[Apparently the oil fields are barely functioning and generate only a few thousand barrels a day. Prior to tossing them under the bus I believe the Kurds occupied the fields and they got what little money that came from them.]
Trump estimates the number at $45m per month. Not a substitute for a functioning economy, but until a durable peace involving Kurdish autonomy is found, and a peacetime economy resurrected, it’s the only game in town for the Kurds. The Russkies are getting involved because they’d like to scale back their aid to Assad. We want to help the Kurds keep it, because it helps us minimize our aid to them.
https://en.wikipedia.org/wiki/Petroleum_industry_in_Syria#Oil_production_during_the_Syrian_Civil_War