Determination of the pressure and composition of wet gas fluid inclusions: An in situ Raman spectroscopic approach

Abstract

Carbonaceous fluid within mineral-hosted inclusions provides important information for carbon cycle in deep Earth. In addition to CH₄ and CO₂, heavy hydrocarbons (e.g., C₂H₆ and C₃H₈) are frequently observed in carbonaceous fluid inclusions (i.e, wet gas inclusions with C₁/∑C_i < 0.95). However, determination of the composition of such complex volatiles is difficult based on traditional microthermometric measurements. Here we carried out experimental calibrations on Raman spectroscopic measurements of the pressure (P) and composition of the CH₄ ± C₂H₆ ± C₃H₈ ± H₂S system at room temperature and 0.1-130 MPa. We confirmed that the C-H symmetric stretching vibration band of CH₄ [ν₁(CH₄), ∼2917 cm^-1] shifted to lower wavenumber with rising pressure, thus the P-ν₁(CH₄) relationship could be applied to calculate the pressure of wet gas. It should be noted that the presence of C₂+ and/or H₂S will shift the [ν₁(CH₄)] to lower wavenumber at constant pressure (with the order of C₃H₈ ≥ H₂S > C₂H₆). Obviously, the P-ν₁(CH₄) relationship derived from pure CH₄ system could not be simply applied to wet gas inclusion, otherwise the pressure would be overestimated. To avoid the overlap of the C-H vibrations of CH₄, C₂H₆ and C₃H₈, the peak areas and peak heights of the overtone vibration of CH₄ [2ν₄(CH₄), ∼2580 cm^-1], C-C symmetric stretching vibrations of C₂H₆ [ν₃(C₂H₆), ∼995 cm^-1] and C₃H₈ [ν₈(C₃H₈), ∼868 cm^-1], and S-H symmetric stretching vibration of H₂S [ν₁(H₂S), ∼2612 cm^-1] were fitted using Gaussian + Lorentz functions. The obtained peak areas and peak heights were then used to calculate the Raman quantification factors (F factor and G factor, respectively) of C₂H₆, C₃H₈ and H₂S relative to CH₄, respectively. Both the F factor and G factor increased with rising pressure, whereas the FC₂H₆, FC₃H₈ and GH₂S kept nearly constant at ∼5.69, 6.39 and 153.8, respectively in high pressure gas mixtures (e.g., >30 MPa). Therefore, for inclusions with higher internal pressure, the molar ratio of CH₄, C₂H₆, C₃H₈ and H₂S could be determined by the aforementioned F and G factors. This method was applied to the calcite-hosted single-phase gas inclusions in the Upper Permian Changxing Formation carbonate reservoir from the eastern Sichuan Basin (South China). Our results indicated that the trapping pressure would be obviously overestimated if the presence of heavy hydrocarbons was not taken into account.

Keywords: Experimental calibration; Fluid inclusion; Raman spectroscopy; Trapping pressure; Wet gas.