Sweet vs Sour Crude Oil
Sweet crude has a low sulfur content, usually below 0.5% by weight. Sour crude has more. Sulfur must be removed during refining to meet fuel standards, so sour crude normally sells at a discount to sweet crude of similar density. Brent and WTI are sweet. Dubai, Urals, Mars and WCS are sour.
Where the line between sweet and sour sits
There is no single legal definition. The most common market convention is:
- Sweet: less than about 0.5% sulfur by weight.
- Sour: more than about 0.5%. Grades between roughly 0.5% and 1% are sometimes called "medium sour" or "light sour" when they are also light.
Exchange contracts set exact limits. Crude delivered against NYMEX WTI futures must have no more than 0.42% sulfur and an API gravity between 37° and 42°. The names date from the 19th century, when prospectors tasted and smelled oil. Low-sulfur crude had a sweeter taste, while high-sulfur crude smelled of rotten eggs from hydrogen sulfide.
Sulfur and density of major crude grades
| Grade | Sulfur | API gravity | Classification |
|---|---|---|---|
| WTI (NYMEX spec) | <0.42% | 37–42° | Light sweet |
| Brent | ~0.4% | ~38° | Light sweet |
| LLS | ≤0.4% | 34–41° | Light sweet |
| Murban | ~0.74–0.78% | ~40° | Light, low sour |
| Urals | ~1.4–1.7% | ~30° | Medium sour |
| Mars | ~1.8–2% | ~30° | Medium sour |
| Dubai | ~2% | ~31° | Medium sour |
| WCS | ~3–3.7% | ~21° | Heavy sour |
Sources: CME (WTI), ADNOC and Platts (Murban), Reuters 2022 loading data (Urals), RBN Energy (LLS, Mars), Crude Monitor and Oil Sands Magazine (WCS). Typical values; cargoes vary. Density is covered in API gravity explained.
Why sour crude trades at a discount
A refiner values crude by the products it yields minus the cost of making them. Sulfur adds cost at several stages:
- Hydrotreating. Gasoline and diesel sold in the US, Europe and most of Asia must contain very little sulfur. Refiners remove it by reacting fuel streams with hydrogen, which requires hydrotreaters and a hydrogen supply.
- Sulfur recovery. The removed sulfur, as hydrogen sulfide, has to be converted into elemental sulfur in dedicated plants.
- Heavier residue. Sour crudes are often also heavier, leaving more residue that must be cracked or coked into lighter products.
- Corrosion and handling. Sulfur compounds corrode equipment and add safety requirements.
Only refiners with that equipment can run large volumes of sour crude. To attract them, sour grades must sell cheaply enough to cover the extra processing cost and give the refiner a return on its investment. That gap is the sweet-sour differential.
Refinery complexity: simple vs complex refineries
| Refinery type | Main units | Preferred crude | Example regions |
|---|---|---|---|
| Simple (hydroskimming) | Distillation, reforming, basic hydrotreating | Light sweet | Older plants in Europe and Africa |
| Cracking | Adds fluid catalytic cracking | Light and medium, sweet or moderately sour | Many European and Asian refineries |
| Coking (complex) | Adds cokers, hydrocrackers, sulfur plants | Heavy and sour, bought at a discount | US Gulf Coast, US Midwest, India, newer Middle East and Chinese plants |
Complex refineries earn more when the sweet-sour spread is wide, because their cheaper crude still yields high-value fuels. That is why US Gulf Coast refiners favour Mars, Mexican and Canadian crude, and why Indian refiners have bought large volumes of discounted Urals since 2022. The crack spread guide explains how refinery margins are measured.
What sets the sweet-sour spread
- OPEC+ policy. Most OPEC+ output, especially from the Gulf, is medium sour. Production cuts shrink sour supply and narrow the spread. Output increases widen it. See OPEC+ explained.
- Sanctions. Restrictions on Iran, Venezuela and Russia, all large sour exporters, change how much sour crude reaches open markets.
