For all its bloodshed, how much oil did the Islamic State ultimately produce? With the last ISIS strongholds under attack, the question is perhaps more academic than practical. By the end of 2017, the likely amount of oil produced in ISIS-held territory is bound to be what it was pre-2013: zero barrels.

But in the long run, it’s still a good idea to understand how such a group can harness the resources it commands, and turn them into the fuel for a grotesque war machine. To that end, on Halloween the World Bank published a study of ISIS’s oil production, based on what can be inferred from remote sensors.

Before we get to the findings, it’s interesting to the methods for tracking that production. From the report:

Careful study of news outlets’ reports, agencies’ press releases and Institute for the Study of War (ISW) maps allow us to assign individual oil wells to the Islamic State group’s control at the daily level. We use data from the Visible Infrared Imaging Radiometer Suite (VIIRS) sensors deployed on the NOAA/NASA Suomi NPP satellite, to assess the status of each of the sites and then impute the appropriate levels of production.

Notably, the VIIRS sensors captured gas flares from active wells. The researchers than compared production data from prior-ISIS records. Helping the process along, in 2015, the capture of an ISIS official responsible for logistics yielded a trove of papers and internal accounting, against which researchers could check their production estimates. (ISIS, it turns out, very much tried to standardize production within its borders, complete with quality control inspectors for everything from explosives to consumer goods.)

The study confirms what most informed observers of the war against ISIS could expect: production peaked in the six months after the fall of Mosul, and then declined a lot in 2015, and declined further still in 2016. The numbers themselves, too, are small: at its brief July 2014 peak, ISIS produced only 80,000 barrels per day, and throughout the entire run of the Islamic State, it produced only about as much oil as Abu Dhabi makes in 10 days.