CAARI 2006 IBA08 Abstract 80
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MICROSTRUCTURE OF NATURAL HYDRATE HOST SEDIMENTS
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K.W. Jones1, P.B. Kerkar2, D. Mahajan1, 2, W.B. Lindquist2, H. Feng3
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Brookhaven National Laboratory, Upton, NY 11793-5000, USA
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Stony Brook University, Stony Brook, NY 11794, USA
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Montclair State University, Montclair, NJ 07043, USA
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PACS Codes: 91.50.Hc, 92.20.Uv
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Keywords: methane hydrate, tetrahydrofuran hydrate, sediments, microstructure, synchrotron-
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computed microtomography
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Corresponding Author: K. W. Jones, Brookhaven National Laboratory, Building 901A, Upton,
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NY 11973-5000, FAX 631 344-5271, Phone 631 344-4588, Email jones@bnl.gov
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Abstract
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There is worldwide interest in the study of natural gas hydrate because of its potential impact on
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world energy resources, control on seafloor stability, significance as a drilling hazard, and
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probable impact on climate as a reservoir of a major greenhouse gas. Gas hydrates can: (a) be
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free floating in the sediment matrix, (b) contact, but do not cement, existing sediment grains, or
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(c) actually cement and stiffen the bulk sediment. Seismic surveys, often used to prospect for
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hydrates over a large area, can provide knowledge of the location of large hydrate concentrations
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because the hydrate location within the sediment pores can have a profound influence on its
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seismic properties. The ability to image a sample at the grain scale and to determine the
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porosity, permeability, and seismic profile is of great interest since these can help determine the
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location of hydrates with certainty. We report here on an investigation of the properties of
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methane hydrate sediments at the grain-size scale using the synchrotron radiation-based
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computed microtomgraphy (CMT) technique. Work has started on the measurements of the
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changes occurring as tetrahydrofuran hydrate, a surrogate for methane hydrate, is formed in the
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sediment.
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Introduction
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Methane Hydrates as a Future Energy Source: The United States currently consumes about
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21 trillion cubic feet (Tcf) of natural gas per year. The U.S. Geological Survey (USGS) [1]
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estimates that taken collectively from all sources, there is enough methane (~ 200,000 Tcf) in the
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form of hydrates- methane locked in ice- to supply energy for the U.S. for hundreds, maybe
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thousands, of years. Of this, the recoverable methane hydrates buried under the U.S. waters and
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Alaska hold some 200 Tcf of natural gas, which would be enough methane to supply the entire
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nation's energy needs for over a decade at its present rate of consumption. This leads to the hope
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that methane hydrates could potentially provide a solution to our dwindling fossil fuel supply
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provided an environmentally compatible extraction method is developed.
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Distribution of Hydrates in Sediments: There are a number of rock physics models in the
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literature that attempt to quantify gas hydrates bearing sediment. The cementation models of
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Dvorkin and Nur [2] treat the grains as randomly packed spheres with the gas hydrates occurring
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at contact points (model 1) or coating the grains (model 2). Models 3 and 4 are variations of the
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cementation models, but consider the gas hydrate as either a component of the load-bearing
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matrix or as filling the pores [2, 3]. A pore-space hydrate fills intergranular, interconnected pores
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of sands and sandstones, which clearly contrasts with nodule and disseminated types (Model 6).
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A pore-filling hydrate is small-sized and ranges up to 10 mm.
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continuously and effectively under conditions to produce methane. Model 5 is an inclusion-type
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model that treats gas hydrate as the matrix and grains as inclusions. Models 1-5 all consider gas
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hydrate as homogeneously distributed in the sediments. However, evidence of gas hydrate coring
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It is likely to decompose
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within the DSDP, ODP and Mallik 2L-38 gas hydrate projects [4] reveals that hydrates often
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exist as pure aggregation (massive bodies, nodules, layers) and disseminate as fracture fillings in
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shallow sediments. This geometry is illustrated in model 6 where layered hydrate and massive
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hydrates is horizontally extensively continuous and related to strata in which the hydrate
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thickness should exceed 100 mm.
