Methane-bearing aqueous fluid inclusions: Raman analysis, thermodynamic modelling and application to petroleum basins

Calibration for the determination of the CH4/H2O ratio using Raman spectroscopy is carried out using synthetic fluid inclusions with 0 m NaCl. Spectra of the symmetric stretching band of methane (ν1,CH4), and the bending (ν2,H2O) and stretching (νS,H2O) bands of water were obtained in the aqueous phase co-existing with the vapour phase at variable temperatures from 25° to a few degrees above the homogenisation temperature. A software program, based on the model of Duan et al. (1992a) [Duan, Z., Moller, N., Greenberg, J.H., Weare, J.H. 1992a. The prediction of methane solubility in natural waters to high ionic strength from 0°C to 250°C and from 0 to 1600 bar. Geochim. Cosmochim. Acta, 56, 1451–1460.] has been developed to calculate the composition of the aqueous liquid phase co-existing with the vapour phase in the synthetic fluid inclusions. Application to natural samples from the Alwyn South area (North Sea) shows that Raman analyses permit discrimination between inclusion populations that are not recognised using only microthermometric measurements. These results provide important constraints on the P–T conditions of inclusion formation and suggest that oil migration occurred at fluid pressures of approximately 240 bars.