Skip past header information
USGS Logo with link to USGS Home Page
Coastal and Marine Geology Program
USGS Woods Hole Science Center

Open-File Report 96-272, Offshore Gas Hydrate Sample Database with an Overview and Preliminary Analysis
Skip past index

Home Page /  Synopsis  / Introduction  / Profile  / Relationships  / Salient Tendencies  / Acknowledgments  / Bibliography  / Appendix



Appendix- Column descriptions for Database (Database, Plate 1, Excel format 5 MB)

Column A GEOGRAPHIC AREA

  1. General geographic area (yellow fields)
      In alphabetical order, this column gives the name of the ocean or sea in which the gas hydrate sample was recovered This geographic name is, where appropriate, linked by a dash to the name of the physiographic province or another geographic name that serves as an aid to further locate the site. The name of the nearest country, state/province, city, or prominent feature to the site is given in parenthesis. If the sampling was a part of a Deep Sea Drilling Project (DSDP) or Ocean Drilling Program (ODP) activity and only one gas hydrate sample site was in the area, the leg and hole number are also provided.

  2. Listing of sites within the aforementioned area (white fields)
      Gas hydrate sites are numerated and, in cases where downhole sampling was involved, each separate zone of hydrate recovery is identified. Additional information is given for the site in the form of local names or core identification, as warranted. Note that a site is defined in terms of geographic position (latitude and longitude) and depth, in that individual hydrate zones associated with the sites, are also represented. Accordingly, DSDP or ODP leg and hole number corresponding to the site are provided as they apply.

Column B - CORE/SAMPLE COUNT

    The number of distinct gas hydrate occurrences sampled in the specified geographic area
      All samples were recovered using over-the-side coring equipment ("gravity" or piston) or downhole samplers, where the former class of samplers was effective only in sampling hydrates in surface sediments. Given that the literature cited does not'distinquish multiple hydrate zones in these cores, each core was taken to represent one gas hydrate sample.

Column C- PHYSIOGRAPHIC PROVINCE

    Classification of the area or subarea in terms of its physiography
      In most instances the classification is only taken to the second order (e.g., continental slope), although modifiers are used where possible (e.g., lower and upper continental slopes have a discernible difference in geologic setting).

Column D TECTONIC SETTING

    A binary assessment of regional tectonism: active or passive (yellow fields)
      The terms "active" and "passive" are applied within the context of plate tectonics.

Column E GEOGRAPHIC POSITION

    Latitude and longitude of the gas hydrate sample sites

Column F - WATER DEPTH

    Water depth (in meters) for each site
      Depths were taken directly from the literature.

Column G SUBBOTTOM DEPTH

    Vertical distance (in meters) from the sea floor ("mudline") to the position of the gas hydrate sample or the top of the sampled gas hydrate zone
      If the sediments are assumed to be normally consolidated (i.e, pore pressures are hydrostatic pressures), then this value plus the water depth (Column F) would equal the total hydrostatic pressure at the position of the gas hydrate occurrence; that is, pressure expressed in units of meters of seawater (1 m seawater ≈ 10 kPa).

Column H TEMPERATURE AT HYDRATE:GEOTHERMAL GRADIENT

  1. Temperature at the level of gas hydrate occurrence or at some other level within the sediment that is relevant to establishing an applicable geothermal gradient
    1. In cases where hydrate occurrence is at the sea floor, the temperature given is bottom water temperature.

  2. The geothermal gradient (in °C/km)
      The temperature at a given subsurface level may be estimated if the geothermal gradient is known. Thus, in the absence of downhole temperature data, the subsurface position of the combination of T and P that would be the phase boundary may be estimated using the geothermal gradient.
    In conjunction with total pressure, the temperature sets the position of the phase boundary; i.e., data in columns G'and H permit determination of the regional gas+water:gas hydrate phase boundary.

Column I ESTIMATED DEPTH TO PHASE BOUNDARY

    The vertical distance (in meters) from the sea floor to the regional phase boundary
      The temperature and pressure combinations required, and hence the water plus subseabed depth to the boundary (assuming sediments are normally consolidated) were estimated using information from four references, which are identified by the following superscripts:


        C&K -Claypool and Kaplan, 1974. Value shown based on curve for phase boundary in seawater plotted to a pressure of 55 MPa (hydrostatic pressure equivalent to total depth of ≈5,500 m).

        D&Q - Dickens and Quinby-Hunt, 1994. Value shown calculated from phase boundary equation for seawater to a pressure of 10 MPa (hydrostatic pressure equivalent to total depth of 1,000 m).

        E&B - Englezos and Bishnoy, 1988. Value shown calculated from phase boundary equation for seawater to a pressure of 13 MPa (hydrostatic pressure equivalent to total depth of 1,300 m).

        S - Sloan, 1990. Value shown calculated from phase boundary equation for pure water to a pressure of X33 MPa (hydrostatic pressure equivalent to total depth of 3,300 m).

      None of the estimates shown in Column I are based on extrapolations: all are within the P, T ranges presented in the cited studies. Results are rounded to 10-m increments.

Column J BOTTOM SIMULATING REFLECTOR PRESENT?

    Yes or No
      If yes, subbottom depth to bottom simulating reflector (BSR) at or proximal to the geographic position of the sample is 'given. If no, BSR may not be present or may not have been detected.

Column K OBSERVED THICKNESS OF HYDRATE ZONE

    Thickness of the interval of gas hydrate occurrence (thickness of relatively uninterrupted sequence of hydrate-bearing sediment)
      The thickness of the zone in which hydrates were physically present and ubiquitous, regardless of apparent relative abundance (default units are meters). In cases where gravity-driven coring devices were used, thicknesses given may be conservative estimates because corers may not have been able to penetrate the entire zone.

    Subseabed depths to upper and lower,zone boundaries (in meters) are shown in parentheses, where known.

Column L THICKNESS/SIZE OF PURE HYDRATE LAYER/GRAINS

    Measured or estimated dimensions, or other characterization, of the mass or size of a sample of pure gas hydrate recovered from the site (hydrate with trace to minor sediment content is considered pure).

Column M HABIT OR MODE OF OCCURRENCE

    Descriptors either used in or based on the cited references with respect to the appearance of the gas hydrate in the field
      With regard to the descriptions, no attempt was made to standardize terminology or provide an indication of scale. In addition, no attempt was made to discrimate between terminology applied to the gas hydrate alone versus the aggregate appearance of sediment+hydrate.

Column N SEDIMENT DESCRIPTION

    Descriptions of the sediment associated with the gas hydrate; that is, the host material for the hydrate
      These descriptions are largely_ excerpts from the cited references.

Column O APPARENT ORIGIN OF INCLUDED GAS

    Origin of methane in the hydrate: biogenic or thermogenic.

Column P OBSERVATIONS AND COMMENTS

    Additional 'information that may be relevant to understanding gas hydrate occurrence at this site or at this subbottom depth

Column Q REFERENCES

    Source of data and information for the row. Some temperature data were estimated from literature or from U.S. Naval Oceanographic Data Center profiles and no specific reference is provided. If reference(s) is in a yellow field, it applies to all samples in the subset; if in a white field, the reference only appplies to that specific row.

Prev Up
6. Bibliography Home