Hydrogeochemical classification of deep formation waters

•Formation waters from Russia, Western Europe and North America are compared.•Two main water types are identified: Na–Cl and Na–Ca–Cl waters.•Mg–Cl brines are rare.

Deep formation waters from Western Europe, Russia and North America are classified by the major water components (Ca2+, Cl−, Mg2+, Na+) and barium/sulphate ratios. The data are used to identify important hydrogeochemical processes (e.g., halite dissolution and albitisation) that lead to the different composition of formation waters. Two significant water types are identified: Na–Cl water and Na–Ca–Cl water. Furthermore, differences in formation water according to stratigraphical units are shown for deep reservoirs in the North German Basin and the North Sea. Based on the collected hydrogeochemical data, development trends are stated for the formation waters, and albitisation is favoured as the main process causing Ca enrichment.