火山-幔源与有机成因CO2气混合模型探讨——以南海北部边缘盆地为例

以南海北部边缘盆地为例，选取典型火山-幔源型CO₂气和不同有机成因CO₂气的碳同位素值作为端元，模拟计算了2种成因CO₂气按不同比例混合后碳同位素值变化规律。结果表明，无论有机成因CO₂的同位素值（-10‰～-25‰）和初始有机成因CO₂气含量（0.5%～2.0%）如何变化，当CO₂气含量小于10%（摩尔分数）时，混合后的δ¹³C_CO2值随总的CO₂气含量增加而迅速增大；而当CO₂含量超过约10%（摩尔分数）时，δ¹³C_CO2值增加缓慢，最后都迅速趋近火山-幔源型CO₂的端元碳同位素值（约-4‰）。该模拟结果合理地解释了为什么世界上所发现的火山-幔源型CO₂气为主天然气藏，其CO₂含量多高于10%（摩尔分数）或15%（重量分数），其碳同位素值大都介于-4‰~-7‰之间。模拟结果与北部湾盆地福山凹陷和珠江口盆地实测CO₂气的碳同位素值变化趋势吻合很好。

A Preliminary Study on the Model for Mixing of Volcanic-Mantle-derived and Organic CO2 -Northern Margin Basins of South China Sea as a Case

It has been widely accepted that the CO2 is mainly of inorganic origin when its content is higher than 10% (v %) or 15% (wt.%) in natural gas pools. Taking northern shelf margin basins of South China Sea as an example, this paper established a mixing model of typical volcanic mantle and organic CO2 to explain above rule for the first time. In despite of the differ ent δ13CCO2 values (-10‰~-25‰) and contents of organic CO2 (0.5%~2.0%), δ13C values of all mixed CO2 increase quickly with the increasing of the content of the total CO2 before 10%. When the CO2 content is higher than 10% (mol %)，the δ13CCO2 values increase very slowly and gradually approach to the end member value of volcanic mantle CO2. This model reasonably explained the phenomena that most of the observed volcanic mantle derived CO22 gas pools have a CO2 content of more than 10% (mol %) and have a diagnostic δ13CCO2 value of -4‰~-7‰. The calculated curve is finely consistent with the measured data of the Zhujiangkou Basin and the Fushan Depression of the Beibuwan Basin.