I posted a link to a very informed and detailed article some 2 months ago on FR. I need to look for it. But here is another related one and other links within it about ISIS (Daesh) and its economy — http://edition.cnn.com/2014/08/18/business/al-khatteeb-isis-oil-iraq/
Another link, hope it works. If it doesn’t, then google for “Islamic State Economy Runs on Extortion, Oil Piracy in Syria, Iraq” (WSJ) since you need to subscribe — http://online.wsj.com/articles/islamic-state-fills-coffers-from-illicit-economy-in-syria-iraq-1409175458