Characteristics of Natural Gas Hydrates Occurring in Pore-Spaces of Marine Sediments Collected from the Eastern Nankai Trough, off Japan

Pore-space gas hydrates sampled from the eastern Nankai Trough area off of Japan were minutely characterized using several instrumental techniques. Gas chromatographic results indicated that the natural gas in the sediment samples studied comprises mainly CH₄. The concentrations of minor components varied according to depth. The powder X-ray diffraction patterns showed that the pore-space hydrates were of structure I (sI); the lattice constants were 1.183−1.207 nm. Both ¹³C NMR and Raman spectra confirmed that CH₄ molecules were encaged in sI hydrate lattice. The average cage occupancies were calculated, respectively, from the Raman data as 0.83 for small cages and 0.97 for large cages. The hydration numbers were determined as 6.1−6.2