Structure-II Clathrate Hydrates in the Daini-Atsumi Knoll of the Nankai Trough, Japan

We examined the crystallographic properties of natural gas hydrates (GHs) in hydrate-bearing sandy sediment sampled from new wells near the second offshore gas-production wells in the Daini-Atsumi knoll region of the eastern Nankai Trough (NT) area, Japan. The sediment layers in the Daini-Astumi knoll area include a silt-dominant (thin turbidite) unit, a sand-mud alternation sequence (upper side), and a thick sandy turbidite sequence (deeper side). GHs are concentrated in the sand-mud alternation and thick sandy turbidite sequences. In the literature, all GH crystals in the eastern NT are structure I (sI) methane (CH4) hydrates. In the sand layers of the sand-mud alternation sequences, we similarly observed sI CH4 hydrate crystals, but in the thick sandy turbidite sequence, Raman spectroscopy revealed sI hydrates enclosing both CH4 and ethane (C2H6). In this sequence, we also observed the C-C vibrations of C2H6 in structure II (sII) large (5(12)6(4)) cages and the C-H vibrations of CH4 in sII small (5(12)) cages. The sII hydrates enclosing CH4 and C2H6 were discovered in the deeper, thick sandy turbidite sequence near the bottom surface reflection. As the C2H6-to-CH4 composition ratio increased, the hydrate structure changed from sI to sII. Our new discovery of sII hydrates in the deeper layers of the eastern NT area is consistent with our previous study, which showed that the C2H6 composition ratio increases at deeper sampling depths.