Through ultraviolet laser argon-argon dating of potassium feldspar cements containing fluid inclusions, we determined temperature-composition-time data for paleofluids in a sedimentary basin, including data for an evolving episode of fluid flow recorded in distinct phases of cement. The fluid evolved from mixed aqueous oil 83 million years ago to purely aqueous by 76 million years ago, thus dating the time of oil charge in this reservoir.