Word for someone who finds oil reservoirs

Solution 1:

Petroleum geologist.

A petroleum geologist is a earth scientist who works in the field of petroleum geology, which involves all aspects of oil discovery and production. Petroleum geologists are usually linked to the actual discovery of oil and the identification of possible oil deposits or leads. [Wikipedia]

Petrogeologist is used also but it is less common.

Solution 2:

Both my father and his father made their livings at precisely this job. The term used for it within the petroleum industry (in the United States, anyway) is exploration geologist.

A person who conducts seismic mapping of the strata of rock and other deposits in a particular area in order to help assess their potential as a source for oil or natural gas is called a stratigrapher.