Mars crude is the most actively traded medium-sour crude grade in the United States and the de facto benchmark for U.S. Gulf of Mexico offshore production. It takes its name from the Shell-operated Mars platform, a tension-leg platform installed in 1996 in approximately 900 meters of water in the Mississippi Canyon area of the Gulf of Mexico, about 130 miles southeast of New Orleans. Mars output and crude from a number of other offshore fields commingle in the Mars pipeline system and are delivered to onshore terminals as a single named stream.
Although Mars rarely appears in retail financial news — overshadowed by Brent and WTI in the public conversation — it is one of the most institutionally important U.S. crude grades. Mars-equivalent crude is the feedstock that the great majority of complex U.S. Gulf Coast refineries are configured to process, and the Mars-WTI differential is one of the most-watched indicators of U.S. sour-refining economics.
Mars is quoted as a differential to WTI or to the WTI/HLS-equivalent benchmark depending on the assessment context. The chart above shows the WTI reference; current Mars differentials are published daily by Argus.
The Mars Production System
The Mars production hub consists of several stacked oilfields developed across multiple platforms over roughly three decades. The original Mars discovery was made by Shell in 1989 and the field came online in 1996 — the first deepwater tension-leg platform in the Gulf and a landmark project for U.S. offshore engineering. Production from Mars has been supplemented over the years by:
- Olympus — a second tension-leg platform installed in 2014, allowing the development of additional Mars-area reserves
- Ursa — Shell's adjacent deepwater development
- Various subsea tieback fields that route production through the Mars infrastructure
Production from this complex flows through the Mars Oil Pipeline System — operated by a Shell-led joint venture — to onshore delivery points at Clovelly, Louisiana, where it enters the Louisiana Offshore Oil Port (LOOP) system or moves into other pipeline networks serving Gulf Coast refineries. Aggregate flows through the Mars pipeline routinely exceed 500,000 barrels per day, with the specific volume varying as Gulf of Mexico fields are added or retired from the stream.
Quality Specifications
Mars is classified as medium sour. Its API gravity is approximately 28° to 30°, considerably heavier than WTI (39.6°) or Brent (38°) but not as heavy as WCS (20–22°) or Mexican Maya (22°). Sulfur content is approximately 1.9% to 2.1% by weight, comparable to Dubai and well above the 0.5% sweet/sour threshold.
The combination — medium gravity, medium-to-high sulfur — gives Mars a refining yield profile rich in middle distillates (diesel, jet fuel, heating oil) and heavy fuel components, with proportionally less gasoline than a light sweet crude. Mars processing requires meaningful hydrotreating capacity and benefits from coking units that can convert the heavy residue fractions into transport fuels.
U.S. Gulf Coast refining was substantially rebuilt during the 1990s and 2000s to handle exactly this kind of feedstock — driven initially by Venezuelan and Mexican heavy crude imports and later supplemented by Canadian heavy crude. The result is a refining complex whose preferred slate is medium-to-heavy sour, with Mars sitting near the sweet spot of the lighter end of that preferred range.
The Argus Sour Crude Index
Unlike WTI or Brent, Mars does not have its own futures contract. Its daily price is assessed by price-reporting agencies — principally Argus Media — through the Argus Sour Crude Index (ASCI). ASCI is a volume-weighted average of physical trades of three major U.S. Gulf medium sour grades: Mars, Poseidon, and Southern Green Canyon. The index has been published since 2009 and was developed in response to Saudi Aramco's 2010 decision to use ASCI rather than WTI as the basis for its Official Selling Prices for U.S.-bound crude exports.
This was a significant institutional event. Saudi Aramco's adoption of ASCI signaled that WTI's pricing relevance for sour-crude trade had eroded — partly because WTI is itself light sweet, and partly because WTI's Cushing-specific delivery point can introduce regional dislocations that have nothing to do with the broader U.S. sour market. ASCI provides a more accurate proxy for U.S. Gulf Coast sour-crude conditions, and it remains the principal pricing reference for Middle Eastern sour-crude flows into U.S. Gulf refineries.
For most market participants, the "Mars price" quoted on any given day is the Argus Mars assessment, derived from observed Mars and ASCI-eligible cargo trades.
