Comparison of the microstructures of natural and synthetic gas hydrates by cryo scanning electron microscopy and its relevance to our understanding of the formation process

We present results of cryo field-emission scanning electron microscopic investigations of gas hydrates from shallow marine sediments of Cascadia margin and Black Sea as well as of synthetic methane hydrate samples formed under a wide range of pressures and temperatures. 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 as well as from the Sorokin trough area of the Black Sea. The hydrates occur as pure white ice-like layers in otherwise soft sediment deposits. The synthetic gas hydrates were prepared from pure CH4 gas at pressures ranging from decomposition to 100 MPa and temperatures from -40 to 10°C including experimental conditions similar to the natural situation. All samples were prepared with an excess of gas (free gas) over periods of a few days up to several weeks. Samples were recovered and stored in liquid nitrogen before they were transferred to a pre-cooled cryo-stage field-emission scanning electron microscope via an interlock. No decomposition was observed during the electron microscopic work, which was carried out always below temperatures of -165°C. The electron microscope is equipped with an EDX system for a detection of light elements. In this way residual frozen water can be distinguished from genuine gas hydrate. Both, natural and synthetic gas hydrates show undistinguishable porous microstructures with pore diameters of several tens to a few hundred nm and grain sizes of a few micron. Grain boundaries are clearly visible in the natural samples by thin linings containing higher chloride contents. The microporosity that is characterized by pores between 100 and 400 nm is probably affecting the physical properties of gas hydrates like thermal conductivity and dynamic elastic properties. The similarity of the observed microstructures suggest that similar self-organized processes are involved in the gas hydrate formation both in natural settings and in the laboratory. Based on our laboratory experiments a phenomenological model for the growth of porous gas hydrates will be presented from which the involved time scales can be deduced and related to the geological phenomena.