i think by the present calculations, there are about 80 years left of oil and gas exploration within North sea fields and this will only grow, as ub7rm said, technology is improving at such a rate that they are finding new ways to get the oli and gas out of fields that they have already left - this means that a lot of the smaller exploration companies are getting in on the act, at they can exploit these fields more economically than the bigger guys (ie shell, BP, etc)
The whole agrument of england bailing out scotland is carp - you can't ignore that benefit of the revenue from oil and gas to the UK economy over the past X years.
and before any acuses me of being anti english, i do not think scotland would benefit from independance as its too late to make the most of the oil and gas revenues