Chemical and crystallographic characterizations of natural gas hydrates recovered from a production test site in the eastern Nankai Trough

Abstract

This study describes the chemical and crystallographic properties of natural gas hydrates recovered from a methane production test site in the eastern Nankai Trough. Gases released from the hydrate-bearing sediments contain methane as the main hydrocarbon component. The hydrate-bound gas includes small amounts of ethane and heavier hydrocarbons (less than ∼300 ppm). Concentrations of minor hydrocarbon components decrease in sediment cores recovered from shallower subseafloor depths. Molecular and isotopic analyses suggest a microbial origin for the natural gas distributed at this site. The 13C NMR and Raman spectra provide evidence that methane molecules are encaged in two distinct polyhedral cages of the structure I hydrate with a hydration number of 6.1. The powder X-ray diffraction profile shows that the crystal type of the gas hydrate is structure I (sI), with lattice constants estimated at 1.1841(2) nm at 83 K. At widely varying temperatures, the lattice constants of the pore-space natural gas hydrate crystals agree well with those of massive natural gas hydrate and artificial methane hydrate, suggesting that the mode of hydrate occurrence does not significantly affect the physical dimensions of the crystal lattice. The small amounts of ethane and heavier hydrocarbons that form sI hydrate have no influence on the lattice expansion of the pore-space hydrate. The density of the natural gas hydrate crystals in the hydrate-bearing sediment sample is estimated at 0.95 g/cm3 at 83 K.