Raman spectroscopic densimeter for pure CO2 and CO2-H2O-NaCl fluid systems over a wide P-T range up to 360 °C and 50 MPa

Abstract

Density of fluid inclusions is one of the most important parameters to reconstruct the P-T conditions of entrapment of geo-fluids. Co-existing with microthermometry methods, Raman spectroscopy also became a useful tool to determine the density of CO2-bearing inclusions via Fermi diad splitting (Δ). In this study, a new CO2 densimeter which takes into account the effects of temperature, pressure and H2O (±NaCl) on Fermi diad splitting was built from experiments using a High Pressure Optical Cell. Raman spectra of CO2 were collected in the range −20 to 360 °C and 0.51 to 50 MPa, i.e. a density range from 0.0057 to 1.1600 g·cm−3. Our results show that temperature and pressure have significant impact on Δ-density relationship, with Δ lower by up to about 0.5 cm−1 with increasing pressure and temperature at a given density. On the contrary, dissolved H2O (±NaCl) had no effect on the Δ-density relationship, so the function defined for pure CO2 system can also be applied to calculate the density of CO2-H2O-NaCl system (up to 360 °C, 50 MPa and 20.35 wt% NaCl). The new densimeters presented in this study provide a useful tool to determine the pressure of CO2-rich fluid inclusions all along the isochoric path for any water concentration.