It is not easy, but I have seen some pretty unimpressive people make it through a Petroleum Engineering course.
There is a major turnover going on now in the petroleum engineering world. People are retiring at a faster rate than they are being replaced. Thus, you will get a job out of school, and get paid a lot more than many other engineering students.
Down sides are that you have to go where the oil is (unless you can get into upper management), and that is never the same place for very long. Also, the oil is not always in the most wonderful parts of the world. Middle East, Malaysia, Khazakstan, North Sea, are all places that regularly turn up on a petroleum engineer's resume.
The new world of oil shale means more opportunities in North America. But most of those are in middle of nowhere places in Alberta CA, North Dakota, West Texas, and Wyoming.
The last consideration is what will happen in 30 years. Either, the world will still be hooked on oil and gas and you will be making piles or someone will have perfected cold fusion and your profession will be obsolete. My bet is with the former. But it is something to think about long term.