We present here bromide, chloride, and sodium data for fluids from reservoirs of Ordovician through Pennsylvanian age in the Illinois basin which suggest that remnant marine fluids contribute significantly to fluid Cl budgets. Cl/Br and Na/Br ratios for Ordovician through Devonian formation fluids are relatively uniform and near those for seawater, despite greater than a factor of ten range in Cl concentration. In contrast, fluids from Mississippian and Pennsylvanian reservoirs, separated from older reservoirs by the New Albany Shale Group, have more variable fluid Cl/Br and Na/Br ratios, most of which are significantly greater than those of seawater. The 1:1 stoichiometry of Cl and Na increases for Mississippian and Pennsylvanian formation fluids is consistent with halite dissolution. Nevertheless, Br systematics and mass-balance considerations indicate that the overall Cl budget of Illinois basin formation fluids appears to be more significantly influenced by the contribution from subaerially evaporated seawater than by halite dissolution.