中国天然气水合物气的成因类型

Genetic types of gas hydrates in China

基于中国祁连山冻土带、南海北部珠江口盆地、台西南盆地的陆坡等天然气水合物样品资料,进行了天然气水合物气的成因类型分析。研究结果表明,祁连山木里地区中侏罗统江仓组发现的天然气水合物气主要是油型气,为自生自储型,δ^(13)C_1值为-52.7‰^-35.8‰,δ^(13)C_2值为-42.3‰^-29.4‰;还发现了少量煤成气,气源岩可能主要为中侏罗统木里组含煤地层,δ^(13)C_1值为-35.7‰^-31.3‰,δ^(13)C_2值为-27.5‰^-25.7‰。南海珠江口盆地与台西南盆地天然气水合物气主要是CO_2还原型生物气,δ^(13)C_1值为-74.3‰^-56.7‰,δD1值为-226‰^-180‰;还发现热成因气遗迹,δ^(13)C_1值为-54.1‰^-46.2‰。综合国内外20个地区(盆地)相关天然气水合物气地球化学资料,提出世界天然气水合物热成因气既有油型气也有煤成气,以油型气为主,在中国祁连山和加拿大温哥华岛附近识别出了少量煤成气,煤成气δ^(13)C_1值重即大于等于-45‰,δ^(13)C_2值大于-28‰;油型气δ^(13)C_1值为-53‰^-35‰,δ^(13)C_2值小于-28.5‰。世界天然气水合物气主要是生物成因气,并以CO_2还原型生物气为主,仅在俄罗斯贝加尔湖盆地发现乙酸发酵型生物气。CO_2还原型生物气δD1值重即大于等于-226‰,乙酸发酵型生物气δD1值轻即小于-294‰。世界天然气水合物的生物气δ^(13)C_1值最重的为-56.7‰,最轻的为-95.5‰,其中-75‰^-60‰是高频段。世界天然气水合物气δ^(13)C_1值最重为-31.3‰,最轻的为-95.5‰;δD_1值最重的为-115‰,最轻的为-305‰。

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. δ^13C1values range from-52.7‰ to-35.8‰, and the δ^13C2values 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 with δ^13C1of-35.7‰ –-31.3‰ and δ^13C2of-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, with δ^13C1of-74.3‰ –-56.7‰ and δD1 of-226‰ –-180‰. A trace amount of thermogenic gases were also found in these basins with δ^13C1of-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 heavy δ^13C1≥-45‰ and δ^13C2-28‰, while the oil-derived gas has δ^13C1from-53‰ –-35‰ and δ^13C2-28.5‰. 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 heavy δ D1 ≥-226‰, while gases of acetate fermentation have δD1 -294‰. The bacterial gas of gas hydrates in the world has the highest δ^13C1value of-56.7‰ and lowest of-95.5‰, with a peak range of-75‰ –-60‰. Gas hydrate in the world has the highest δ^13C1of -31.3‰ and lowest of-95.5‰ and the highest δD1 of-115‰ and lowest of -305‰.