Genetic types of gas hydrates in China

摘要

Researches were carried out on the origin of gas hydrate samples from the tundra in the Qilian Mountain, Pearl River Mouth Basin in the northern South Sea and the continental slope of Taixinan Basin in China. Gases of the gas hydrate samples from the Jurassic Jiangcang Formation in the Muli County in Qilian Mountain are mainly of oil-derived origin, characterized by self-generation and self-preservation.δ13C1values range from ?52.7‰ to ?35.8‰, and theδ13C2values vary from ?42.3‰ to ?29.4‰. There was a small amount of coal-derived gases, which might source from the coal-bearing Middle-Jurassic Muli Formation withδ13C1of ?35.7‰ – ?31.3‰ andδ13C2of ?27.5‰ – ?25.7‰. Gases of the gas hydrate samples from the Pearl River Mouth Basin and Taixinan Basin are dominated by bacterial origin of carbonate reduction, withδ13C1of ?74.3‰ – ?56.7‰ andδD1of ?226‰ – ?180‰. A trace amount of thermogenic gases were also found in these basins withδ13C1of ?54.1‰ – ?46.2‰. This study combined the geochemical data of gas hydrates from 20 areas (basins) in the world, and concluded that thermogenic gases of the gas hydrates in the world can be either of coal-derived or oil-derived origin, but dominated by oil-derived origin. A small amount of coal-derived gas was also found in the Qilian Mountain in China and the Vancouver Island in Canada. The coal-derived gas has relatively heavyδ13C1≥ ?45‰ andδ13C2> ?28‰, while the oil-derived gas hasδ13C1from ?53‰ – ?35‰ andδ13C2< ?28.5‰. Gas hydrates in the world mainly belong to bacterial origin of carbonate reduction. Methanogensesis of acetate fermentation was only found in some gas hydrates from the Baikal basin in Russia. Bacterial gases of carbonate reduction have relatively heavyδD1≥ ?226‰, while gases of acetate fermentation haveδD1< ?294‰. The bacterial gas of gas hydrates in the world has the highestδ13C1value of ?56.7‰ and lowest of ?95.5‰, with a peak range of ?75‰ – ?60‰. Gas hydrate in the world has the highestδ13C1of ?31.3‰ and lowest of ?95.5‰ and the highestδD1of ?115‰ and lowest of ?305‰.