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北部湾盆地福山凹陷油气钻探中发现了高含CO2气的天然气气藏。对CO2气稳定碳同位素、伴生稀有气体氦和氩同位素进行了分析研究,结果显示,福山凹陷CO2气稳定碳同位素偏重,(13CCO2为-5.01~-10.08‰,绝大多数样品大于-7.0‰,为无机成因CO2气特征;伴生稀有气体氦同位素3He/4He值为(4.74~5.03)×10-6,R/Ra值为3.38~3.59;伴生稀有气体氩同位素40Ar/36Ar值为1881~2190,也显示出幔源或壳幔混合CO2气的特征。综合判定认为,福山凹陷CO2为壳幔混合成因。始新统流沙港组岩浆岩体分布特征与CO2气藏分布范围基本一致,也表明幔源—岩浆可能是福山凹陷CO2气的主要来源。与南海北部边缘盆地其它地区如珠江口盆地西部、琼东南盆地东部CO2气成因一致,都为幔源—岩浆来源,或壳幔混合来源。
Natural gas reservoirs with high CO2 content have been found during oil and gas drilling in the Fukuyun Sag in the Beibu Gulf Basin. The stable carbon isotopes of CO2, helium and argon isotopes of associated noble gases were analyzed. The results showed that the stable carbon isotopes of CO2 in Fukuai Sag were (-5.1 ~ -10.08 ‰), most of them were larger than -7.0 ‰, (3He / 4He) of associated rare gas (4.74 ~ 5.03) × 10-6 and R / Ra value of 3.38 ~ 3.59. The 40Ar / 36Ar values of argon isotopes of associated noble gas ranged from 1881 to 2190, But also shows the characteristics of mantle source or crust-mantle mixed CO2 gas.The synthetical judgment indicates that the CO2 in Fushan Sag is caused by crust-mantle mixing.The distribution characteristics of the magmatic rocks in the Liusha Formation of the Eocene are basically consistent with the distribution range of the CO2 gas reservoirs, Source magmatism may be the main source of CO2 gas in the Fukuya Sag, which is similar to the origin of the CO2 gas in the western margin of the Pearl River Mouth Basin and the eastern Qiongdongnan Basin in other parts of the northern marginal basin of the South China Sea, including mantle-magma source or crust-mantle source.