Hello aspirant. Computer science is an ever green branch.. it was, it is and it will be in future too. Whatever the time might be, in terms of career, courses, opportunities,  job etc, computer science will always remains at the top. Coming to the branch of petroleum engineering. I would personally suggest to go for it for masters not as a bachelors degree because, atleast in india the scope for this is pretty less. Almost 75 percent graduates from this stream are struggling to get the job in that sector. Since this sector is still in basic stage and is emerging , there are not many opportunities in this. So go for bachelors degree and you can take this for masters.

Hope this helps

Regards

Nikhitha