This paper is the first in a sequence which develops a model of hot, high-pressure geothermal brines based on a common salt solution. In this paper, there are proposals for T-p-X state-space delineations of such a model fluid. Using experimental and calculated data, correlations for these delineations are given for a wide range of temperatures, pressures and solute mass fractions, which are based on the primary variables temperature T, pressure p and mass fraction of sodium chloride X. These correlations are approximate, particularly at higher temperatures, but are qualitatively correct and can be used in subroutines suitable for use in numerical simulation programs. Correlations will be presented in subsequent papers for some of the thermodynamic properties of such a brine system.