Natural gas hydrates are a potential source of energy1 and may play a role in climate change2 and geological hazards3. Most natural gas hydrate appears to be in the form of ‘structure I’, with methane as the trapped guest molecule4, although ‘structure II’ hydrate has also been identified, with guest molecules such as isobutane and propane, as well as lighter hydrocarbons5,6. A third hydrate structure, ‘structure H’, which is capable of trapping larger guest molecules, has been produced in the laboratory7, but it has not been confirmed that it occurs in the natural environment. Here we characterize the structure, gas content and composition, and distribution of guest molecules in a complex natural hydrate sample recovered from Barkley canyon, on the northern Cascadia margin8. We show that the sample contains structure H hydrate, and thus provides direct evidence for the natural occurrence of this hydrate structure. The structure H hydrate is intimately associated with structure II hydrate, and the two structures contain more than 13 different hydrocarbon guest molecules. We also demonstrate that the stability field of the complex gas hydrate lies between those of structure II and structure H hydrates, indicating that this form of hydrate is more stable than structure I and may thus potentially be found in a wider pressure–temperature regime than can methane hydrate deposits.
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Supplementary Figure 1 shows gas chromatographic data of the gas sample recovered from the dissociated gas hydrates from Barkley Canyon, offshore Vancouver Island. Supplementary Figure 2 shows the stability zones of methane hydrate and naturally occurred sII-sH complex gas hydrates from Barkley Canyon, offshore Vancouver Island. * The influence of pore water chemical compositions on sII and sH complex hydrates has been roughly adjusted according to the empirical equation of Lu and Matsumoto (2005). Supplementary Table 1 shows the X-ray Powder Data for sII and sH gas hydrates recovered from Barkley Canyon, offshore Vancouver Island Supplementary Table 2 shows the chemical shifts of hydrocarbons in their pure gas (or liquid) and hydrate phases. (PDF 183 kb)
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Lu, H., Seo, Y., Lee, J. et al. Complex gas hydrate from the Cascadia margin. Nature 445, 303–306 (2007). https://doi.org/10.1038/nature05463
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