Analysis of produced water is one of few direct sources of information to the subsurface processes active in an oil field. However, revealed geochemical patterns are typical complex and require a multidisciplinary approach to successfully unlock and decode. The Halfdan field initially contained two main types of formation brine but now, after 15 years of extensive water flooding, the field contains mostly a seawater modified type. The initial water was a low salinity sulphate-depleted type and a medium high saline sulphate-rich type that varied in a geographically systematic manner. The water treatment resulted in a net deposition of sulfate and Mg and a net export of Ca and Cl ions from the field. This pattern is similar to core flooding results, suggesting that a substantial alteration of the wettability of the chalk occurred. Spatial and temporal variation in water composition are mapped based on 3880 analysis from 75 wells and are related to water flooding history and sweep efficiencies obtained from 4D seismic. Produced water analysis is thus an important asset for designing water sweep in mature fields. Furthermore knowledge about the initial water types may optimize water treatment and prediction of geographically distribution scale and corrosion risks.