Most oilfield targets are soft targets. It shouldn't be hard to disrupt the flow of oil. The question, though, is one of who 'owns' the oil? (controlling mineral interests) or is ISIS getting all of the money?
Then, who is buying it?
Islamic State Smuggles Oil Into Turkey—With Hostages as Insurance
http://www.businessweek.com/articles/2014-09-11/islamic-state-smuggles-oil-into-turkey-with-hostages-as-insurance
September 11, 2014