Although the oil field is underground, due to diffusion, many hydrocarbons in the oil layer can penetrate to the crust surface, thus revealing the clues of the oil field. These hydrocarbons are good food for some microorganisms. The more hydrocarbons, the faster reproduction. At this time, as long as these microorganisms are found underground, after artificial cultivation and determination of their numbers, we can know whether there are oil fields underground.