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 spectros copy is carried out using synthetic fluid inclusions with 0 m NaCl. Spectra of the symmetric stretching band of methane (nu (1,CH4)), and the bending (nu (2,H2O)) and stretching (nu (S,H2O)) bands of water were obtained in th e aqueous phase co-existing with the vapour phase at variable temperatures from 25 degrees to a few degrees above the homogenisation temperature. A so ftware 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 solubil ity in natural waters to high ionic strength from 0 degreesC to 250 degrees C and from 0 to 1600 bar. Geochim. Cosmochim. Acta, 56, 1451-1460.] has bee n developed to calculate the composition of the aqueous liquid phase co-exi sting with the vapour phase in the synthetic fluid inclusions. Application to natural samples from the Alwyn South area (North Sea) shows that Raman a nalyses permit discrimination between inclusion populations that are not re cognised using only microthermometric measurements. These results provide i mportant constraints on the P-T conditions of inclusion formation and sugge st that oil migration occurred at fluid pressures of approximately 240 bars . (C) 2001 Elsevier Science B.V. All rights reserved.