A petroleum engineer is an engineer that is involved in the exploration for oil, processing, transport mechanisms and distribution. This includes the exploration and searching likely areas, extracting what's found, knowing geology, a lot of safety considerations and understanding the construction, theory, maintenance and use of all the voluminous hardware -- and today, software -- used to support it.
There are multiple sources of petroleum. There are land-based drill sites and water sites with floating and stationary rigs. A petroleum engineer needs to be familiar with all of these.
Given the concerns regarding the environment, the petroleum engineer must also be very familiar with these and the various safeguards and systems involved in protecting our ecology. As an engineer, you are under great scrutiny and have a significant responsibility. Petroleum engineers are highly paid for this very reason. It's fascinating work.