Earlier quoted context omitted.
But is their ability to produce or sell refined vs unrefined oil effected by the sanctions? Maybe not directly; maybe in how they're able to access funding for infrastructure, for example?
Iran had a 69% decrease in oil exports (In terms of barrels exported) between 2018 and 2019, as a result of US sanctions [1]. I don't think refined vs unrefined is the biggest issue, given that a reduction in quantity would be hard to recover in terms of quality. [1] https://fred.stlouisfed.org/series/IRNNXGOCMBD
However, for sake of understanding: currently about 4-6% of global oil and gas is used to produce plastics. This represents about $600 Billion in total market value.
The total global crude oil market is about $1,800 Billion if oil is $50/bbl.
So it seems that you get roughly 5x higher revenue on a barrel of oil turned into plastic than sold in its unrefined state. Probably much more if oil is $20/bbl.