start with some oxygen your dupes breath it then we feed the Co2 to the slicksters they make oil we refine that oil we get nat-gas and petroleum we burn off the gas, we get dirty water and Co2 we filter the dirty water, we turn it into oxygen (via electrolizers) and the cycle repeats
if you have a petrol-boiler instead of just an oil refinery then use the petroleum for power instead
you should also take the Co2 you get from the generators and give that to the slicksters too, in order to increase efficiency.

Co2 -> oil -> petrol or nat-gas -> generators -> water -> oxygen -> Co2 -> REPEAT
↓ ↓
plastic? Co2
↓
REPEAT