Natural Gas Hydrates - from the Microstructure towards a Geological Understanding

The dissertation addresses mineralogical characteristics of natural gas hydrates from cold seeps in the Gulf of Mexico and the eastern Black Sea. The investigated properties are the crystal structure, the crystallite sizes and size distributions, the compositions of the hydrate-forming gases, the hydrate porosity as well as the grain boundary networks. That was accomplished using X-ray diffraction, gas chromatography, Raman-spectroscopy, and scanning electron microscopy. "Bragg tomography" was applied for crystallite size measurements of hydrates.Computer tomography using Synchrotron radiation was used for the first time for gas hydrate inspections on a micro-scale.The crystallite sizes of natural hydrates are governed by the velocity of crystal growth, therefore time is an important factor. One important result of this thesis is that the gas hydrate crystallite size evolution is also affected by more factors, which are impurities in the grain boundary network such as oil and salts. Gas hydrates from the northern Gulf of Mexico crystallize to structure type II hydrate (sII) from methane and a fraction of up to 30 mol% of C2 through C5 hydrocarbons. One important result of this thesis is that the lower molecular mobility of C2-C5 hydrocarbons compared to methane incorporated in hydrates is displayed phenomenologically by dense, non-porous surfaces of the gas hydrates on a micrometer scale. Instead, hydrates which were formed from volatiles like methane, often crystallizing to sI, are characterized by a micro-porosity. A further objective of this thesis was to investigate the coexistence of sI and sII at the Chapopote asphalt volcano in the southern Gulf of Mexico. sI and sII hydrates trapped below viscous asphalt are intimately associated on short distances within individual crystallite-agglomerates. Micro-computer tomograms of the Chapopote hydrates showed small bubbles as well as genetic changes of the hydrate outside hydrate stability. The presence of bubbles indicates that free gas co-occurs together with the gas hydrate. An important outcome is that also well-preserved hydrates can be accompanied by an ice fraction not deriving from hydrate dissociation; accordingly, ice is not an exclusive indicator for hydrate decomposition. The potential effect hydrates may exert on climate was investigated at the Batumi gas seep, eastern Black Sea. In the study area (0.5 km2) the hydrate-bound carbon within the upper 2.65 m below seafloor was estimated on 10.7 kt. The rough seafloor topography suggests episodic detachment of gas hydrate chunks from the seafloor, which float upwards due to the positive buoyancy.