I'm a Petroleum Engr working in the Industry. My background is Chemical Eng first degree and an Msc in Pet Eng. I think is better to have your first degree in another engineering specialty before an Msc in Pet Eng even if you want to work as a Pet Eng. Pet Eng is a bit narrow and with an Msc you can cover all you need to know in one year and the rest comes with exp. Most of my colleagues took the same root as I did ( not necessarity Chem Eng, some EE, maths, Mech or even geology). As per getting a good job, a good degree is a good starting point, most companies require a 2.1 and above and pray to God as well. I hope this helps. Good luck. it pays to work hard!!!
It depends on what you want to do in the future.
For the oil sector, it doesn't really matter what engineering you do at undergrad, I think. These days, it's a lot more competitive, with most engineering fresh intakes having masters degrees already.
I'd say you can do either, but if you are particularly interested in the petroleum sector, just do pet engineering straight.