In the 1970s the government commissioned a report on the impact of North Sea Oil. It concluded that Scotland would be very weathly as an independent country. And the UK government feared it would be bankrupt without the oil revenue. The report was concealed for 30 years.
http://news.bbc.co.uk/1/hi/scotland/4661584.stm
Indeed Norway, with a similar population and oil reserves, has become one of the wealthiest countries in the world.
Repeated requests to set up an oil fund in Scotland were denied by the UK government. Instead receipts from oil revenue funded Thatcher's monetary policy, resulting unemployment benefit, and deindustrialization in the 1980s.
http://www.newstatesman.com/politics/2013/04/thatcher-and-north-sea-oil-–-failure-invest-britain’s-future
Scotland is the only country in the world that discovered oil and saw little economic improvement as a result - because of UK government mismanagement.
It is a matter of public record that UK governments have lied about and mismanaged North Sea oil resources.
The UK government's own report concluded that an independent Scotland would be a very wealthy small country and Norway proves their point.
Since the UK government mismanaged and lied about Scotland in the past, why do you trust them to be any more competent or trustworthy in future?
What makes you think that London will send extra money to Scotland now the oil revenue is in decline? Boris Johnson (next Pm? Who knows) is well known for his view that Scotland gets too much money. Many other London politicians have said the same in less colourful terms. With the oil revenue gone he may actually have a point. Until now London politicians have moaned about Scotland getting extra money while conveniently ignoring the tax receipts from oil. When that vanishes what's left? We just hope London will subsidise Scotland forever? You call that a realistic plan for the future?