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“In a single oil field there can be hundreds of these makeshift operations,” said Lamrani, citing aerial imagery showing a constellation of tiny furnaces around a Mosul field that was mostly just sand a year ago. “It’s not the ideal way to do it, so their revenue is going down. But it still works.” The images were provided to The Washington Post by Stratfor and AllSource Analysis, a Colorado firm that specializes in geospatial research.

The tiny refineries are partly offsetting huge losses in income resulting from the disruption of traditional oil production in northern Iraqi fields controlled by ISIL since mid-2014. After capturing the facilities, the group’s leaders initially attempted to run them as businesses, retaining enough workers to keep the refineries operating and hauling the finished products by tanker truck to independent dealers in Turkey, Syria and Iraq’s Kurdish provinces, U.S. officials say.

At its peak, ISIL’s oil operations were netting an estimated $50 million a month. But the group’s oil income has plummeted in recent months, through a combination of poor management and a steady drumbeat of airstrikes that have targeted refineries and storage depots as well as tanker convoys.

It’s not the ideal way to do it, so their revenue is going down. But it still works

The proliferation of micro-refineries is the latest sign of strain in the group’s self-declared caliphate, which has lost half its territorial holdings in Iraq since late 2014.

At the same time, the use of low-tech alternatives also reflects a certain resilience by an organization that also depends on self-generated oil to run its military operations and electric generators, Lamrani said. Stratfor estimates that oil contributed about $20 million a month to ISIL’s coffers as recently as March, with much of the petroleum coming from makeshift facilities.