It seems a little presumptuous to think that just by claiming independence that any of this money would revert to a new Scottish government. The rights are all granted by the Crown and royalties are paid pursuant to licensing agreements. It seems to me that the UK government would have little interest in assigning those rights and in the absence of any amendments to treaties it would be quite entitled to maintain its interest in any oil fields outside Scottish territorial waters.
The point is that a country is usually entitled by international agreements to the mineral rights oin its half of the median line between two countries. The median line in the North Sea was reached by negotiation between the parties involved. Scotland was not one of the parties involved in the treaty negotiations over the North Sea, so if it did secede from the United Kingdom and was not a signatory to any treaty, I doubt that it would have any rights under international law over those oil fields.
And we can hang onto them. If the Scots want to take them they would have to send the navy that they haven't got.