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Objective: Sediment properties such as pore-size distribution, wetability, and mineralogy can
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affect the kinetics of growth and dissociation of hydrates in porous media. The hydrate growth
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model, sediment morphology, and the distribution of phases in pores are critical for
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understanding seafloor stability, and modeling the dissociation kinetics of production methods
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such as depressurization, thermal stimulation and chemical inhibition [5]. THF is miscible with
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water and forms structure II hydrate with water at 19% by weight. Natural gas hydrates require
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high pressure for their formation since the natural gas molecules are only slightly soluble in
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water. The THF-hydrate serves as a surrogate for methane hydrate as it melts at 4.4oC at ambient
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pressure, has comparable density (0.94 g/cc with hydrate (0.91 g/cc) [6]. As the thermal
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conductivities of water (0.6 W/moK) and hydrate (0.5 W/moK) are very small, the rate of
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freezing is constant. However, ice or hydrate phase formation results in expansion of the sample,
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increasing the porosity and microcracks. In rapid freezing, there can be abrupt change in
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morphology and unreacted THF/water or air in the microcracks can decrease the overall thermal
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conductivity even though the thermal conductivity of ice (2.2 W/moK) is four times larger than
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that of water.
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Experimental Approach: In the present study, the investigated samples were taken from the
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Blake Ridge (BLR) 178 Leg in the Atlantic Ocean off the coast of North Carolina and from the
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Keathley Canyons (KC) 151-3 location in the Gulf of Mexico (GOM). The BLR material was
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obtained from deeply buried sediments at 667 m below the ocean floor. The Gulf of Mexico
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material was obtained at a total depth of 1887 m (<1 m below the mud line). Measurements
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were made on both wet and dry samples. CMT data were collected at the National Synchrotron
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Light Source (NSLS) at Brookhaven National Laboratory (BNL) using the bending magnet beam
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lines X2B and X27A. X-rays passing through the sample struck a scintillation detector viewed
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by charge-coupled device detectors with sizes approximately 1300 x 1000 pixels. The effective
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pixel sizes used for the measurements were in the range of 3 μm to 8 μm. The data collection
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involved collection of individual frames taken in angular increments of about 0.15°.
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Tomographic sections through the sample were then created using several different
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reconstruction programs. Two methods of sample cooling were used. One approach used a jet
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of cooled nitrogen [7] to produce a sample temperature of 28°C. The other used a jacketed
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container and a laboratory chiller to produce a sample temperature of 6oC. Both values were
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low enough to produce THF hydrates.
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Results and Discussion : The gross effects of freezing the sediment/THF/H2O mixture can be
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determined by making radiographs as a function of time following the start of cooling. We used
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a sieved sample of GOM sediment with nominal grain sizes between 52 and 108 µm compared
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with the effective pixel size of the CCD camera of 4 µm. The distribution of attenuation
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coefficients in the sample is then found from the radiographs. The results of the measurements
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are displayed in Figure 1. The peak found for the sediments shift to lower values of attenuation
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as a function of time as expected from the creation of frozen material. Also note that the shape
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of the distribution is affected by the freezing process with width of the main peak from the
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sediments showing an increase in width during the freezing process. There is no evidence for
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creation of large void space or space filled with a hydrate/ice mixture devoid of sediments.
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Tomographic measurements were made on both Blake Ridge and GOM sediments.
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Sections through samples of the Blake Ridge material that sieved into different size groups and
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for the as-acquired material are shown in Figure 2. The sediment is fine grained and the sections
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of the raw material and material passing 50 μm are strongly affected by the smaller grains. This
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suggested the use of samples with larger grain sizes for the initial tomographic measurements for
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investigation of the hydrate formation process.
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Complete tomograms were also acquired for warm and frozen sieved and unsieved GOM
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sediment.
