Where on Earth do we expect to find very old groundwater (infiltrated thousands of years ago)?
In large intracontinental basins where the main rock formations are exposed in adjoining highlands and rare deeply buried within the basin itself. The Madison Limestone is an example.
The Madison and its equivalent strata extend from the Black Hills of western South Dakota to western Montana and eastern Idaho, and from the Canada–United States border to western Colorado and the Grand Canyon of Arizona.
Ground-water ages vary from virtually modern to about 23,000 yr. The 14C ages indicate flow velocities of between 7 to 87 ft/yr. Hydraulic conductivities based on average carbon-14 flow velocities are similar to those based on digital simulation of the flow system (Downey, 1984).
Virtually every oil / gas well contains old water. However , surface water , mixed with produced water, is injected in some to facilitate production .Last number I saw was that onshore wells in the US averaged 90 % water in the liquid phases.