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

Carbonaceous fluid within mineral-hosted inclusions provides important information for carbon cycle in deep Earth. In addition to CH ₄ and CO ₂ , heavy hydrocarbons (e.g., C ₂ H ₆ and C ₃ H ₈ ) are frequently observed in carbonaceous fluid inclusions (i.e, wet gas inclusions with C ₁ /∑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 CH ₄  ± C ₂ H ₆  ± C ₃ H ₈  ± H ₂ S system at room temperature and 0.1-130 MPa. We confirmed that the C-H symmetric stretching vibration band of CH ₄ [ν ₁ (CH ₄ ), ∼2917 cm ^-1 ] shifted to lower wavenumber with rising pressure, thus the P-ν ₁ (CH ₄ ) relationship could be applied to calculate the pressure of wet gas. It should be noted that the presence of C ₂ + and/or H ₂ S will shift the [ν ₁ (CH ₄ )] to lower wavenumber at constant pressure (with the order of C ₃ H ₈  ≥ H ₂ S > C ₂ H ₆ ). Obviously, the P-ν ₁ (CH ₄ ) relationship derived from pure CH ₄ 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 CH ₄ , C ₂ H ₆ and C ₃ H ₈ , the peak areas and peak heights of the overtone vibration of CH ₄ [2ν ₄ (CH ₄ ), ∼2580 cm ^-1 ], C-C symmetric stretching vibrations of C ₂ H ₆ [ν ₃ (C ₂ H ₆ ), ∼995 cm ^-1 ] and C ₃ H ₈ [ν ₈ (C ₃ H ₈ ), ∼868 cm ^-1 ], and S-H symmetric stretching vibration of H ₂ S [ν ₁ (H ₂ S), ∼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 C ₂ H ₆ , C ₃ H ₈ and H ₂ S relative to CH ₄ , respectively. Both the F factor and G factor increased with rising pressure, whereas the FC ₂ H ₆ , FC ₃ H ₈ and GH ₂ S 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 CH ₄ , C ₂ H ₆ , C ₃ H ₈ and H ₂ S 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.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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