Aerial Extent Of Hydrate Formation

Hydrate, often with gas below, has been recognized in continental slopes and rises between about 500 and 3,500 m water depth (Eiken and Hinz, 1989, Mienert et al., in press). All Arctic and northern North Atlantic continental slope areas where sediment thicknesses are greater than about 3 km are potentially areas in which significant gas generation and consequent hydrate formation is possible. Prediction of gas production and the presence of hydrate, however, is uncertain.

A number of criteria are used in determining the area and thickness of potential hydrate development:

1. An average heat flow value of 30°/km for this region.

2. Biogenic gas is produced by the deep biosphere (Chapters 7 & 8).

3. Sedimentary successions throughout the region are regarded as having good gas generation potential.

4. 500 m is a minimum depth for finding hydrate-bearing areas.

5. Bottom water temperature at between the -1.5° C to +1.5° C. Three main areas where hydrate is likely to be found have been identified using these criteria (Fig. 5):

Little River State Park

Figure 5. Hydrate likelihood areas, from Max and Lowrie (1993). Slope and abyssal areas separated in Arctic at about the 3,000 m contour. Sediment thickness data not included for areas to the south of dashed line passing across south Iceland. Area I. Continental slope areas between 500 m and 3,000 m follow recognized hydrate development in continental slope north of Alaska (Grantz et al., 1989). Area II. Abyssal areas and sedimentary basins not tied to continental shelf structure where sediment thickness exceeds 3 km. Area III. Areas of abnormally high heat flow associated with plate margin where gas could be sourced from thin sediments or where there is juvenile gas. Area III minimum sediment thickness of between 0.75 km -1 km in ridge vicinities (Knipovich and Laptev).

Figure 5. Hydrate likelihood areas, from Max and Lowrie (1993). Slope and abyssal areas separated in Arctic at about the 3,000 m contour. Sediment thickness data not included for areas to the south of dashed line passing across south Iceland. Area I. Continental slope areas between 500 m and 3,000 m follow recognized hydrate development in continental slope north of Alaska (Grantz et al., 1989). Area II. Abyssal areas and sedimentary basins not tied to continental shelf structure where sediment thickness exceeds 3 km. Area III. Areas of abnormally high heat flow associated with plate margin where gas could be sourced from thin sediments or where there is juvenile gas. Area III minimum sediment thickness of between 0.75 km -1 km in ridge vicinities (Knipovich and Laptev).

Area I. This area includes continental slopes between 500 m and 3,000 m water depth where sediment thickness is greater than 3 km. This region the water depth range in which hydrates have been identified along the North Slope of Alaska (Grantz et al., 1989). About 818,000 km2 of the Arctic Basin and about 154,000 km2 of the Northern North Atlantic are underlain by areas with these physical characteristics.

Area II. This area includes abyssal regions below 3,000 m (except in the continental slope-ward margin of the Wrangel Abyssal Plain where the 2,500 m contour has been mainly used) where sediment thickness is greater than 3 km. In abyssal areas, no identification of hydrate has yet been made, but if sufficient methane has been generated, there is a thick HSZ in which hydrate could form. About 1,403,000 km2 of such area occurs in the Arctic.

Area III. Two relatively small areas of thin sediment overlie active transform/ridge systems in which it is known that gas and hot hydrothermal fluids are generated as a by-product of magmatic and volcanic activity. It is possible that hydrothermal fluids would deliver methane to sediments that would otherwise be too thin to generate its own gas. This area consists of about 35,000 km2 of the Arctic Basin and about 80,000 km2 of the northern North Atlantic.

The HSZ increases in thickness with increasing depth (Chapter 6) (Miles, 1995). 450 m represents a reasonable average thickness for the HSZ in Area I. For Areas II and III, in deep and very deep water, a thickness of about 700 m is used as a reasonable average thickness.

The thickness of the HSZ may be less important than a mechanism that allows hydrate to concentrate. The HSZ off SE Japan, for instance, is relatively thin (Chapter 18), but the hydrate there is highly concentrated.

Estimations of hydrate and gas were made by Max and Lowrie (1993), but are now regarded as being high (Kvenvolden, 1998).

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