北部湾盆地福山凹陷CO2气成因探讨

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，(13C_CO2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。

ORIGIN OF CO2 GAS IN FUSHAN DEPRESSION IN BEIBU GULF BASIN

Natural gas pools with high content of CO2 were discovered during exploration in Fushan depression, Beibu gulf basin. Stable carbon isotopic composition and associated helium and argon isotopic composition were analyzed. The stable carbon isotope of the CO2 in the study area is relatively heavy, with values of δ13CCO2 ranging from -5.01‰ to -10.08‰ and mostly over -7.0‰, indicating an inorganic origin. The 3He/4He value of associated helium gas ranges from 4.74×10-6 to 5.03×10-6, with a R/Ra value of 3.38～3.59, while the 40Ar/36Ar value of associated argon gas ranges from 1881 to 2190, also indicating a mantle source or mixed crust mantle source of the CO2 gas. Based on comprehensive analysis, it is believed that the CO2 gas in Fushan Depression is of crust mantle source. The occurrence of volcanic rocks in Eocene Liushagang Formation is basically consistent with that of the natural gas pools with high content of CO2, which also confirm the crust mantle source. It is similar in origin with the CO2 gas in other basins on the northern margin of South China Sea, such as the western Pearl River Month Basin and Eastern Qiongdongnan Basin. They are all of mantle magmatic origin or mixed crust mantle origin. A comprehensive study of the genesis and reservoiring characteristics of the CO2 gas in the study area is of great significance to lower the risk of hydrocarbon gas exploration or to search for commercial pure CO2 gas pools with high resources abundance.