Scanning Electron Microscopic Investigations on Natural and Synthetic Gas Hydrates: New Insights into the Formation Process

We present results of field-emission scanning electron microscopic investigations of gas hydrates from shallow marine sediments of Cascadia margin as well as from synthesis experiments. The natural hydrates were taken by TV-grab sampling during the TECFLUX project on RV SONNE cruises, SO143 and SO148 on the southern summit of Hydrate Ridge. The samples are dominantly methane hydrates with a low content of H2S (1.5-3.0 vol%). The hydrates develop as pure white ice-like layers in otherwise soft sediment deposits. The synthetic gas hydrates were prepared from pure CH4 gas at variable pressure and temperature including experimental conditions similar to the natural situation. All synthetic hydrates show a porous microstructure with pore diameters of a few hundred nm (see figure) and grain sizes of a few †m[1]. Samples were transferred to a pre-cooled cryo-stage field-emission scanning electron microscope via an interlock. No decomposition was observed during our work, which was carried out below -165° C in a vacuum of <10-5 mbar by using an electron beam of 1.0-1.5 keV. The microscope is connected with an energy-dispersive X-ray spectrographic analyzer, which can clearly identify methane in the clathrate structure by detecting the carbon peak in the elemental spectrum. The microstructures of the natural gas hydrates vary greatly with the magnification. In general, large pores between a few to hundreds of †m in diameter are observed, and these have been also documented in thin sections. These pores are interpreted to originate from gas bubbles that ascend from deeper in the sediment. The pores develop in the pore water as skins of hydrate around the former gas bubbles. We investigated the inner part of the former bubble walls by FE-SEM and could document tiny filaments that often form a network of honeycomb-like structures. EDX- analyses show that these filaments have Cl-Peaks, and we think the filaments are remnants of pore water salt that cannot be incorporated into the cage structure of the hydrates. Such ion exclusion may mark the boundaries of the gas hydrate crystal grains. Crystal diameters between 15-40 †m are in accordance with X-ray diffraction results of the same material. Higher magnifications under the FE-SEM (2000 fold and higher) revealed porous parts of the natural gas hydrates on a sub micron scale similar to the microstructure observed in synthetic hydrates with a more or less homogeneous three-dimensional sponge-like porosity. The pore size is typically 100-400 nm, again being in close agreement with the synthetic material. The areas with this porosity seem to always be surrounded by dense gas hydrate material. Only isolated pores and channels with diameters of 1-5 †m occur in the dense gas hydrate matrix. They appear to form a system of connected open pores through which water or gas could be transported. A first tentative explanation of the formation of porous hydrates on the basis of a physico-chemical microperforation model will be given. [1] Kuhs, W.-F., Klapproth, A. Gotthardt, F., Techmer, K. und Heinrichs, T. (2000): The formation of meso- and macroporous gas hydrates.- Geophysical Research Letters Vol. 27 No 18: 2929 - 2932