ABSTRACT

Methane hydrates from both sub-permafrost and sea-floor occurrences show a very particular microstructure as evidenced by scanning-electron- microscopy. These hydrates are frequently porous with typical pore sizes ranging from 100 nm to 500 nm, occasionally reaching 1 μm. The pores are predominantly closed with only occasional openings between them and filled with methane gas. The gas-filling will affect the physical properties of gas hydrates. In particular, it can be expected that the attenuation of elastic waves is increased. We suggest that the repeatedly observed combination of high seismic velocities and high attenuation in gas-hydrate-bearing sediments may well be attributed to the presence of gas in the porous microstructures.

INTRODUCTION

The general picture of gas hydrate is that of a dense ice-like substance acting as a cement in sedimentary material although very little is known about the actual microstructures in geological settings. Laboratorygrown gas hydrates of methane, carbon dioxide and nitrogen have been found to be meso- to macroporous (Kuhs, Klapproth, Gotthardt, Techmer and Heinrichs, 2000). Meanwhile there is accumulated evidence that natural gas hydrates from both continental and sea-floor occurrences show identical microstructures. In fact, all natural samples investigated so far by means of cryo scanning electron microscopy show a very pronounced sub-micron porosity. In particular, samples from Hydrate Ridge (Suess, Bohrmann, Rickert, Kuhs, Torres, Trehu and Linke, 2002) as well as samples from the continental Mallik site, NWT, Canada (Techmer, Heinrichs and Kuhs, 2004) exhibit such porous microstructures together with denser parts. The occurrence of porous microstructures may be indicative for the occurrence of excess free gas during the formation (Staykova, Kuhs, Salamatin and Hansen, 2003). Here, we present the results of microstructural investigations of several marine and continental gas hydrates, comparing the observed structures with laboratory-made material.

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