| |
| ||||||||||||||||||||||||||||
Proactive disclosure Print version   | |  Gas Hydrates Gas hydrate studies on the west coast of Vancouver Island
R.D. Hyndman Contents of this page:
Deep sea gas hydrate off the west coast of Canada has been studied by seismic techniques and by Ocean Drilling Program boreholes. The bottom-simulating reflector (BSR) occurs in subduction zone accreted sediments in a 30 km wide band beneath much of the continental slope. The BSR temperature data here and elsewhere agree with the base of the hydrate stability field for pure methane and seawater (~2°C). The BSR results from the contrast between high-velocity disseminated hydrate concentrations above the BSR and an underlying low-velocity layer containing free gas. The top of the hydrate layer and the base of the gas layer are gradational. The free gas concentration is less than a few percent. The hydrate concentration from multichannel and downhole log velocities, and from pore fluid chemistry in ODP cores increases downward to about 20% of the pore space just above the BSR. 
Comprehensive data on deep sea gas hydrates have been obtained for the northern Cascadia subduction zone along the west coast of Vancouver Island, including studies with a variety of seismic techniques (1) and data from Ocean Drilling Program boreholes (ODP Site 889/890) (2). The bottom-simulating reflector (BSR) that marks the base of the hydrate stability field, mapped by multichannel and single channel seismic data, is common in a 30 km wide band beneath the mid-continental slope (1) (Figure 2 and Figure 3). The highest hydrate concentrations lie beneath the anticlines of the folded accretionary sedimentary prism (3). The gas in the ODP cores is primarily biogenic methane (2). The methane appears to be generated in the sediments over a depth range of several km and carried upward to the hydrate stability field by the fluid expulsion associated with sediment tectonic thickening (4) (Figure 3).
The BSR results from the impedance contrast between high-velocity hydrate concentrations in the pore spaces above the BSR and an underlying low-velocity gas layer. The BSR reflection is generally a simple symmetrical pulse over a range of source frequencies (3,5) (e.g., Figure 4). There is no reflection from the top of the hydrate layer or from the base of the underlying gas, so both of these boundaries are interpreted to be gradational (Figure 5). At the ODP site, the impedance contrast is about 2/3 due to the velocity increase of the hydrate and 1/3 due to the velocity decrease of the underlying free gas. Multichannel (MCS) amplitude-versus- offset (AVO) and seismic modeling results indicate that Poisson's ratio is not substantially reduced by the gas.
This result and the velocity below the BSR from borehole vertical seismic profile (VSP) data (6), indicate that the concentration of free gas must be less than a few percent. The sediment section is relatively homogeneous (on a seismic wavelength), and the critical reference velocity-depth profile for no hydrate and no gas is well constrained from upward extrapolation of the deeper multichannel interval velocity data (7) (Figure 6). On a seismic wavelength scale the deeper data show a simple increase in velocity with depth from increase in consolidation and decrease in porosity in the mainly clastic turbidite sediments. Above the BSR at the ODP site, the velocities from the MCS interval velocities (7), downhole velocity logs (2) and a downhole vertical seismic profile (VSP) (6) are in excellent agreement. The velocity enhancement due to hydrate increases downward from the seafloor to just above the BSR, where the velocity is about 1800 m/s compared to a reference velocity of about 1650 m/s (Figure 7). The low velocity underlying free gas layer is thin and is only detected in the VSP data and the full waveform modeling. It has a minimum velocity of about 1470 m/s.
The hydrate concentration has been estimated from the multichannel, downhole log and VSP velocities using several models for the velocity increase vs hydrate concentration (8). At the ODP site the concentration is estimated to increase downward to a maximum of 20% of the pore space (~10% of sediment) just above the BSR (Figure 7b). A similar concentration is estimated from a simple hydrate dissociationinterpretation of the low chlorinity of the pore fluid in the ODP cores (Figure 7c).
The continental slope hydrate contains a very large amount of methane; at the ODP site about 1 x 109 mæ/kmÙ (at STP). Additional methane is contained in the underlying trapped free gas. The free gas concentration is difficult to determine because velocity is insensitive to gas concentration for concentrations less than a few percent. However, the free gas layer probably contains much less methane than the overlying hydrate. If the estimated hydrate concentration at the ODP site is taken as representative of the areas of the Vancouver Island continental slope where there is a strong BSR (30 km by 200 km), the total gas is about 1013 mæ (350 Tcf). This is a 200 year supply for Canada at the present rate of natural gas consumption. Data from several other subduction zones suggest that this concentration of methane in hydrate may be representative of many of the larger clastic accretionary prisms around the world.
The BSR at ODP Site 889/890 (water depth 1300 m) occurs at a subbottom depth of about 230 m. The temperature is slightly lower than, but within the uncertainty of, the base of the hydrate stability field for seawater and methane, as determined by laboratory measurements. A compilation of DSDP/ODP temperature-pressure (depth) data for BSRs is shown in Figure 8. None of the data are ideal; for most of the points some extrapolation is required or there are disturbing effects, such that the uncertainties are about ~2 °C. As expected, most points are within the uncertainties of the seawater pure methane stability field. However, some unexplained differences are present.
Hyndman, R.D., G.D. Spence, T. Yuan and E.E. Davis, ODP, Initial Reports, 146, 399-419 (1995). Westbrook, G.K., B. Carson, R.J. Musgrae, et al., Proc. ODP, Initial Reports, 146 (1994) Spence, G.D., T.A. Minshull, and C. Fink, Proc. ODP, Scientific Results, 146 (1995). Hyndman, R.D., and E.E. Davis, J. Geophys. Res., 97, 7025-7041 (1992). Hyndman, R.D. and G.D. Spence, J. Geophys. Res., 97, 6683-6698 (1992). MacKay, M.E., R.D. Jarrard, G.K. Westbrook, and R.D. Hyndman, Geology, 22 459-462 (1994). Yuan, T., G.D. Spence, and R.D. Hyndman, J. Geophys. Res., 99, 4413-4427 (1994). Yuan, T., R.D. Hyndman, G.D. Spence, and B. Desmons, J. Geophys. Res., 101, 13,655-13,671(1996). Hyndman, R.D., J.P. Foucher, M. Yamano, and A. Fisher, Earth Planet Sci. Lett., 109, 289- 301 (1992).
|
