Exploration Atlantic gas hydrates target of Ocean Drilling Program leg

Oct. 16, 1995
Charles Paull University of North Carolina Chapel Hill Researchers will investigate the properties of natural gas hydrates in marine sediments by drilling a series of 50 to 800 m deep holes into the continental rise off the Southeastern U.S. The drilling will be conducted during November and December on Leg 164 of the Ocean Drilling Program.

Charles Paull
University of North Carolina
Chapel Hill

Researchers will investigate the properties of natural gas hydrates in marine sediments by drilling a series of 50 to 800 m deep holes into the continental rise off the Southeastern U.S. The drilling will be conducted during November and December on Leg 164 of the Ocean Drilling Program.

The past few years the earth science community has developed an increased awareness of gas hydrates. One might think that these minerals, composed of water and low molecular weight gases (usually methane), would be of interest only to chemists because of their odd crystal structure and to gas companies because of their nasty tendency to plug pipelines.

However, the attention of a broad section of the earth science community stems from the realization that enormous volumes of natural gas may be stored as gas hydrate in marine sediments.1 In fact, the conditions appropriate for gas hydrate formation (low temperature, high pressure, and adequate gas concentrations2), are very common in the upper few hundred meters of rapidly accumulated continental margin sediments.3

Current albeit crude estimates suggest there are about 10,000 gigatons of carbon stored in gas hydrates, which is about twice the estimate for the carbon in all other fossil fuel deposits.1

Although gas hydrates may be a common phase in the shallow geobiosphere, surprisingly little is known about them in their natural environment. Our ignorance about natural gas hydrates can be attributed to their instability under normal surface conditions. Natural gas hydrates in drill cores rapidly dissociate, frequently before they are observed or at least are adequately described during routine core processing. Therefore, most geologists have never seen a sample of gas hydrate.

Much of what we know about natural gas hydrates comes from seismic reflection surveys of continental margins which frequently record a prominent bottom-simulating-reflector (BSR). BSRs are believed to be associated with the base of gas hydrate stability and may correspond with the interface between the hydrate bearing sediments above and potentially gas charged sediments below. However, the exact nature of BSRs is in need of ground-truthing.

While the ephemeral nature of natural gas hydrates makes them difficult to study, their estimated volume suggests that they may significantly affect the sediment sections in which hydrates occur and even the earth's climate since methane is a greenhouse gas. The formation and subsequent decomposition of gas hydrates within sediments influence the mechanical properties of the sediment, sediment stability, continental margin porosity and permeability structure, fluid and gas migration pathways, pore fluid composition, and solid phase diagenesis.

The resource potential of gas hydrates is currently unknown, but the estimated size of the reservoir suggests that gas hydrates are a potentially important future energy resource. Also, if these natural gas hydrate deposits are dynamic reservoirs, the potential to affect the earth's climate by releasing methane to the atmosphere has to be considered.

ODP Leg 64 objectives

ODP Leg 164 is devoted to furthering our understanding of the in situ characteristics of gas hydrates and gas hydrate-bearing sediments.

The Blake ridge-Carolina rise gas hydrate field has been targeted for drilling because it is associated with an extensive and well-developed BSR (Fig. 1)(57728 bytes). The first drilling of a BSR occurred on the Blake ridge,4 and this area may be considered the "type section" for BSRs that are related to gas hydrates.5 6

The area is also well surveyed and lacks the regional tectonic influences that complicate hydrate distribution in accretionary prisms.

The objectives for Leg 164 include:

  1. better estimating the amounts of gas trapped in extensively hydrated sediments,

  2. understanding the lateral variability in the extent of gas hydrate development,

  3. investigating the distribution and fabric of gas hydrates within sediments,

  4. establishing the physical property changes associated with gas hydrate formation and decomposition in continental margin sediments,

  5. determining whether the gas captured in gas hydrates is produced locally or has migrated from elsewhere,

  6. measuring changes in the porosity (and permeability?) structure associated with gas hydrate cemented sediments,

  7. determining the role of gas hydrates in stimulating or modifying fluid circulation, and

  8. establishing the influence of the Carolina Rise diapirs on the gas hydrates as well as the origin of the diapirs themselves.

Drilling program

Drilling is planned along three transects on the Blake ridge-Carolina rise (Figs. 1(57728 bytes), 2(59685 bytes). Most of Leg 164 will be spent drilling four ~750 m deep holes across the Blake ridge where the geology and topography are relatively simple. These transects will enable comparison of the physical properties of individual sediment layers between closely-spaced drillsites where geophysical data indicate a lateral transition from non-hydrated to hydrated sediments.

Drilling is also scheduled on the crest and flanks of two diapirs (Cape Fear diapir and Blake Ridge diapir) where formerly or currently gas hydrate-bearing sediments are exposed near the seafloor.

The Blake ridge transect (Figs. 2(59685 bytes), 3(133458 bytes) consists of three closely-spaced holes across the flank of the Blake ridge, a large sediment drift of Tertiary age. All three holes will extend through the base of gas hydrate stability within the same stratigraphic unit. This short (~3.5 km long) transect extends from a place where there is no BSR to one where a very strong BSR exists in the seismic profiles.7 The tight spacing of these holes may provide insight into the cause of variation in BSR development.

The Cape Fear diapir transect is on the upper rise in the same geologic setting as the Carolina rise transect (Fig. 1(57728 bytes) but is associated with the sole of a major slump scar and the crest of an exhumed diapir.8 Piston core and dredge samples from the top of the diapir contain hemipelagic materials of diverse Tertiary ages. Because the continuity of the BSR is lost near the diapir, the emplacement of the diapir is believed to have produced a "hole" in the regional gas hydrate field.

