Scotland owns no oil until it becomes independent. It's not Scottish, just as it's not English or Welsh. It's United Kingdom oil. The argument should also be true when reversed - i.e Scotland doesn't get more government cash per person, rather an area of the UK does. Unfortunately the governments refusal to allow people to cross the border to receive this greater benefit makes this an arrangement of inequality. This is the major problem when you effectively have two governments for one state - an idiotic state of affairs tbh.
Even if they did move for independence, it's unlikely they'd be given 100% rights to it - a territorial border would have to be agreed, and obviously assets would be prime motivation for England and Wales to try and keep as much as possible. It's very unlikely that pure geography would dictate this.