Thermodynamic modeling of ternary CH4H2ONaCl fluid inclusions

This paper reports the application of thermodynamic models, including equations of state, to ternary CH4H2ONaCl fluid inclusions. A simple equation describing pressure–temperature–salinity relations on the CH4 hydrate-liquid-vapor surface has been developed to calculate the NaCl contents (salinities) of inclusions, where the dissociation pressure of CH4 hydrate coexisting with vapor and liquid at a given temperature is calculated with a pressure equation of pure CH4. The pressure equation is a function of temperature and CH4 Raman peak position shift corrected by Ne lamp. With these relations and the latest CH4 solubility and PVTx models, a new iterative approach is presented to calculate the CH4 contents of CH4H2ONaCl inclusions on the assumption that the bulk molar volume of an inclusion at the melting temperature of CH4 hydrate and at the vapor bubble disappearance (homogenization) temperature are identical. A prominent merit of this method is that the compositions, molar volumes and homogenization pressures of CH4H2ONaCl inclusions can be simultaneously obtained without having to use volume fractions of vapor bubbles at the dissociation temperatures of CH4 hydrates determined based on optical observations or measurements. The homogenization pressures and isochores of CH4H2ONaCl fluid inclusions from updated models are briefly discussed. The code to estimate PVTx properties of inclusions in the ternary system CH4H2ONaCl, based on microthermometric and Raman data, can be obtained from Chemical Geology or the corresponding author (maoshide@163.com).

► An iterative approach is presented to calculate CH4 content of CH4H2ONaCl inclusion.
► Salinity of CH4H2ONaCl inclusion is calculated from a simple empirical equation.
► Volume fraction and homogenization pressure can be obtained with this method.