which branch will be best to study in engineering civil or petroleum

Out of the two that you have mentioned I would suggest for Civil as it has wider & far more spread out scope than the other one.

Both have a wide range of opportunities. in B.E(civil engineering) there are a no.of opportunities like constructions which have a major demand and petroleum chemical has also has a major depend because today the refining of crude oil is so expensive it is upto you to decide based on your interest