Thermodynamic modeling of ternary CH4H2ONaCl fluid inclusions

Abstract This paper reports the application of thermodynamic models, including equations of state, to ternary CH 4 H 2 O NaCl fluid inclusions. A simple equation describing pressure–temperature–salinity relations on the CH 4 hydrate-liquid-vapor surface has been developed to calculate the NaCl contents (salinities) of inclusions, where the dissociation pressure of CH 4 hydrate coexisting with vapor and liquid at a given temperature is calculated with a pressure equation of pure CH 4 . The pressure equation is a function of temperature and CH 4 Raman peak position shift corrected by Ne lamp. With these relations and the latest CH 4 solubility and PVTx models, a new iterative approach is presented to calculate the CH 4 contents of CH 4 H 2 O NaCl inclusions on the assumption that the bulk molar volume of an inclusion at the melting temperature of CH 4 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 CH 4 H 2 O NaCl inclusions can be simultaneously obtained without having to use volume fractions of vapor bubbles at the dissociation temperatures of CH 4 hydrates determined based on optical observations or measurements. The homogenization pressures and isochores of CH 4 H 2 O NaCl fluid inclusions from updated models are briefly discussed. The code to estimate PVTx properties of inclusions in the ternary system CH 4 H 2 O NaCl, based on microthermometric and Raman data, can be obtained from Chemical Geology or the corresponding author ( maoshide@163.com ).