An equation for determining methane densities in fluid inclusions with Raman shifts

Abstract

Raman shift of the C–H symmetric stretching band (v1) of methane can be used to calculate the pressure and density of methane in fluid inclusions. However, previous numerical functions for Raman band shifts and methane density are only suitable for methane with densities lower than 0.3g/cm3. In this study, Raman shifts for pure CH4 were systematically measured at 25°C and pressures up to 150MPa. Parameters of the long range attractive forces were fitted, and Raman shifts of CH4 were calculated with hard sphere model for density up to 0.55g/cm3. Based on experimental data and theoretical calculation, a unified equation was established to calculate the density of methane gas from Raman shifts over a wide density ranges up to 0.55g/cm3:D=vd−vo=211.3ρ4−73.238ρ3+24.477ρ2−29.0632ρwhere ρ represents the density of methane in g/cm3; D(cm-1)is the difference between the measured peak position of methane in the fluid inclusion (vd) and the known peak position of methane at quite low density (vo). The equation will work regardless of which machine the measurements of Raman shifts are done on, as long as the user knows or finds the zero-density peak position.