Toys – there are several sources of the claim for “its Scotlands oil” outlined in the wiki link.
1) when the oil exploration started there was a need for establishing who had jurisdiction over what areas. thus “The Continental Shelf Act 1964 and the Continental Shelf (Jurisdiction) Order 1968 defines the UK North Sea maritime area to the north of latitude 55 degrees north as being under the jurisdiction of Scots law[6] meaning that 90% of the UK’s oil resources were under Scottish jurisdiction.[7]”
So thats puts most of the oil under Scotlands jurisdiction.
2) There there are various international treaties that set out how the seabed should be divided – IIRC the principles from this law were used to give the results above
United Nations Convention on the Law of the Sea is probably the main one involved here but there are others IIRC that are pertinent. Again these give a similar result. Its the same rules that allow Norway to claim a large part of the north sea.
There is a group of political thinkers who want the oil to remain English. They use all sorts of manoeuvres and odd interpretations to try to show that these various provisions and treaties do not apply but its hard to see that they don’t
Its all terribly hypothetical anyway as there will not be a majority in Scotland for independence I doubt and if there was it would be a foolish English government that challenged well understood international law. In the event of a dispute there are international mechanisms for dealing with them Is there not a dispute at the moment over the attempt by Russia to lay claim to the Arctic? The same laws have been used to divide up the Antarctica seabed and around the Falkland Islands.
As regards oil not being included in the devolution settlement – of course it was not. that would have given Scotland financial independence and cut a large source of revenue to the exchequer. There are a large number of similar things that remain “reserved matters” – reserved for Westminster not Holyrood. Such as energy policy, tax policy