Mars Crude Oil

What Mars crude is

Mars is a blend of crude from deepwater fields in the Mississippi Canyon area of the US Gulf of Mexico (renamed the Gulf of America by the US government in 2025). It takes its name from Shell's Mars field and is gathered by pipeline to onshore terminals in Louisiana. With an API gravity of about 30° and sulfur near 2%, Mars is similar in quality to many Middle East export grades, which is why it became the US reference for sour crude.

Medium sour crude is the main diet of complex US Gulf Coast refineries, which have cokers and hydrotreaters to upgrade heavier, high-sulfur oil into gasoline and diesel. For background, see sweet vs sour crude and API gravity.

The Argus Sour Crude Index

The ASCI is a daily volume-weighted average of trades in three Gulf Coast medium sour grades, treated as a single blend:

Before 2009, Middle East producers priced US sales against WTI. When WTI began to disconnect from coastal markets because of inland supply gluts, a coastal sour index was a better match for what their crude competed with. Mars was the obvious anchor because it trades in large, regular volumes.

How Mars trades against WTI

Mars is assessed daily by price reporting agencies as a differential to WTI. Physical deals are often priced at the WTI calendar-month average plus or minus that differential. In most years Mars trades at a discount to WTI because it is heavier and more sulfurous, yielding less gasoline and diesel and needing more processing. The discount changes with the balance of sour crude supply and demand:

Mars in 2026: from discount to premium

The 2026 Middle East disruption removed millions of barrels a day of medium sour crude from the market. According to the IEA's April 2026 Oil Market Report, Mars rose by $5.76/bbl month-on-month to average a $4.45/bbl premium in March 2026, and reached $13.21/bbl by the end of the month, as refiners worldwide sought alternatives to Gulf grades. Heavy sour Canadian crude moved the other way, weakening as the US Strategic Petroleum Reserve release diverted some Gulf Coast refiners away from it.

The SPR matters for Mars because much of the reserve is sour crude. The IEA reported that discounted US reserve barrels competed in Europe in April 2026, pulling down sour differentials there. See Strategic Petroleum Reserve for how releases work.

Mars, Dubai and global sour pricing

Mars is linked to the world's other sour markers through trade flows:

• Dubai sets the value of Middle East sour crude in Asia. When Dubai rises relative to Brent, Gulf producers send less crude west and Mars tends to strengthen.
• Saudi, Iraqi and Kuwaiti OSPs for US buyers are set against ASCI, so Mars feeds directly into what US refiners pay for Middle East crude.
• WCS and Mexican Maya are heavier sour grades that compete with Mars for coking capacity on the Gulf Coast.
• LLS is the Gulf Coast's light sweet marker, and the LLS-Mars spread is the standard US sweet-sour differential.

IMO 2020 and the sour discount

From 1 January 2020 the International Maritime Organization cut the maximum sulfur content of ship fuel outside emission control areas from 3.5% to 0.5%. Before the rule took effect, refiners expected demand for high-sulfur fuel oil to collapse, which would make sour crude (rich in high-sulfur residue) less valuable and widen the Mars discount. In practice the effect was soon overtaken by the 2020 pandemic and the deep OPEC+ cuts that followed, which reduced sour crude supply. Ships fitted with exhaust scrubbers can also still burn high-sulfur fuel. The sweet vs sour guide covers this in more detail, and the crude oil benchmarks guide compares Mars with other grades.