to -39.9 ?& PDB, somewhat heavier than vent gases, possibly because of bacterial activitylo. The Cz and C3 hydrocarbons are both present in similar but relatively high percentages compared with the vent gas (Table 1). Preliminary NMR of an intact hydrate sample preserved in liquid nitrogen is consistent with structure I1 hydrate (Ripmeester, J., pers. communication).

Structure H gas hydrate Structure H hydrates produced in the laboratory can enclose larger molecules than structure I or I1 hydrates, including common thermogenic hydrocarbons such as i- C5. Given the widespread occurrence of petroleum, it was postulated in 1993 that structure H hydrate could co-exist in nature with structure Il hydratel2.

Evidence for the natural occurrence of structure H gas hydrate at the Bush Hill locality was first reported in 19946. Massive amber-colored gas hydrate breached the sea-floor. It had been exposed when a buoyant lobe of hydrate broke free of the sediment and floated upwards into the water column. Identification of structure H hydrate was based on abundant i-C5, which represented 41.1% of the total C1-C5

hydrocarbon distribution of the sample (Table 1). The 8l3 C of C1 from the sample is heavy (-29.3 %O PDB),possibly because of bacterial activity6.

Experimentally-Precipitated gas hydrate Gas hydrate was experimentally precipitated at the crest of Bush Hill in 1995 using natural vent gases as the starting materiallo. Water temperatures during experiments were 9.0-9.2OC. Precipitation of white to yellow gas hydrate was noted to occur within minutes.

The hydrocarbon compositions of experimentally precipitated gas hydrates are similar to vent gas compositionslO. The c 1 - C hydrocarbons of the experimentally precipitated gas hydrates gases are dominated by methane (C1 = 87.7-93.9%),and the

813C values of C1 are withii the 40.5 to -45.3 %O PDB range.

CONCLUSIONS Thermogenic gas hydrates occur on the Gulf of Mexico continental slope because of active vertical migration of oil and gas to the sea floor within their stability zone.

The Bush Hill seep site on the Gulf slope is an important case history. Massive thermogenic gas hydrates occur in association with the orifices of hydrocarbon

vents.

Both

structure

I1

and

structure

H

hydrates

appear

to

co-exist

in

this

473

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