It depends how independent Shetland would become - if an island territory of Scotland, then the 12 mile rule would apply. If fully independent, ie not aligned to either Edinburgh or London, then Shetland could claim a far greater share of territory. And given that the newer fields are west of Shetland this could make an impact.
However, the oil and gas reserve figures being bandied around are a bit meaningless unless you know exactly what is being quoted - reserves are classified in various ways based on class and certainty. The actual answer for proved and probable reserves is around 8 to 9 million boe, nowhere near the figures of 16 and 24 million that are currently being bandied around - these would most likely include unproven reserves and resources that are hoped to exist and are likely to be very difficult and currently uneconomic to extract.
42 boe has already been extracted, meaning that 80% of North Sea oil and gas has already gone. What is left needs a huge amount of investment to retrieve - at current spending rates it will take 50 years and the infrastructure won't last that long. It's hard to see companies investing in a mature field to prop up the infrastructure when there are better pickings elsewhere. For the oil and gas industry to continue to flourish it needs investment and a more generous fiscal regime, ie less taxation. The UK government is already providing incentives to delay decommissioning of fields.
More importantly, the question to answer is how will Scotland fund itself? In 2013 the direct tax take from oil and gas production for the whole of the UK was £4.67 billion and falling. This compares with annual spending of the Scottish government (plus UK spending on Scotland) running at £65.2 billion. Hence, direct taxation of oil and gas production may account for less than 7% of the Scottish budget. Where the other 93% is going to come from?