I found this interesting article which claims that the fuel we import into Nigeria is of a lower quality than that produced by illegal refineries in the Niger-Delta

Black market fuel made from stolen oil in rudimentary “bush” refineries hidden deep in the creeks and swamps of the Niger delta is less polluting than the highly toxic diesel and petrol that Europe exports to Nigeria, new laboratory analysis has found.

Shell, Exxon, Chevron and other major oil companies extract and export up to 2m barrels a day of high quality, low sulphur “Bonny Light” crude from the Niger delta. But very little of this oil is refined in the country because its four state-owned refineries are dysfunctional or have closed.

Instead, international dealers export to Nigeria around 900,000 tonnes a year of low-grade, “dirty” fuel, made in Dutch, Belgian and other European refineries, and hundreds of small-scale artisanal refineries produce large quantities of illegal fuel from oil stolen from the network of oil pipelines that criss-cross the Niger delta.