I worked in the oil and gas business for 35 years. I worked with all kinds of engineers including mechanical, civil, electrical, chemical, instrumentation/control, structural, accoustic and metallurgical.
It is a highly technical business that is always looking for the best and most up to date way to pursue the discovery, extraction and processing of hydrocarbons.
You are correct that petroleum engineers are primarily interested in the below ground reservoirs and how to maximize the extraction of the oil and gas. Once it is on the surface, the other disciplines take over.