Geological Controls On Gas Hydrate Distribution 41 Sediment Deposition

The most significant control on hydrate accumulation appears to be sediment deposition rate. Of the four concentrations of hydrate noted above, three are associated with areas of high deposition. The northernmost (Fig. 3) occurs in a region of undisturbed recent sedimentation (Schlee et al., 1992) between the Hudson and Wilmington sea valleys. The valleys form two major channelways for transportation of sediments across the continental rise. The area between the valleys is a depocenter where more than 300 m of late Pliocene to present sediment accumulated (Mountain and Tucholke, 1985).

The southeastern, deep water hydrate concentration occurs on the Lower Rise Hills, a region of sediment wave deposition that also was active in Late Pliocene to present. Hydrates have not been sampled in either this or the drape area, but their abundance in these areas, as indicated by seismic interpretation, is not surprising. Most marine gas hydrates have been formed from biogenic methane and areas of rapid hemipelagic deposition tend to accumulate considerable amounts of organic detritus and, by rapid burial, preserve it from oxidation at the sea floor, so that it is converted to methane by bacteria within the sediments. Therefore abundance of gas hydrate in areas of rapid deposition would be expected.

The third concentration of gas hydrate in the mapped area is at the Blake

Ridge (Fig. 3), a location that is well-known for high concentrations of gas hydrate, identified both in seismic profiles and scientific drilling (Paull et al. 2000). The source of gas in the Blake Ridge has been bacterial decay of organic material that accumulated in the sediment. A profile across the Blake Ridge (Fig. 1) shows marked blanking of reflections. Blanking is strongest in the lower part of the hydrate zone just above the BSR. The blanking generally seems to be concentrated at the crest of the ridge. The great volume of hydrate within the Blake Ridge sediments, which is the highest concentration mapped, probably results from the shape of the ridge and the capacity of hydrate-cemented sediments to trap gas. The Blake Ridge is a thick (1200 m) sediment drift deposit that accumulated relatively rapidly due to current patterns on the ocean floor; accumulation rates reached 160-190 m/million years (Mountain and Tucholke, 1985). As the sediment accumulated, the ridge built upward and the deeper sediments became warmer; in essence, the surfaces of constant temperature — the isotherms — migrated upward as the sediment surface rose. This warming caused the hydrate in the deeper, older sediments to break down, releasing gas that moved upward through the sediments. The gas accumulated at the relatively impermeable base of hydrate-cemented sediments, which, by its configuration, acted as a trap. The trapped free gas is inferred to have penetrated the GHSZ by diffusion or along pathways provided by small compaction faults and it was immediately converted to hydrate, thus causing a concentration of hydrate at the base of the hydrate-stable zone.

4.2. Diapirism

Off North and South Carolina a linear group of salt diapirs extends along the seaward side of the deep Carolina Trough, one of the four major continental margin basins of the eastern United States (Dillon et al., 1983). Gas hydrates are concentrated around the diapers (Fig. 6). The BSR is observed to rise over each diapir, indicating that the hydrate-stable layer becomes thinner (Paull and Dillon, 1981). The structure of the hydrate-stable layer is very complex in the vicinity of the diapirs (Fig. 7). Actually the correspondence of hydrate-layer structure to diapir location probably is much closer than indicated in Figure 7, as the mapping of diapirs and mapping of hydrate were, of necessity, done on different seismic profiling data sets. The effects on hydrates at salt diapirs is reviewed in detail in Chapter 6 (Dillon and Max). The effects result from the greater thermal conductivity of salt compared to sediment, which produces a warm spot at a diapir, and the presence of salt ions in interstitial fluids, which would act as inhibitors (antifreeze; Taylor et al. 2000). Both effects cause the base of gas hydrate stability to be warped up over the diapir, forming a trap for gas, and the trapped gas tends to nourish the gas hydrate in the vicinity of the diapirs, causing gas hydrate concentrations.

Figure 6. Locations of landslides and diapirs plotted on the map of gas hydrate volume (Fig. 3) for the diapir and Blake Ridge regions. Volume of hydrate is indicated as isopach contours of the amount of hydrate existing in the sedimentary pores.

4.3. Mass Movement

Sea floor mass movements seem to have significant effects on structure of the hydrate layer and on the abundance of hydrate. The broad area where there is a dearth of gas hydrate in the central region of The U.S. Atlantic margin from Wilmington sea valley southward to off Cape Hatters (Fig. 3) is a region marked by overlapping sea floor slide and slump scars. Note the raggedness of the 2000 m contour in this region, which displays the irregularity of the sea floor produced by mass movements of sediment. The continental rise in this region, especially in the depth range of about 3,000 to 4,000+ m, where slides are concentrated, shows almost no indication of gas hydrate in the seismic profiles.

OIAPIH

METESS BELOW SEA -7M 6O0-7W S00-600 «500

floor

Gas Hydrate Landslide

Figure 7. Locations of landslides and diapirs plotted on the map of thickness of the gas hydrate layer (Fig. 4) for the diapir and Blake Ridge regions. Thickness of the hydrate layer is equivalent to the depth of the BSR below the seafloor.

OIAPIH

METESS BELOW SEA -7M 6O0-7W S00-600 «500

floor

Figure 7. Locations of landslides and diapirs plotted on the map of thickness of the gas hydrate layer (Fig. 4) for the diapir and Blake Ridge regions. Thickness of the hydrate layer is equivalent to the depth of the BSR below the seafloor.

Figure 8. Profile across the Cape Fear slide scar. The BSR becomes shallower and weaker beneath the slide scar.

