A re-evaluation of the effects of temperature and NaCl concentration on quantitative Raman spectroscopic measurements of dissolved CH4 in NaCl aqueous solutions: Application to fluid inclusion analysis

We re-evaluated the effects of temperature and NaCl concentration on quantitative Raman spectroscopic measurements of dissolved CH4 in NaCl aqueous solutions with the newly developed Unsaturated Homogenized Solution Method (UHSM). The Raman peak area ratios (PARs) of the symmetric stretching vibrational mode of methane (at ~ 2917 cm− 1) to the OH stretching band of water in homogeneous CH4-containing aqueous solutions were obtained at temperatures from 273.15 to 603.15 K, pressures from 10 to 120 MPa, and NaCl concentrations from 0 to 5 mol/kg H2O. The pressure and concentration of CH4 (mCH4) have little effect on the Quantitative Factor (QF = PAR / mCH4). However, the QF shows both temperature and NaCl concentration dependencies; QF increases linearly with temperature from 273 to 603 K at constant NaCl concentration, and the rate of increase of QF to temperature decreases with NaCl concentration. After normalization to QF at 298.15 K, the QF at elevated temperature (T, in K) and NaCl concentration (mNaCl, in mol/kg H2O) can be represented as a function of temperature and mNaCl:QF[T, mNaCl] / QF[298.15 K, mNaCl] = fn (T, mNaCl) = anT + bnwhere an = − 4.898E − 04 × mNaCl1/2 + 2.011E − 03 and bn = 1.377E − 01 × mNaCl1/2 + 4.042E − 01. This function has been applied successfully to determine the methane concentration in natural fluid inclusions in quartz, and it can be used in other laboratories for CH4–NaCl–H2O ternary systems, where QF at 298.15 K is the only parameter that needs to be obtained for the quantitative analyses of dissolved CH4 in NaCl aqueous solutions.