The Mars-WTI Differential
Mars is conventionally quoted as a differential to WTI, typically in the range of WTI minus $1 to WTI plus $3 per barrel — wider ranges occur during stress episodes. The differential reflects a combination of:
Quality differential. A medium sour barrel and a light sweet barrel of the same volume yield different product slates with different market values. In normal conditions, Mars carries a small quality discount to WTI of approximately $2 to $4 per barrel based on refining yield economics.
Logistics adjustment. Mars is delivered to the U.S. Gulf Coast, while WTI is priced at Cushing, Oklahoma. The freight cost from Cushing to the Gulf Coast — typically $2 to $4 per barrel through the Seaway and Marketlink pipelines — partially offsets the Mars quality discount.
Sour-refining margin signal. When U.S. Gulf Coast complex refineries are running flat-out on strong product cracks, they bid aggressively for medium sour feedstock and Mars can trade at a premium to WTI. When refining margins are weak or Gulf Coast refineries face heavy maintenance, Mars discounts widen.
The Mars-WTI spread is therefore one of the cleanest indicators of U.S. complex refining health and is closely watched by trading desks and refining analysts.
Hurricane Exposure
One of the structural features that distinguishes Mars from onshore-produced grades is its acute exposure to Gulf of Mexico hurricane activity. The Mars and Olympus platforms, along with the offshore pipeline infrastructure that feeds them into shore, sit directly in the path of major Atlantic hurricane systems during the June-to-November season.
Operators routinely shut in production and evacuate platforms ahead of approaching named storms, removing 100,000 to 500,000+ barrels per day of Mars-area production from the market for periods ranging from a few days (for minor storms) to multiple weeks (for major hurricanes that cause physical damage). Hurricane Katrina in 2005, Hurricane Ida in 2021, and several other major storms have caused production outages measured in months.
Hurricane-driven supply disruption typically widens the Mars-WTI differential in two opposing ways. Mars prompt prices may spike on physical scarcity, while WTI may also rise on broader U.S. supply concerns. The net differential effect depends on the storm's specific impact path. Analysts watching Mars prices through hurricane season factor a structural risk premium into their forward views from June through November.
Who Refines Mars
The buyer base for Mars is heavily concentrated in U.S. Gulf Coast refining, including:
- Major integrated refiners — Shell (which produces much of the underlying crude), Marathon, Phillips 66, Valero, and Chevron all run substantial Mars volumes through Gulf Coast facilities.
- Motiva — the Saudi Aramco-owned Port Arthur, Texas refinery is one of the largest single refineries in North America and a major Mars consumer.
- Independent refiners — including specialty operators with appropriate sour-processing capability.
Mars also competes directly with imported Middle Eastern sour grades (particularly Saudi Arab Medium and Iraqi Basrah Medium), Mexican Maya, Canadian heavy crude, and post-2015 a small share of imported Latin American sour barrels. Saudi Aramco's adoption of ASCI as a pricing reference for U.S.-bound crude has created a structural linkage between Mars pricing and Saudi marketing decisions.
What Drives the Mars Price
WTI. Mars prices off WTI in absolute terms, so headline price moves track WTI.
U.S. Gulf Coast refining margins. Strong diesel and gasoline cracks tighten Mars differentials.
Hurricane season activity. Active Atlantic seasons widen Mars premia during shut-in episodes.
OPEC+ sour crude availability. Saudi, Iraqi, and Kuwaiti production cuts reduce alternative sour imports and support Mars differentials.
Canadian heavy crude flows. Increased Canadian heavy via TMX or pipeline expansion provides substitute Gulf Coast sour feedstock and competes with Mars.
SPR sour crude operations. Strategic Petroleum Reserve sour barrels released or refilled directly affect U.S. Gulf sour market balance.
Mars in One Sentence
Mars is the U.S. Gulf of Mexico medium sour benchmark — assessed by Argus, anchored by Shell-operated deepwater production, and watched by complex Gulf Coast refiners as the cleanest signal of domestic sour-refining economics.
Continue Reading
- What is WTI crude — the pricing reference for Mars
- What is Western Canadian Select — competing heavy sour supply
- What is Dubai crude — the Asian sour benchmark
- Oil market glossary