Draw a line NNW from Berwick-upon-Tweed for 200 miles. It reaches up to the Orkey Islands. Now imagine everything east of that line as remaining under UK jurisdiction whilst the legally-brand-new country of Scotland gets the rest. Now compare this division of the North Sea to
this map of oil deposits, and you begin to see how the 200 mile limit can be used by the UK to royally screw over an independent Scotland vis-a-vis North Sea oil.
Scottish Lawyer: We want our EEZ.
UK Lawyer: Sure, there's lots of Arctic Ocean you can claim.
SL: We mean in the North Sea.
UKL: Hmm. Well, how about we give up some of our EEZ for you. *Draws map to exclude 90%+ of all the oil fields*
SL: *Cries foul*
UKL: Sorry old bean, but my job is to protect the UK's rights, and you're not part of the UK any more. Still, I guess we can give you a bit more. We want to keep our nuclear subs in Scotland for starters, and then...
Bear in mind the UK can simultaneously be giving up all that Arctic sea north and west of Scotland without any fuss, and in addition can make the very reasonable claim that this situation is without precedent - when was the last time a single country with an extensive EEZ was divided in two? Sudan doesn't count given South Sudan has no coastline at all, so... time to set a precedent, and in a manner that countries concerned with secession will just love to back. Most, if not all, of the permanent Security Council members will like the idea for example. Finally, the UK can throw up all sorts of annoyances to win the argument - think NATO for example - and if it really wanted to be underhanded, could offer the Orkey & Shetland Islanders benefits if they remained in the UK instead of Scotland. Do that and you can entirely cut Scotland off from the North Sea beyond its own narrow strip of coastal waters. Point is, there's lots of possibilities for using access to the North Sea oil fields for Scotland as a great bargaining tool.