徐家围子断陷火山岩包裹体组分和同位素组成分析及应用

Analysis and application of components and isotopic compositions of fluid inclusions in volcanic reservoirs of the Xujiaweizi fault depression

应用真空机械破碎法对徐家围子断陷火山岩包裹体的组分和同位素组成进行了热爆前、后的对比分析,进而结合气井产出天然气的相应特征及热模拟实验结果对天然气的成因进行了探讨。结果表明,徐家围子地区火山喷发过程中的岩浆脱气烃类含量较高,暗示该区烃类天然气存在无机成因的可能,有必要对火山喷发脱气与普通岩浆活动脱气的差别做进一步研究;徐家围子断陷烃类天然气的碳同位素序列倒转主要是由不同期次/源天然气的混合作用造成,而非后期盖层散失过程中的组分和同位素分馏,同位素序列倒转不能单独用于判断天然气的成因;相对有机成因气,无机成因气(甲烷、乙烷和CO2)碳同位素值常较重,用甲烷碳同位素值重于-20‰作为无机成因气的判别标准有一定的适用性,但过于苛刻;徐家围子断陷CO2以无机成因为主,主要源于坳陷期地壳活动过程中的地幔岩浆脱气,无机成因CO2能够聚集成藏与其未经历初次运移,与地层水接触少等因素有关。上述认识有益于对徐家围子断陷天然气的成因作出正确判断。

A comparative analysis of components and isotopic compositions of gas inclusion in volcanic reservoirs of the Xu -jiaweizi fault depression before and after thermal decrepitation was performed by using the vacuum mechanical crushing method.The origin of natural gas was further discussed in combination with the corresponding characteristics of natural gas produced from wells and the results of thermal simulating test .The hydrocarbon content of gas from magma degassing is high,indicating that some hydrocarbon gases may be of inorganic origin , thus it is necessary to further research on the differences between gas from volcanic action and that from magmatic action .The main cause of reversal carbon isotopic se-quence of hydrocarbon gas in the study area is the mixing of natural gases generated in different times or from different sources ,rather than the component and isotopic fractionation during dissipation of natural gas through the caprock .There-fore,the reversal phenomenon can not be used individually to confirm the genesis of natural gas .In comparison with organic gas,inorganic gas（including CH4,C2H6,CO2）often has a heavier carbon isotopic value .It is applicable to some extent to use the the δ13 C1 value over -20‰as the criteria for determining inorganic gas ,but this criteria is over strict .The CO2 in Xujiaweizi fault depression is mainly sourced from magma degassing .The inorganic CO 2 did not experience primary migra-tion and had no chance to contact with formation water ,being favorable for their accumulation .These understandings are helpful to determine the origin of natural gas in Xujiaweizi fault depression .