C-H-O-N体系挥发分的拉曼定量分析:压力、温度和流体组成的影响

Raman spectroscopic measurements of the composition and pressure of C-H-O-N volatiles:Influences of pressure,temperature and fluid composition

拉曼光谱是定量分析流体包裹体中挥发分组成和压力的重要手段,而这又是恢复流体捕获温度、压力的前提。本文收集了CH4-N2、CH4-N2-H2O和CH4-N2-CO2-CO体系在21~350℃和1~60 MPa条件下的原位拉曼光谱,获取了CH4、N2、CO2和CO特征拉曼光谱的峰位和峰面积等光谱参数,并以上述体系为例,探讨了应用拉曼光谱定量分析C-H-O-N体系挥发分组成的关键技术问题,即压力、温度和流体组成对分析结果的影响。首先,在温度恒定的条件下,v1-CH4、v1-N2、(v1-2v2)-CO2高频峰和v1-CO峰位随着压力的升高而发生负向偏移。在压力恒定的条件下,v1-CH4、(v1-2v2)-CO2峰位随温度升高向高频偏移,但v1-N2和v1-CO峰位随温度变化未发生明显偏移,因而可以应用这些组分的特征峰位来反映流体压力。其次,CH4、CO2和CO相对于N2的拉曼定量因子随压力升高而增大,分别在大于10 MPa、10 MPa和2.5 MPa以上达到稳定值。温度对CH4和CO的定量因子影响不显著,但CO2的定量因子随温度升高明显降低。水对CH4定量因子影响仅在低压条件下显著,在200℃以下的高压条件下影响较弱。最后,以含未知CH4/N2比例的CH4-N2混合气体的熔融毛细硅管合成包裹体为例,提出了应用拉曼光谱定量分析流体包裹体中复杂挥发分比例和内压的一般方法。受实验条件和仪器设置的影响,各个实验室测定的拉曼定量因子存在较大差异,在应用拉曼光谱定量分析包裹体挥发分组成时应予以注意。

Raman spectroscopy is an important method for the quantitative determination of the composition and pressure of volatiles in fluid inclusions,which in turn are a prerequisite for obtaining the fluid trapping temperature and pressure.In this study,the in-situ Raman spectra of CH4-N2,CH4-N2-H2O,and CH4-N2-CO2-CO systems were collected over a wide range of temperature(21–350℃)and pressure(1–60 MPa).The spectral parameters including the peak position and peak area of the v1-CH4,v1-N2,upper(v1-2v2)-CO2,and v1-CO bands were obtained.By taking the above systems as examples,we investigated the key technical problems related to the Raman quantification of the volatile components in the C-H-O-N system,i.e.,the effects of pressure,temperature,and fluid composition on the analysis results.The major contributions of this study are as follows:(1)The wavenumbers of v1-CH4,v1-N2,(v1-2v2)-CO2,and v1-CO can be used to measure the pressure in complex C-H-O-N volatile components.At a constant temperature,the v1-CH4,v1-N2,upper(v1-2v2)-CO2,and v1-CO shifted to lower wavenumbers with increasing pressure.At a constant pressure,the v1-CH4 and upper(v1-2v2)-CO2 bands increased in wavenumbers across the entire investigated temperature range,whereas the v1-N2 and v1-CO did not show an obvious shift.(2)The effects of pressure,temperature,and fluid composition on the Raman quantification factors were evaluated.The relative Raman quantification factors of CH4,CO2,and CO with respect to N2 increased with rising pressure,and reached a stable value at pressures above 10 MPa,10 MPa,and 2.5 MPa,respectively.The influence of temperature on F(CH4)and F(CO)was not obvious,whereas F(CO2)decreased significantly with increasing temperature.The influence of water on F(CH4)was obvious only under low-pressure conditions and was weak under high-pressure conditions below 200℃.(3)A general method is proposed for the quantitative analysis of complex volatile fractions and internal pressures in fluid inclusions by Raman spectroscopy.Due to the influences of experimental conditions and instrument settings,the Raman quantitative factors determined by each laboratory are quite different.Attention should be given when applying Raman spectroscopy to the quantification of the volatile compositions and pressures of fluid inclusions.