In situ Raman spectroscopic quantification of CH4–CO2 mixture: application to fluid inclusions hosted in quartz veins from the Longmaxi Formation shales in Sichuan Basin, southwestern China  

作　　者：Ye Qiu Xiao-Lin Wang Xian Liu Jian Cao Yi-Feng Liu Bin-Bin Xi Wan-Lu Gao 

机构地区：[1]School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,Jiangsu,China [2]Ocean College,Zhejiang University,Zhoushan 316021,Zhejiang,China [3]Wuxi Research Institute of Petroleum Geology,SINOPEC,Wuxi 214151,Jiangsu,China

出　　处：《Petroleum Science》2020年第1期23-35,共13页石油科学（英文版）

基　　金：financially supported by the National Natural Science Foundation of China(Grant Nos.41922023 and 41830425);the Fundamental Research Funds for the Central Universities(Grant Nos.020614380056 and 020614380078).

摘　　要：We re-evaluate the Raman spectroscopic quantification of the molar ratio and pressure for CH4–CO2 mixtures.Firstly,the Raman quantification factors of CH4 and CO2 increase with rising pressure at room temperature,indicating that Raman quantification of CH4/CO2 molar ratio can be applied to those fluid inclusions(FIs)with high internal pressure(i.e.,>15 MPa).Secondly,the v1(CH4)peak position shifts to lower wavenumber with increasing pressure at constant temperature,confirming that the v1(CH4)peak position can be used to calculate the fluid pressure.However,this method should be carefully calibrated before applying to FI analyses because large discrepancies exist among the reported v1(CH4)-P curves,especially in the highpressure range.These calibrations are applied to CH4-rich FIs in quartz veins of the Silurian Longmaxi black shales in southern Sichuan Basin.The vapor phases of these FIs are mainly composed of CH4 and minor CO2,with CO2 molar fractions from4.4%to 7.4%.The pressure of single-phase gas FI ranges from 103.65 to 128.35 MPa at room temperature,which is higher than previously reported.Thermodynamic calculations supported the presence of extremely high-pressure CH4-saturated fluid(218.03–256.82 MPa at 200°C),which may be responsible for the expulsion of CH4 to adjacent reservoirs.We re-evaluate the Raman spectroscopic quantification of the molar ratio and pressure for CH4–CO2 mixtures. Firstly, the Raman quantification factors of CH4 and CO2 increase with rising pressure at room temperature, indicating that Raman quantification of CH4/CO2 molar ratio can be applied to those fluid inclusions（FIs） with high internal pressure（i.e., > 15 MPa）.Secondly, the v1（CH4） peak position shifts to lower wavenumber with increasing pressure at constant temperature, confirming that the v1（CH4） peak position can be used to calculate the fluid pressure. However, this method should be carefully calibrated before applying to FI analyses because large discrepancies exist among the reported v1（CH4）-P curves, especially in the highpressure range. These calibrations are applied to CH4-rich FIs in quartz veins of the Silurian Longmaxi black shales in southern Sichuan Basin. The vapor phases of these FIs are mainly composed of CH4 and minor CO2,with CO2 molar fractions from4.4% to 7.4%. The pressure of single-phase gas FI ranges from 103.65 to 128.35 MPa at room temperature, which is higher than previously reported. Thermodynamic calculations supported the presence of extremely high-pressure CH4-saturated fluid（218.03–256.82 MPa at 200 °C）, which may be responsible for the expulsion of CH4 to adjacent reservoirs.

关 键 词：Raman spectroscopy Molar ratio FLUID pressure CO2–CH4 MIXTURE FLUID inclusion Paleo-overpressure 

分 类 号：O657.37[理学—分析化学] P618.13[理学—化学]