Sour Crude Oil
Crude petroleum with sulphur content above 0.5% — the majority of global crude production, traded at a discount to sweet crude due to its higher refinery processing cost.
Definition
Sour crude oil is crude petroleum with a sulphur content greater than 0.5% by weight. The name derives from the presence of hydrogen sulphide (H₂S) — a toxic, flammable gas with a characteristic rotten egg smell — and other sulphur compounds (mercaptans, thiophenes) that give high-sulphur crude its "sour" character.
The contrast is sweet crude — crude oil with sulphur below 0.5% — which is easier and cheaper to refine into low-sulphur products. See our sweet crude glossary entry for the complementary definition.
Sour crude accounts for the majority of global crude oil production. Saudi Arabia — the world's largest oil exporter — predominantly produces sour crude grades. For Saudi production context, see our Saudi Arabia fuel prices analysis and OPEC glossary entry.
Major sour crude grades (2026)
| Crude grade | Origin | Sulphur (%) | API gravity | Classification |
|---|---|---|---|---|
| Arab Heavy | Saudi Arabia | ~2.85 | ~28° | Heavy sour |
| Arab Medium | Saudi Arabia | ~2.59 | ~31° | Medium sour |
| Arab Light | Saudi Arabia | ~1.77 | ~33° | Medium-light sour |
| Dubai/Oman | UAE / Oman | ~2.0 | ~31° | Medium sour; Asian benchmark |
| Basra Heavy | Iraq | ~3.5 | ~24° | Heavy sour |
| Urals | Russia | ~1.3 | ~31° | Medium sour; key European benchmark |
| Canadian oil sands (WCS) | Canada | ~3.5–4.5 | ~20–22° | Heavy sour; heavily discounted |
| Merey | Venezuela | ~2.3 | ~16° | Extra-heavy sour |
For the sweet crude benchmarks used in international pricing, see our Brent crude entry and WTI entry.
Why sour crude trades at a discount
The processing of sour crude into low-sulphur refined products requires hydrodesulphurisation (HDS) — a catalytic process in which hydrogen is reacted with sulphur compounds under high temperature and pressure to produce hydrogen sulphide (H₂S), which is then removed and converted to elemental sulphur. The cost of this process creates the sour crude discount:
- Capital cost: HDS units cost $200 million–$1 billion to install; only "complex" refineries have them
- Operating cost: Hydrogen feedstock (typically produced via steam methane reforming, a gas-intensive process) adds $1–5/barrel of processing cost
- Catalyst cost: HDS catalysts (cobalt-molybdenum or nickel-molybdenum) require periodic regeneration or replacement
The sour-sweet differential — the price discount of sour crude relative to Brent (sweet) — varies with refinery capacity utilisation and the balance of sweet vs sour crudes available:
| Period | Typical sour-sweet spread (Arab Light vs Brent) | Driver |
|---|---|---|
| 2019 (pre-COVID) | ~$3–5/bbl | Ample complex refinery capacity; normal IMO 2020 anticipation |
| 2020 (COVID demand collapse) | ~$1–3/bbl | Demand collapse hit all crudes; crude quality less relevant |
| 2022 (Russia crisis) | ~$5–10/bbl | Urals (medium sour) discounted by sanctions; sour supply looser |
| 2024–2026 (normalised) | ~$3–6/bbl | Complex refinery capacity adequate; moderate crude quality demand |
Refinery complexity and sour crude processing
Not all refineries can process sour crude economically. The "Nelson Complexity Index" measures refinery upgrading capability — higher complexity means the refinery can handle heavier, sourer crudes and extract more value:
- Simple / hydroskimming refineries: Complexity 1–5; primarily process sweet crude; limited ability to handle sour crude
- Complex refineries: Complexity 6–10; hydrotreaters, fluid catalytic crackers (FCC); can process medium sour crude
- Very complex / conversion refineries: Complexity 10+; cokers, hydrocrackers; can process heavy sour crude and extract maximum distillate yield
Gulf Coast US refineries are among the world's most complex — originally built to process heavy sour Venezuelan and Mexican crude. The Russian Urals crude sanctions after 2022 forced European refineries (which had been optimised for medium-sour Urals) to reconfigure for alternative crude diets, contributing to margin volatility. For the refinery context, see our refinery glossary entry and refinery margin entry.
Frequently asked questions
-
What is sour crude oil?
- Sour crude is crude oil with sulphur content above 0.5% by weight. Most Saudi, Iraqi, Russian Urals, and Canadian oil sands crudes are sour. Sour crude requires hydrodesulphurisation (HDS) processing to remove sulphur before making low-sulphur road fuels, which adds processing cost and is reflected in a price discount versus sweet crude (low-sulphur crude like Brent or WTI).
-
Why is sour crude cheaper than sweet crude?
- Sour crude requires additional refinery processing — hydrodesulphurisation — to reduce sulphur for low-sulphur road fuels. This processing requires capital investment in HDS units and expensive hydrogen feedstock, costing $1–5/barrel more than processing sweet crude. The sour crude discount (typically $3–6/barrel vs Brent for Arab Light) compensates refiners for this extra cost. Simple refineries without HDS units cannot process sour crude economically.
-
What is the difference between sour crude and sweet crude?
- Sweet crude has <0.5% sulphur (Brent ~0.37%, WTI ~0.24%); sour crude has >0.5% sulphur (Arab Heavy ~2.85%, Urals ~1.3%). Sweet crude is easier and cheaper to refine into compliant low-sulphur products and trades at a premium. Sour crude is more abundant globally but trades at a discount reflecting additional processing cost. The light/heavy and sweet/sour dimensions together determine a crude's quality premium or discount vs benchmark.