My answer is go for it! Being a girl has nothing to do with your capabilities as an engineer. Yes, it will be difficult but not because it may be a course with more boys than girls. It will be tough because it's a STEM course and they are as challenging as they are rewarding. You can do anything you set your mind to and work for.
Don't think that your gender should play a role in the decisions you make when it comes to your education, your dreams, your aspirations. Work hard in the petroleum engineering courses and don't let anyone tell you that you can't do it. You can do it! :)
Just find a support group- be it here on EngineerGirl.org, your friends, or your family. Keep them close and lean on them when you need to. Good luck in your studies!