Scope of petroleum engineering will never end because it has the power of OIL and oil has remained a topic for politics. It will definitely be versatile.

Also the disadvantage is  you might have to work in dangerous situations and therefore that can be threat to life.

Otherwise scope is good.


Joining merchant navy on the other hand is like having no social life but in terms of finance, you will be playing in it.
There is job stability and high income for sure.
So here along with scope stability in job is present.

Accordingly you can choose in which you have interest in.
Good luck.