Summary A total of 41 chalk core plugs, cut with the same orientation from large blocks of outcrop chalk, have been aged in crude oil at 90 °C for different time periods, in duplicate sets. Different filtration techniques, filtration temperatures and injection temperatures were used for the crude oil. Oil recovery by spontaneous, room temperature imbibition, followed by a waterflood, was used to produce the Amott water index for cores containing aged crude oil and for cores where the aged crude oil was exchanged by fresh crude oil or decane. The main objective was to establish a reproducible method for altering the wettability of outcrop chalk. A secondary objective was to determine mechanisms involved and the stability of the wettability change. The aging technique was found to be reproducible and could alter wettability in Rordal chalk selectively, from strongly water-wet to nearly neutral-wet. A consistent change in wettability towards a less water-wet state with increased aging time was observed.