A comparative Raman spectroscopic study of natural gas hydrates collected at different geological sites

Intact natural gas hydrates recovered on the West African margin in the South Atlantic Ocean (ZaiAngo and Neris II projects) and from the Norwegian Sea (Hakon Mosby Mud Volcano) are investigated by micro-Raman spectroscopy at ambient pressure and low temperature. The gas hydrates collected at different geological sites contain a high methane concentration relative to other minor components that are slightly dispersed in the samples. They crystallize in a type I cubic lattice structure as also confirmed by our preliminary synchrotron diffraction results obtained on the ZaiAngo specimen. However, detailed analysis of selected microscopic areas reveals a variation in the gas distribution among the different specimens. Trace amounts of CO2 and H2S can be identified by their characteristic vibrational signatures in the 1000–3800 cm− 1 spectral range. They are found to be co-clathrated with methane. Their presence produces a compositional effect on the relative cage occupancy of CH4, as determined from the integrated band intensity ratio corresponding to the molecular stretching modes of methane in the hydrate. The comparative Raman analysis of synthetic hydrates of H2S, CH4 and CH4-deuterohydrates allows the unambiguous assignment of weak band overtones of trapped methane and co-clathrated H2S molecular vibrations.