Sweet versus sour
The sweet/sour split is a rough industry convention rather than an exact law. A common threshold is about 0.5% sulphur by weight: below it a crude is treated as sweet, above it as sour. Many Middle Eastern export grades are sour, while several North Sea and West African grades are sweet.
Why sulphur matters in refining
Sulphur must be removed to make finished fuels that meet modern environmental limits. Processing sour crude therefore needs hydrotreating and sulphur-recovery capacity, which costs money and energy. A refinery without that capacity will either avoid sour crude or pay less for it, which is why sulphur is a key pricing factor.
Fuel sulphur limits and demand
Regulations cap sulphur in finished fuels. Road diesel in many markets is limited to 10 parts per million, and since 2020 the International Maritime Organization has capped sulphur in marine fuel at 0.5% outside designated control areas. These limits raise demand for low-sulphur products and for the refining capacity that can make them from higher-sulphur crude.
- Road diesel: 10 ppm sulphur in many markets; see EN590 10ppm diesel.
- Marine fuel: 0.5% global cap since IMO 2020 (lower in control areas).
Sulphur in a trading decision
For a buyer, sulphur is weighed against the discount on offer and the refinery's ability to process it. Basrah grades are sour; the practical difference between them is covered in Basrah Medium vs Basrah Heavy.
Sources & references
This article paraphrases widely established facts from the organisations below. It does not reproduce their text. Always confirm current figures against the primary source.
- International Maritime Organization (IMO) — 2020 marine fuel sulphur cap
- U.S. Energy Information Administration (EIA) — sweet and sour crude