Natural Gas Hydrates Recovered from the Umitaka Spur in the Joetsu Basin, Japan: Coexistence of Two Structure-I Hydrates with Distinctly Different Textures and Gas Compositions within a Massive Structure

This study evaluated natural gas hydrates recovered from three different wells (J13R, J07RB, and J23R) at the Umitaka Spur in the Joetsu Basin, via Raman spectroscopy. The hydrates in two of the samples (J13R-13P and J07RB-5P) were of the particle displacement type, whereas sample J23R-3P displayed hydrates of the massive type. The hydrate crystals in all three samples contained a structure-I (sI) hydrate that primarily captures methane (CH4) molecules with ethane (C2H6), carbon dioxide (CO2), and/or hydrogen sulfide (H2S), which were observable using powder X-ray diffraction and Raman spectroscopy techniques. In the massive type J23R-3P sample, two hydrate crystals with different textures were found to coexist, with one displaying an icy appearance and the other a columnar shape. Interestingly, the icy J23R-3P crystals were sI hydrates containing mostly CH4 with C2H6, CO2, and H2S, whereas the columnar J23R-3P crystals were sI CH4 hydrates containing few other guest molecules. The coexistence of two diff...