Hi,

I am currently a senior petroleum engineer. I followed the same path more than 10 years ago when I graduated as a process engineer and I'm really satisfied with what I've done. Petroleum/Reservoir engineering is much of fluid mechanic in porous media. So it is process engineering after all. Remember that Darcy was a process engineer (hydraulics). With a good knowledge in reservoir geology and some field training you are good to go.
So go ahead and good luck.