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

Abstract

AbstractWe 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 high-pressure 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 from 4.4% 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.