^^ i think principal engineers is those that already an expert in their filed,
those taikos who's been in the industries >25 years
they can be from any branch of engineering, mechanical (static, rotating, package), piping
structural, process, safety, control & instrumentation & electrical, pipeline...

i could be wrong tho.. 

Thanks for your explanation alvinkhorfire
Btw, does a master degree in geophysics help a lot in getting job in oil and gas industry? Not sure whether want to further my studies or just start working