Adaptation of the Bowers and Helgeson (1983) equation of state to the H2O–CO2–CH4–N2–NaCl system

The equation of state developed by Bowers and Helgeson ([Bowers, T.S., Helgeson, H.C., 1983. Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H2O–CO2–NaCl on phase relations in geological systems: equation of state for H2O–CO2–NaCl fluids at high pressures and temperatures. Geochim. Cosmochim. Acta, 47, 1247–1275.] and [Bowers, T.S., Helgeson, H.C., 1985. Fortran programs for generating fluid inclusion isochores and fugacity coefficients for the system H2O–CO2–NaCl at high pressures and temperatures. Computers and Geosciences, 11, 203–213.]), which was originally designed for H2O–CO2–NaCl fluids, has been extended to CH4 and N2 bearing fluids. Available experimental P–V–T–X data in the H2O–CO2–CH4–N2–NaCl fluid system are accurately reproduced by this equation of state, and, therefore, isochores and fugacity coefficients can be accurately calculated up to 1000 MPa and 1300 K. This equation of state cannot be used in and near immiscibility regions and critical points, like any modified Redlich–Kwong equation of state. The empirical modifications allow isochore construction for realistic geological fluid systems which are often found in fluid inclusions, and which involve several gases and salts.