Research On Quantitative Method For The Determination Of Carbon Isotopic Compositions Of CO₂ In Individual Fluid Inclusions With Micro-laser Raman Spectroscopy

Stable isotope of individual fluid inclusion has great significance in researching the formation of rock and mineral paleo-fluid,geology of ore deposits,oil/gas mineral deposits and geodynamics of tectonic evolution.Carbon dioxide is a common and important component in fluid inclusions.Because carbon isotopic ratio of CO₂ in fluid inclusion can provide geochemical characteristics of source rocks,many works have been conducted to measure the carbon isotopic ratio?13C/12C?of CO₂.However,mass spectroscopy is a destructive analytical technique,and can not be applied to measure individual fluid inclusion.There is not many technology or method for the analysis of stable isotopes in individual fluid inclusions both at home and abroad.Raman spectroscopy is the most effective method for stable isotope analysis of individual fluid inclusion.In this paper,based on the study of principles and feasibility of the method of Micro-laser Raman spectroscopy,it is proposed that Micro-laser Raman testing method for stable isotopes of individual fluid inclusions could be established by Raman spectroscopy testing and researching of the standard materials with known isotopic ratios.In this way,Raman parameters and experimental conditions of stable isotopes in individual fluid inclusions could be determined.This paper introduced an instrument designed for online testing 13CO₂ and 12CO₂ under different pressures by Micro-laser Raman spectroscopy.The spectral characteristics of CO₂ carbon isotopes including peak position,peak area and half peak width were further analyzed and studied.This will provide standard Raman spectra for identification and analysis of CO₂ carbon isotopes in individual fluid inclusions.12CO₂ and 13CO₂ were mixed with N2 at various molar fraction ratios respectively through self-deviced experimental apparatus,and a series of 12CO₂/N2 and 13CO₂/N2 binary mixtures with various molar fraction ratios were synthesized.It was found that the Raman peak area ratios were proportional to the molar fraction ratios.The linearity of the working curves were very good.Moreover,the slopes of working curves were regarded as Raman quantification factor?F12CO₂ and F13CO₂?.It is concluded that F12CO₂ and F13CO₂ are equal to 0.523?0<C12CO₂/CN2<2?and 1.11998?0<C13CO₂/CN2<1.5?by using the area of characterized single peak?A[12]+ and A[13]+?.However,F12CO₂ and F13CO₂ are equal to 1.16259?0<C12CO₂/CN2<2?and 1.61086?0<C13CO₂/CN2<1.5?when using the sum of resonance peaks?A[12] and A[13]?through linear fitness.In addition,the values of ?12CO₂ and ?13CO₂ are 1.49 and 1.437.So the determination of Raman quantification factors offers a theoretical foundation for the research on computing the value of ?13C.Based on the study of principles and feasibility of the method of laser Raman spectroscopy,the molar fraction ratio C13/C12 would be calculated using the product of A12CO₂/A13CO₂?the ratio of Raman peak area?and F13CO₂/F12CO₂.The artificial inclusions with known molar fraction ratios were presented to testify the validity and precision of the method.So it is possible to measure the carbon isotopic composition??13C value?of artificial inclusions in the range of 0.1 MPa to 20 MPa through F12CO₂ and F13CO₂,which obtained by the area of characterized single peak.The ?13C values of natural CO₂ gas from Shengli Oil-field is-5.6 ‰ by using isotope ratio mass spectrometry?IRMS?.Then,measurement of ?13C values by Micro-Laser Raman analysis were carried out on natural CO₂ gas at room temperature under different pressures.It is found that the intensity of Raman peak at 1370 cm-1??+[13]?decreases when the gas pressure increases?> 2 MPa?.So the natural CO₂ gas were used by the method of Gaussian Multipeak fitting to obtain the Raman peak of 13CO₂??+[13]?with the program Origin 6.0.?13C values obtained by Micro-Laser Raman spectroscopy technology and IRMS are in good agreement with each other?relative errors <7%?.It is concluded that the representative Raman peak area are better to be used for the determination of ?13C values.This research provides fundamental analysis for determining gas carbon isotope composition??13C values?quantitatively by Micro-Laser Raman spectroscopy,and experiment results demonstrate that this method has the potential for obtaining ?13C values in natural CO₂ gas reservoir.In addition,the individual fluid inclusions?volume fraction of CO₂ > 70%?collected from the Yanchang formation sandstone reservoir,Ordos Basin,have been applied to estimate their carbon isotopic compositions??13C values?.The individual fluid inclusions have been analyzed by Micro-Laser Raman analysis method,it showed that the Raman peak of 13CO₂ at about 1370 cm-1??+[13]?was overlapped at high pressure?>2 MPa?.Then we used program Origin 6.0 by the method of Gaussian Multipeak fitting to achieve Raman peak of 13CO₂??+[13]?.Hence,this research provides fundamental analysis for determining gas carbon isotope composition??13C values?quantitatively by Micro-Laser Raman spectroscopy,and experiment results demonstrate that this method has the potential for obtaining ?13C values in natural individual fluid inclusions.It will help to free from restriction and uncertainty in tracing the origins and sources of paleo-fluids caused by the analysis on group fluid inclusions.