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 CH4 and CO2, heavy hydrocarbons (e.g., C2H6 and C3H8) are frequently observed in carbonaceous fluid inclusions (i.e, wet gas inclusions with C-1/& sum;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 [nu(1)(CH4), similar to 2917 cm(-1)] shifted to lower wavenumber with rising pressure, thus the P-nu(1)(CH4) relationship could be applied to calculate the pressure of wet gas. It should be noted that the presence of C-2+ and/or H2S will shift the [nu(1)(CH4)] to lower wavenumber at constant pressure (with the order of C3H8 >= H2S > C2H6). Obviously, the P-nu(1)(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 [2 nu(4)(CH4), similar to 2580 cm(-1)], C-C symmetric stretching vibrations of C2H6 [nu(3)(C2H6), similar to 995 cm(-1)] and C3H8 [nu(8)(C3H8), similar to 868 cm(-1)], and S-H symmetric stretching vibration of H2S [nu(1)(H2S), similar to 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 GH(2)S kept nearly constant at similar to 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.