I would use a resource map to locate oil reserves.
<u>Explanation:</u>
Resource maps are equipped with the locations of resources and reserves situated in the area that the map belongs to.
The purpose of these maps is to show the availability and distribution of different minerals found in the area.
To serve the purpose of the convenience of locating reserves and resources, resource maps and other maps are made by the major geological surveying agencies and authorities functioning in the respective regions