Interesting article from Financial Times that explains much of what's behind the Isis oil connection.
My thoughts below related to these issues.
Militias and farmers, who fight against Isis, still go to Isis run petrol stations to "fill er up." That's how they run the vehicles they fight and farm with. Buying from one's enemy; especially if there's no other supply available.
I've often wondered how Isis can sell oil for funding their terrorism. Who's buying it? Turns out a lot of that oil is sold locally, to the various farmers and militias in the area; even militias that fight against Isis. Oil doesn't necessarily have to get as far as the global market for this cycle to happen.
Kind of makes me think of how dirty the oil dependency cycle can become.
Reminds me of the situation where, people in my own country, buy oil from oil companies that they often deride. The big bad oil companies, but people still need the oil. Where else can one buy it from? The oil companies aren't violent, like Isis (so I may naively believe), but they can still be manipulative for sure. That's how business often works.
In the case of Isis, they sell oil to people who, in some cases fight against Isis. Isis has the bigger guns so they end up shooting the people in the end. It makes me think of arms merchants that benefit from selling weapons to both sided of a conflict.
Here's another thought. Maybe we (the western nations, not just Obama) should have done a better job, early on, supplying more friendly rebel groups with fuel (I guess oil) so they wouldn't buy from Isis?
Possibly the good news is that we are doing more to supply the friendlier rebel groups now. Also Isis isn't good at long term planning. According to this article, their oil fields are drying up without the expertise of international oil companies to rejuvenate their fields through fracking and other advanced technologies. The Isis oil machine may not last for long.