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Abstract

Clathrate hydrates—also known as gas hydrates—are ice-like compounds consisting of gas and water molecules. They occur wherever elevated pressures and low temperatures prevail; and where enough water and hydrate-forming gas molecules are available. Therefore, natural gas hydrates occur at all active and passive continental margins, in permafrost regions, in some deep lakes, and under unfavorable circumstances, also, in pipelines. This article provides an overview of the (thermodynamic) requirements and various models for the nucleation and growth of gas hydrates and the different gas hydrate structures that may occur and which have been detected in nature. Furthermore, this study also shows the influence of the properties of the enclosed gas molecules such as size and shape on the structure and thermodynamic properties of the resulting hydrate phase. Finally, the complexity of a natural environment with regard to the various influences of sediments, microbial activity, and salinity of the pore fluid on hydrate formation is also discussed.