- Fuel oil prices. Sour crude yields more high-sulfur fuel oil. When fuel oil is cheap relative to crude, sour grades lose value.
- Light sweet supply growth. US shale and Guyanese and Brazilian output have added large volumes of light, low-sulfur crude.
How IMO 2020 changed sour crude prices
On 1 January 2020 the International Maritime Organization cut the maximum sulfur content of marine fuel outside emission control areas from 3.5% to 0.5%. Ships had burned large amounts of high-sulfur fuel oil, a residue of sour crude refining, so the rule threatened a major outlet for sour barrels.
In the months before the deadline, high-sulfur fuel oil prices fell sharply relative to crude and low-sulfur products rose. Sour crude differentials weakened as refiners anticipated the change. The effect then faded as the 2020 pandemic and the record OPEC+ cuts that followed reduced sour supply, and as more ships fitted exhaust scrubbers that allow them to keep burning high-sulfur fuel. Emission control areas, such as those around North America and Northern Europe, require 0.1% sulfur.
When sour crude trades at a premium: 2026
The sweet-sour discount is not fixed. In 2026, when the near-closure of the Strait of Hormuz removed millions of barrels a day of Gulf medium sour crude, sour grades that could still be shipped became scarce. The IEA reports that:
- US Gulf medium sour Mars averaged a $4.45/bbl premium in March 2026, reaching $13.21 by month-end.
- Oman, a medium sour grade loading outside the Strait, traded at record premiums to Dubai.
- Asian refiners running sour crude saw record margins in late March, based on Dubai prices, though the IEA cautioned that those calculated margins overstated real profitability.
The lesson is that quality differentials hold only when supply is broadly balanced. In a shock, availability and location matter more than sulfur. For how each benchmark was affected, see the crude oil benchmarks guide.
Sweet vs sour crude FAQ
What is the difference between sweet and sour crude oil?
The difference is sulfur content. Sweet crude usually contains less than about 0.5% sulfur by weight, while sour crude contains more. Sour crude needs extra refining to remove sulfur before fuels can be sold, so it normally trades at a discount to sweet crude of similar density.
Is Brent sweet or sour?
Brent is a light sweet crude, with sulfur content of about 0.4% and an API gravity of roughly 38°. WTI is also light sweet, with a NYMEX limit of 0.42% sulfur. Dubai, Urals, Mars and Western Canadian Select are examples of sour crudes.
Why is sour crude cheaper?
Removing sulfur requires hydrotreaters, hydrogen and sulfur recovery plants, and sour crudes often leave more heavy residue that must be upgraded. Only complex refineries can process large amounts. To attract them, sour crude has to sell at a discount that covers the extra cost and investment.
How did IMO 2020 affect sour crude?
IMO 2020 cut the sulfur limit for ship fuel from 3.5% to 0.5% on 1 January 2020. That reduced demand for high-sulfur fuel oil, a by-product of sour crude refining. Sour crude discounts widened before the deadline, but the effect faded as the pandemic and OPEC+ cuts tightened sour supply.
Why are they called sweet and sour?
The terms come from early oil prospectors, who tasted and smelled crude to judge its quality. Low-sulfur oil had a relatively sweet taste and mild smell. High-sulfur oil smelled of hydrogen sulfide, like rotten eggs, and was called sour.
Related pages
API Gravity
What API gravity measures, the formula, light, medium and heavy thresholds, examples by grade, and why lighter crude usually sells for more.
Crude Oil Benchmarks
Guide to the world's crude oil benchmarks: Brent, WTI, Dubai/Oman, Murban, OPEC basket, Urals and WCS, how each is priced and where it is used.
Mars Crude
Mars crude explained: the offshore Gulf of Mexico medium sour grade, how it trades against WTI, and what the Mars differential says about sour demand.
Dubai Crude
What Dubai crude is, how Platts assesses Dubai/Oman, the Brent-Dubai spread (EFS) and why it prices most Middle East oil sold to Asia.