A petroleum engineer is involved in nearly all of the stages of oil and gas field evaluation, development and production.

Petroleum geologists: who find hydrocarbons by analysing subsurface structures with geological and geophysical methods.
Reservoir engineers: who work to optimise production of oil and gas via proper well placement, production levels and enhanced oil recovery techniques. They use computer simulations to assist in the identification of risks and to make forecasts on reservoir potential.
Production engineers: who manage the interface between the reservoir and the well through such tasks as perforations, sand control, artificial lift, downhole flow control and downhole monitoring equipment. They also select surface equipment that separates the produced fluids (oil, natural gas and water).
Drilling engineers: who manage the technical aspects of drilling both production and injection wells. They work in multidisciplinary teams alongside other engineers, scientists, drilling teams and contractors.

Good luck.