Calibration data for simultaneous determination of P - V - X properties of binary and ternary CO 2 - CH 4 - N 2 gas mixtures by Raman spectroscopy over 5–600 bar: Application to natural fluid inclusions

The P - V - X properties of two-component fluid inclusions (FIs) are generally determined from microthermometry data using appropriate thermodynamic models (i.e., VX diagrams) and/or equations of state (EoS). However, some limitations can hamper the applicability of this technique such as the small size, low density or complex composition of the analyzed FI. Raman spectroscopy is known as the best-suited alternative method to microthermometry for the investigation of natural FIs because it can provide simultaneously non-destructive qualitative and possible quantitative analyses after specific calibrations. The present work aims to provide calibration data to directly determine the P - V - X properties of binary or ternary mixtures of CH 4, CO 2, and N 2. The variation of spectral features as a function of composition and pressure (or density) was investigated by using Raman spectroscopy coupled with an improved High-Pressure Optical Cell (HPOC) system and a customized heating-cooling stage. From our experimental data, the relative Raman scattering cross-section (RRSCS) of CH 4 (ν CH4 ∗ ) was demonstrated to be constant at 7.73 ± 0.16 over the investigated range of pressure (5–600 bars) and for any composition. This parameter can thus be used for the determination of composition with an uncertainty of ~0.5 mol%. Several calibration equations were calculated for different PX domains, linking the Fermi diad splitting of CO 2 (Δ) or the relative variation of the CH 4 peak position (ν CH4 ∗ ) to the pressure (or density) and composition of CO 2 -CH 4, CH 4 -N 2, and CO 2 -N 2 -CH 4 mixtures at 22 and 32 °C. The pressure and density of the fluids can henceforth be directly measured from Raman spectra with an uncertainty of ~20 bars and ~0.01 g·cm −3, respectively. Our calibration equations were then validated on natural FIs by comparing the results obtained from Raman and microthermometry. We also interpreted the variation of the peak position of CH 4 based on the change of intermolecular interaction. Finally, we discussed the applicability of the obtained calibration data into another laboratory by comparing it with the data of pure CO 2 and CH 4 published in literature. A small shift between calibration curves implies a systematic error which is perhaps due to the difference in the configuration or the day-to-day deviation of the instruments. Therefore, standards of well-known P - V - X properties should be regularly measured to prevent and to correct any variation or shifting of the instrumental responses.