Petroleum engineering very speciallized and streamlined course. It is better to chose such courses at Master’s level.If you pursue your degree in mechanical, chemical, civil or any other core branch, the basic fundamentals will remain same. As petroleum engineering is applied branch, fundamentals from your bachelor will be useful. Anyone can enter in Petroleum engineering but vice versa may not be possible. Today 70 % petroleum graduates in India struggling to get job in Petroleum sector. They end up with no option other than going for MS or working with small companies. Don’t go behind fancy packages they show it as survey. I know good engineers working for peanuts (2500 month salary provided by one of the rig contractor).