Density of Natural and Synthesized Gas Hydrates as Estimated from Their Structural Parameters

The density of natural gas hydrates is indispensable in estimating hydrate saturation in sediments, and hydrate saturation is a parameter required for the determination of the resource amount. However, such a key parameter is rarely reported, mainly due to the difficulty in recovering high-quality hydrate samples and the inability to directly measure hydrate density under ambient conditions. The densities of gas hydrates can be estimated based on hydrate structural parameters, which were obtained by instrumental methods such as Raman, nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and neutron diffraction. In this study, mixed methane–propane hydrates are synthesized, and their densities are calculated. As the initial propane content decreases in the range of 1.15–4.07 mol %, the density of the mixed methane–propane hydrates also decreases. The factors that affect hydrate density are discussed, including depth, gas origin, and experimental conditions. The densities of natural and synthesized gas hydrates are estimated and summarized. Generally, the densities of natural sI methane hydrate range from 0.89 to 0.92 g·cm–3 across various geological environments, while sII hydrate from the Cascadia margin has a density of 0.957 g·cm–3, serving as a unified standard for specific environments. The densities of synthesized gas hydrates show strong gas composition dependence, with estimates of approximately 1.1 g·cm–3 for sI CO2 hydrate, approximately 1.9 g·cm–3 for sI xenon hydrate, approximately 0.9 g·cm–3 for sII C3H8 hydrate, and 0.93–0.98 g·cm–3 for different sII mixed hydrocarbon hydrates at 273.15 K, also presented as a unified standard.