Structural Studies of Lake Baikal Natural Gas Hydrates

The structure of near-bottom gas hydrate samples obtained in Lake Baikal during the expeditions in 2005–2018 are reported. The hydrates contain mainly methane and ethane. More than 85% of the samples contain hydrate of cubic structure I (sI) with up to 4.2 mol.% ethane. The concentration of ethane in the samples containing hydrate of cubic structure II (sII) is 12–14 mol.%. Refined unit cell parameters of natural hydrates are in good agreement with the data obtained in the studies of artificially synthesized hydrates. Possible mechanisms for the formation of sII hydrates are discussed. Some arguments are provided in favor of a probable presence of dispersed gas hydrates in the near-bottom layers of Baikal sediments and the possibility of small variations in the composition of hydrate gas in different hydrate fragments taken from the same hydrate layer.