Measurements were made on a specimen of GOM material sieved to produce a
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sample with grains in the 52-108 m range saturated in a liquid mixture composed of 60% of
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THF-40% H2O. Tomograms were obtained for a warm specimen and on a frozen specimen at a
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temperature of –6 °C. Visually, there is no clear indication of differences in the sections and no
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clear indication of void or low attenuation value pixels. This was verified by observation of 50
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different slices from each sample. Tomograms obtained for the as-acquired GOM sediments
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gave similar results. In this case, the voxels were about 8 µm in size so that the details in that
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range would not be resolved. Again, there was no evidence for the formation of sediment-free
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voxels. The frozen material is composed of distinct regions with coarser and finer particles
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found in relatively distinct regions. However, the lowest attenuation coefficients found for the
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sediments are still larger than those found in the plastic sample container showing that the
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hydrates and ices are mixed with the finer sediment particles presumably originally attached to
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the larger particles collected in the sieving process.
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The present study focused on the THF-hydrate system in depleted sediment taken from
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two sites: Blake Ridge and GOM. A low-pressure requirement (1 atm) to form THF-hydrate
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allows THF to serve as a good surrogate for methane hydrate. We, therefore, chose to use THF in
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this study as we develop a system that can handle high pressure at beam line and allow us to
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conduct methane hydrate study. The experimental results gave no evidence for the existence of
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free-floating hydrate/ice at the micrometer size scale. Modification to the experimental design
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will be needed for further investigation of the hydrate-sediment grain interactions.
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Acknowledgments
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Research was supported in part by the US Department of Energy under Contract No. DE-AC02-
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98CH10886 (KWJ and HF) and the Office of Fossil Energy (DM and PK). The specimen cooler
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was provided by the PXRR, Research Resource for Macromolecular Crystallography, financial
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support for which comes from the Offices of Biological and Environmental Research and of
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Basic Energy Sciences of the US Department of Energy, and from the National Center for
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Research Resources of the National Institutes of Health.
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References
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[1] Kvenvolden, K.A., McMenamin, M.A.: “Hydrates of Natural Gas: A Review of their
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Geologic Occurrence, U.S. Geological Survey Circular, 825, pp. 11, 1980
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[2] Dvorkin, J., and Nur, A.: “Elasticity of High-Porosity Sandstones: Theory for Two North Sea
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Datasets”, Geophysics, 61, 1363-1370, 1996.
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[3] Helgerud, M., Dvorkin, J., Nur, A., Sakai, A., Collett, T.: “Elastic Wave Velocity in Marine
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Sediments with Gas Hydrates: Effective Medium Cooling”, GRL, 26, 2021-2024, 1999.
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[4] Dallimore, S.R., Uchida, T., Collett, T.S.: “Scientific Results from JAPEX/JNOC/GSC
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Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada”,
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Geological Survey of Canada Bulletin 544, February 1999.
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[5] Mahajan, D., Taylor, C., Mansoori, G.A., “Natural Gas Hydrate/Clathrate; Major Organic
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Carbon Reserve of the Earth”, Journal of Petroleum Science & Engineering, Special Volume,
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2006
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[6] Koh, C.A., “Towards Fundamental Understanding of Natural Gas Hydrates”, Chem. Soc.
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Rev., 31, pp. 157-167, 2002.
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[7]
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Crystallography”, Oxford Instruments.
htttp://bionsrrc.org.tw/document/CryoJet.pdf,
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“Cryojet–Nitrogen
Jet
for
X-ray
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Figures
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Figure 1. Time sequence of radiographs of a sample of sieved GOM sediments, THF, and H 2O
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taken as the sample was cooled and frozen.
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Figure 2: Tomographic sections through BLR sediments measured with a voxel size of 3.97 µm
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are shown for a series of sieved samples. The sections shown are for: unsieved material (top
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left), less than 50 µm (top right), 107-250 µm (bottom left), and 250-500 µm (bottom right). The
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x-ray attenuation is shown on a gray scale with white as the highest value. The scales vary from
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section to section.
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Figure 3: Tomographic sections through a frozen sieved specimen of GOM sediment/THF and
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H2O. The diameter of the section on the top is 4000 μm. The enlarged portion shown below is
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128 μm x 128 μm in size.
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Figure 4: The histogram for attenuation coefficients measured in the full sample section of
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Figure 3 is shown at the top. A surface plot of the attenuation coefficients found for the
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magnified section given at the bottom of Figure 3 is shown at the bottom of the figure.
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