A transect of four shallow holes (50-75 m deep) extends from near the topographic crest of the diapir, across the area where jointed allochthonous materials are currently exposed on the seafloor, and downslope to the slide scar southeast of the diapir (Fig. 2A)(59685 bytes). These strata were buried at depths of more than 100 m before being exhumed by slumping.

Evaluating the process of re-equilibration to the new, shallower conditions may provide great insight into the sensitivity of hydrated sediment to changes in the physical conditions and the dynamics of gas hydrate venting. Although the Carolina trough diapirs are believed to be salt structures,9 the existence of salt has not been documented.

Drilling of one to four holes up to 50 m in length is proposed along a short transect (~100 m in length) on the crest of the Blake Ridge diapir (Fig. 1)(57728 bytes) where a small fault extends through the Quaternary sediment cover (Fig. 2B)(59685 bytes). Near where this fault crops out, there are dense chemosynthetic biological communities and angular clumps of carbonate-cemented crusts on the sea floor. Gas-charged fluids emanate from this fault.10

The fault is believed to connect the base of the gas hydrate stability zone with the sea floor. The intention is to drill these shallow holes across the area where the fault intersects the sea floor in order to sample the migrating fluids and/or gases.

On Leg 164, emphasis will be placed on downhole measurements and in situ sampling. Vertical seismic profiles will be run in several holes, and an extensive well logging program is planned. Optimism exists that the pressure core sampler's (PCS) problems have been remedied, and thus we expect to make frequent PCS runs. The water sampling temperature probe (WSTP) will be used frequently, because it is critical to establish in situ salinities in order to calculate the amount of hydrate that broke down in the cores.

Studying core sections which contain gas hydrates may stress the normal core sampling and processing procedures that are so well established on the Joides Resolution drill ship. Normally more than an hour elapses before cores are cut. Previous experience with the deeper cores from the Blake ridge (DSDP Legs 11 and 76) indicates that these cores will be very gassy.

Core caps may blow off and materials may be extruded from the ends of the cores while they are warming on the rack. Thus, only the bigger pieces of hydrate will survive. Special care will be required to expedite the sampling process. Ultimately the success of the leg will rest on how well we can reconstruct what the sediments were like before the hydrates decomposed.

Leg 164 promises to be a unique opportunity to study fluids and gas phases within continental margins which potentially have enormous economic and geochemical significance. The ephemeral nature of the gas hydrates under surface conditions is a challenge that will add an element of intrigue to ODP Leg 164.

References

  1. Kvenvolden, K.A., Methane hydrateA major reservoir in the shallow geosphere?, Chemical Geology, Vol. 71, 1988, pp. 41-51.

  2. Sloan, E.D., Clathrate hydrates of natural gases, Marcel Decker, New York, 1990, 641 p.

  3. Claypool, G.E., and Kaplan, I.R., Methane in marine sediments, in Kaplan, I.R., (ed.), Natural gases in marine sediments, Plenum Press, New York, 1974, pp. 99-139.

  4. Hollister, C.D., Ewing, J.I., et al., Sites, 102, 103, 104, Blake-Bahama outer ridge (northern end), initial reports of the Deep Sea Drilling Project, Vol. 11, Washington, D.C., U.S. Government Printing Office, 1972, pp. 135-218.

  5. Markl, R.G., Bryan, G.M., and Ewing, J.I., Structure of the Blake-Bahama outer ridge, Journal of Geophysical Research, Vol. 75, pp. 4,539-55.

  6. Tucholke, B.E., Bryan, G.M., and Ewing, J.I., Gas hydrate horizon detected in seismic reflection-profiler data from the western North Atlantic, AAPG Bull., Vol. 61, 1977, pp. 698-707.

  7. Katzman, R., Holbrook, W.S., and Paull, C.K., A combined vertical incidence and wide-angle seismic study of a gas hydrate zone, Blake outer ridge, Journal of Geophysical Research, Vol. 99, 1994, pp. 17,975-995.

  8. Popenoe, P., Schmuck, E.A., and Dillon, W.P., The Cape Fear Landslide: Slope failure associated with salt diapirism and gas hydrate decomposition, in Submarine landslides: Selective studies in the U.S. Exclusive Economic Zone, USGS Bull. 2002, 1993, pp. 40-53.

  9. Dillon, W.P., Popenoe, P., Grow, J.A., Klitgord, K.D., Swift, B.A., Paull, C.K., and Cashman, K.V., Growth faulting and salt diapirism: Their relationship and control in the Carolina trough, Eastern North America, in Studies of Continental Margin Geology, Watkins, J.S., and Drake, C.L., eds., AAPG Memoir 34, 1982, pp. 21-46.

  10. Paull, C.K., Spiess, F.N., Ussler, W. III, and Borowski, W.A., Methane-rich plumes on the Carolina continental rise: Associations with gas hydrates, Geology, Vol. 23, 1995, pp. 89-92.

Bibliography

Dillon, W.P., and Paull, C.K., Marine gas hydrates: II Geophysical Evidence, in Cox, J.L., ed., Natural gas hydrates, properties, occurrence and recovery, Butterworth, Woburn, Mass., 1983, pp. 73-90.

The Author

Charles Paull is a professor of geology and marine sciences at the University of North Carolina at Chapel Hill. He has a BS in geology from Harvard College, an MS in marine geology and geophysics from the University of Miami, and a PhD in oceanography from Scripps Institution of Oceanography.

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