Once a major source of revenue for the Syrian government, proceeds from oil sales were sharply curtailed in 2011 following the imposition of sanction by its European customers. Sales of Syrian crude oil to those European countries amounted to some $3.6 billion a year, according the U.S. Energy Information Administration.
Since then, the ongoing Syrian civil war has severely curtailed output as the government has lost control of major oil facilities.
As of last year, ISIS controlled as much as 350,000 barrels per day production capacity in Iraq and Syria, but was only able to produce 50,000 to 60,000 barrels a day, according to estimates from IHS.
The oil is sold on the black market, mostly via trucks smuggling it over the border to Turkey, a route first established more than a decade ago by former Iraqi dictator Saddam Hussein, who used the black market to evade sanctions on oil sales.
That supply chain, created to evade the UN's oil-for-food program, still provides a ready market for oil and diesel fuel produced at facilities seized by ISIS.
The group has set up what amounts to its own oil company, recruiting trained engineers and managers through a human resources department and offering competitive salaries; some formerly worked at the oilfields now under ISIS control, according to the Financial Times.
By targeting a cluster of ISIS-controlled oilfields, the U.S.-led coalition is hoping to put a serious crimp in the group's oil production and cut off an important source of cash.
The loss of oil revenues would further strain the terrorist group's ambition to establish a true state and deliver basic services to the people it is trying to conquer. By way of comparison, the Iraqi government budgeted more than $2 billion to provide services in the provinces where ISIS operated last year, Treasury's Cohen noted, far more cash than ISIS is generating.
No matter how well the rest of the world is able to isolate ISIS financially, however, efforts to curb ISIS's terrorist ambitions by restricting its cash flow can only go so far, according to Kroll's Karson.
"Money is only part of it," he said. "You don't need a lot of money to move across borders and you don't need a lot of money to buy explosive devices. So it's a part of the puzzle, but it's by no means the entire thing."