1. Leg 204 Summary1

Shipboard Scientific Party2

ABSTRACT

During Leg 204, we cored and logged nine sites on the Oregon continental margin to determine the distribution and concentration of gas hydrates in an accretionary ridge and adjacent slope basin, investigate the mechanisms that transport methane and other gases into the gas hydrate stability zone (GHSZ), and obtain constraints on physical properties of gas hydrates in situ. A three-dimensional seismic survey conducted from 19 June through 3 July 2000 provided images of potential subsurface fluid conduits and indicated the depth of the bottom-simulating reflector (BSR), a commonly used proxy for the base of the GHSZ. After coring at the first site, we acquired logging-while-drilling (LWD) data at all but one site to provide an overview of downhole physical properties prior to coring. The LWD data confirmed the general position of key seismic stratigraphic horizons and yielded an initial estimate of gas hydrate concentration through the proxy of in situ electrical resistivity. These records proved to be of great value in planning subsequent coring. We also tested the use of infrared (IR) thermal imaging of cores as a new and effective tool to identify gas hydrates as rapidly as possible after core retrieval. The thermal images were used to estimate the distribution and texture of hydrate within the cores. Geochemical analyses of interstitial waters and of headspace and void gases provided additional information on the distribution and concentration of gas hydrate within the GHSZ, the origin and pathway of fluids into and through the GHSZ, and the rates at which the process of gas hydrate formation is occurring. Biostratigraphic and lithostratigraphic descriptions of cores, measurement of physical properties, in situ pressure core sampling, and thermal measurements complement the data set, providing ground-truth tests of inferred physical and sedimentological properties.

Among the most interesting preliminary results are the following:

  1. Near the southern summit of Hydrate Ridge, very high concentrations of gas hydrate are present from the seafloor to ~30 meters below seafloor (mbsf), and they contain significant amounts of C2+ hydrocarbon gases in addition to methane.
  2. High chloride concentrations near the summit indicate that hydrate formation is recent and rapid.
  3. The lateral extent of the near-surface gas hydrate deposit at the summit can be mapped based on its backscatter and seismic signature.
  4. Away from the summit, no gas hydrate is present in the upper ~45 mbsf.
  5. Between ~45 mbsf and the base of the GHSZ, gas hydrates are distributed in lenses that are probably controlled by the physical properties of the sediments.
  6. In a slope basin east of Hydrate Ridge, gas hydrate concentration is quite low, with the probable exception of a 12-m-thick zone of relatively high concentration near the base of the GHSZ.
  7. Different physical and chemical proxies for gas hydrate distribution and concentration give generally consistent results.
  8. Thermal anomalies recorded by systematically scanning cores with an IR thermal-imaging camera provide a robust record of gas hydrate distribution that can be calibrated using estimates of in situ gas hydrate concentration derived from pressure core samples and anomalies in chloride concentration.
  9. An unprecedented number of cores retrieved at in situ pressure provide accurate estimates of subsurface methane concentration.
  10. Density logs of cores retrieved at in situ pressure provide details of in situ gas hydrate distribution and direct evidence for free gas within the GHSZ and can be used to study the response of gas hydrate to depressurization.
  11. The base of the GHSZ represents a discontinuity in C1/C2 and other chemical constituents of pore waters and gas voids, indicating that gas hydrate formation has profound geochemical effects.
  12. Several different geochemical mixing and fractionation signals provide constraints on fluid flow and gas hydrate dynamics.
  13. Ash-rich layers and thick turbidites serve as conduits for fluid flow.
  14. Borehole breakouts provide constraints on tectonic forces and possibly on the in situ strength of gas hydrate-bearing sediments.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 204IR-101

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