Formation water is the water that is trapped in the pore spaces of sedimentary rocks during their formation. This water can be saline and contains dissolved minerals, making it important in the study of hydrocarbon reservoirs, groundwater systems, and the geochemical history of sedimentary basins.
Reference: Hanor, J. S. (1994). Origin of Saline Fluids in Sedimentary Basins. Geological Society.