Fewer slides scars exist south of Cape Hatteras, but those that are present are unusually large (Popenoe and Dillon, 1996). The two major slides are the Cape Fear slide (indicated on Fig. 4) and the Cape Lookout slide. Both slides are mapped in Figure 7 which also shows the details of the structure of the gas hydrate stability zone. Note the marked thinning of the hydrate zone under the slides. As indicated in a seismic profile across the Cape Fear slide (Fig. 8), the BSR becomes indistinct under the center of the slide and the BSR also rises slightly at the sides of the slide scar.

Mapping shows that the BSR-to-seafloor distance commonly decreases by 200 to 300 m from the center of the slide to surrounding, undisturbed areas (Fig. 7), an amount greater than the thickness of slope strata removed by the mass movement. This shallowing of the BSR and the clear association of slide scars to thinning of the gas hydrate-stable zone suggest that displacement of strata by mass movement causes breakdown of hydrate, probably due to pressure reduction resulting from removal of part of the sediment load. The disappearance of the BSR, probably means that both the gas that previously existed there, plus that released when the gas hydrate dissociated, has escaped.

Although mass movement may reduce pressure and cause breakdown of hydrate, the converse is also probably true. That is, if hydrate breaks down for some reason other than mass movement, this can weaken the sediments and foster landslides (Chapter 12, Paull and Dillon). Gas hydrate breakdown (dissociation) will not only remove a cementing medium (that probably is not very important), but also will release both water and gas into sediment pores in volume greater than the volume of gas hydrate that previously existed. If the gas is sealed in, as we would expect, this causes an increase in pressure, known as "overpressure", that reduces the shear strength and essentially buoys up the near-bottom sediments; this can trigger collapses and slides on slopes.

The evidence for such mass movements of sediment tends to be lost when a slide moves off downslope. However, in at least one case on the Blake Ridge (Fig. 4), such a process apparently resulted in a blowout of mobilized sediment and subsequent collapse of the crest of the ridge without causing a slide. The thinning of the GHSZ mapped on the Blake Ridge (Fig. 7) centered at about 31° 55'N, 75° 40'W results from that blowout. A profile across the structure (Fig. 9), a perspective image (Fig. 10), and a series of conceptual diagrams (Fig. 11) indicates how this structure probably formed (Dillon et al., 1998; 2000).

We start with an undisturbed part of the ridge (Fig. 11 A), which is comparable to Figure 1. The process passed through episodes of breakdown of gas hydrate, which was perhaps due to the pressure reduction caused by sea level lowering during glacial stages. That dissociation of gas hydrate generated overpressures and caused flow and inflation of the ridge crest (Fig 1 IB). Eventually rupture occurred and a significant volume (>13 cubic km) of sediment, water and gas escaped (Fig 11C). As this volume escaped, the strata collapsed into the evacuated volume to leave the structure that we see today (Fig. 1 ID; Fig. 10, profile).

-LIMIT OF COULAPSE-

-LIMIT OF COULAPSE-

1500 IOOO taOO

1500 IOOO taOO

Figure 9. Seismic reflection profiles oriented SW-NE (NE to right) across the collapse structure on the crest of the Blake Ridge.
Figure 10. Perspective image of the Blake Ridge, showing the collapse structure at the crest. The area shown is approximately a square, 40 miles on a side and the vertical side of the box extends from 2350 to 3900 m. The profile of Figure 9 crosses the ridge across the center of the collapse.

A INITIAL STAGE

VE = 10 - 1-1

B FLOW AND INFLATION

---C. OF FLOW ^ & _. -

C -C» „ ERUPTION -^ DETACHMENT AND

D COLLAPSED

CHAOTIC STRUCTURE

— -

— -

Fig. 11 Conceptual model of the collapse process at the crestal region of the Blake Ridge. The region of flow in the second panel is inferred to be a region in which fluid-rich sediments were mobilized. The zone of trapped free gas indicated in the first panel may have included this entire region of flow.

Fig. 11 Conceptual model of the collapse process at the crestal region of the Blake Ridge. The region of flow in the second panel is inferred to be a region in which fluid-rich sediments were mobilized. The zone of trapped free gas indicated in the first panel may have included this entire region of flow.

5. SUMMARY AND CONCLUSIONS

Gas hydrates on the Atlantic continental slope and rise have been sampled in drilling, and they also can be detected in seismic reflection profiles. Characteristics that allow hydrate detection in seismic profiles and mapping of the thickness of the GHSZ and possible volume of gas hydrate are:

1. A bottom simulating reflection, known as a "BSR", which is a reflection that parallels the sea floor and cuts through reflections from strata; the BSR is a reflection from the base of the hydrate-cemented layer where free gas is present beneath it;

2. Reduction in amplitude of reflections within the hydrate-cemented zone, known as "blanking", appears to occur as a function of hydrate development where hydrate displaces water in pore space and causes a diminution of the reflection coefficients of strata.

Four regions of gas hydrate concentration have been mapped on the continental rise in the offshore region between New Jersey and Georgia. Three are correlated with sediment depocenters and one occurs along a concentration of diapirs. Concentration at depocenters probably occurs because most gas in hydrate is biogenic, produced by bacteria that feed on organic material that is preserved from oxidation when it is rapidly buried in fast-depositing marine sediments. A major concentration of hydrate, one not related to a distinct depocenter, is associated with a trend of diapirs that create gas traps resulting in the concentration of gas and gas hydrate. Breakdown of gas hydrate and thinning of the hydrate layer has occurred at sites of mass movement, probably caused by reduction of pressure due to removal of sediment during landsliding. Conversely, breakdown of hydrate may cause mass movements by substituting a weak layer of gassy sediment for a strong layer of hydrate-bearing sediment.

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