In 2012, a repeat time-lapse seismic survey was acquired over the Halfdan oil field located in the Danish North Sea. The main oil reservoir is the Cretaceous chalk Tor Formation, and the field is developed with a line-drive water flood. The reservoir saturation changes due to water injection and gas breakout lead to large changes in acoustic impedance, which can be imaged with 4D seismic. The 4D acoustic-impedance changes, in conjunction with production and geologic data, were analyzed on a well-by-well basis and resulted in several interesting correlations between the geology and the 4D response. The 4D was used to update the seismic interpretation of some important layers within the static and dynamic model. In some areas, the impact of local porosity variations on the lateral sweep could be seen. The 4D also provided evidence that some faults serve as potential pathways for formation-water entry into the oil-producing layers. In addition, numerous well-intervention opportunities were identified through integrating the 4D with the production, geologic, and dynamic data. Examples from the two-vintage and three-vintage 4D acoustic-impedance data illustrating these key 3D and 4D learnings and a successful well intervention are shown.