A German report by Die Zeit (paper) and ARD (TV), which aims to:
The goal was to find out exactly how the group works, where its money comes from and who is profiting. We wanted to find out how successful IS really is.
Link:http://www.zeit.de/feature/islamic-state-is-caliphate

Business crosses frontlines and reaches beyond:
The oil is sold down a long chain of middlemen in ever-smaller batches. Criminal bands, tribes, corrupt civil servants, Shiites, Sunnis, Kurds and Turkmen: People from all parts of society seem to be involved. The end users are in the Kurdish regions of Iraq or Syria, in Iran or in Turkey...

(Later) Yet the more unbearable the situation becomes, the greater the risk of revolt will be. At the moment, it is brute force that is holding this caliphate together – not faith in God and certainly not "love and patience." And it is theft that is filling IS coffers, not any kind of functioning economy. The caliphate is unsustainable.