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The Global Sour Barrel

The sour grades WCS competes with, Dubai, Mars, Merey: how they are priced, and what moves the arb.

Read this page as the comp sheet: Dubai, Mars, and Merey are the sour grades that price Western Canadian Select (WCS) by substitution, and the Brent-Dubai spread is the arb gauge between Atlantic and Asian sours. Reported differentials are point-in-time values, not a time series, and every bullet carries its vintage. No single public series tracks the full comp set, so compare like with like.

WCS does not price in a vacuum: it prices against the world sour barrel, and the Brent-Dubai Exchange of Futures for Swaps (EFS) plus the Mars premium to West Texas Intermediate (WTI) are the two numbers that say whether Canadian heavies are cheap or expensive on a relative basis. When Gulf sours vanish and the EFS blows out, substitute sours from the Atlantic Basin firm and WCS rides with them; when Basrah returns or the EFS compresses, the relative bid unwinds. This page is the cross-check on every TMW print: if the sour comp set is strong, a wide WCS differential is a buying opportunity, not a warning.

Updated September 24, 2026 · refreshes every 14 days

Mars: the US Gulf sour benchmark

• Louisiana-delivered Mars settled at $87.65 per barrel in the March 2026 unwind, down about $20 per barrel from the prior session's Argus outright assessment, after comments from US President Donald Trump that the US-Israel war on Iran could end soon triggered a broad selloff. The volume-weighted average premium over West Texas Intermediate (WTI) fell to $4.20 per barrel, about $8.50 per barrel lower than the prior session, with a high-to-low swing of about $10 per barrel. Mars also narrowed from a roughly $13.80 per barrel premium to June Intercontinental Exchange (ICE) Brent futures to about $2.40 per barrel, and Southern Green Canyon traded as low as parity to Domestic Sweet (DSW) crude at Cushing, down from a $7.07 per barrel premium. The snap-back shows how much of the March premium was war risk rather than physical tightness.
  ~Mar 2026 (April trade month context); accessed 2026-09-24 Argus Media

Brent-Dubai EFS: the East-West sour arb gauge

• The Brent-Dubai Exchange of Futures for Swaps (EFS), the spread between Intercontinental Exchange (ICE) Brent futures and Dubai swaps, averaged $1.47 per barrel in the first half of January 2026, surged above $6 per barrel by March 3 as the Hormuz freeze took hold, averaged $9.12 per barrel in March, and oscillated between $3.05 and $16.25 per barrel through June and July. In stable conditions the EFS trades around $1.50 to $2.00 per barrel, and past spikes such as Abqaiq in 2019 and the Russia-Ukraine shock in 2022 barely nudged it above $1.50, making the 2026 move unprecedented in both scale and duration. In 2026 the geographic (routing security) component has overwhelmed the quality component, turning the EFS into a proxy for supply security rather than crude type, and it is the key East-West arb signal for Dubai-basket sour grades.
  Aug/Sep 2026 issue; accessed 2026-09-24 Petroleum Economist (Aug/Sep 2026 issue)

Dubai: the Asian sour benchmark

• Saudi Aramco set its November 2026 official selling prices (OSPs) with Arab Light to East Asia at $4.00 per barrel over the Oman/Dubai average, up from $3.60 for October; to West Europe at $7.20 per barrel over (ICE) Brent, up from $5.70; to the Mediterranean at $6.30 per barrel over (ICE) Brent, up from $4.40; and to North America unchanged at $7.45 per barrel over the Argus Sour Crude Index (ASCI). Arab Medium and Arab Heavy to Asia were held flat at $3.45 and $1.70 over Oman/Dubai. The pattern, much bigger hikes for Europe than for Asia, prices in stronger European competition for sour barrels while testing how much Asian demand can absorb.
  Announced early Sep 2026 for November 2026 loadings; accessed 2026-09-24 Argaam (November table; October comparison via Eye of Riyadh)

Iraq: Basrah's Hormuz workaround

• Iraq's crude exports jumped 73 percent in August 2026 to 73 million barrels for the month (about 2.34 million barrels per day), the highest level since the Iran war began, after the State Organization for Marketing of Oil (SOMO) sold Basrah crude at discounts of $25 to $30 per barrel to offset extreme shipping and insurance costs and a small number of Iraqi tankers received special Iranian permission to transit the Strait of Hormuz. The rebound returned more than $4.5 billion of medium-sour crude to the market in a single month, directly relieving the Dubai-basket sour shortfall that had supported Mars and other Atlantic Basin sours.
  Data: Aug 2026; reported Sep 24, 2026; accessed 2026-09-24 Arab Posts
• Iraq's State Organization for Marketing of Oil (SOMO) offered Basrah Medium and Basrah Heavy for collection by ship-to-ship (STS) transfer near the Omani coast for September delivery via tender, the first time since the Iran war began that buyers could lift Iraqi crude outside the Persian Gulf instead of loading deep inside it. Vitol Group and TotalEnergies were among the largest traders moving Iraqi oil during the war, and SOMO also tendered fuel oil for pickup inside or outside the Gulf. If the outside-Hormuz loading point becomes routine, it structurally lowers the freight risk embedded in Basrah pricing and narrows the crisis premium in Dubai-linked grades.
  Reported Aug 28, 2026; accessed 2026-09-24 Rigzone (citing Bloomberg)

What moves the arb

• The seven core producers of the Organization of the Petroleum Exporting Countries (OPEC+) and its allies (Saudi Arabia, Russia, Iraq, Kuwait, Kazakhstan, Algeria and Oman) met virtually on September 6, 2026 and held October production quotas at September levels, the first month since April without an increase. The decision follows the August 2 approval of a 188,000 barrel per day September increase (Saudi Arabia and Russia 62,000 each, Iraq 26,000, Kuwait 16,000, Kazakhstan 10,000, Algeria 6,000, Oman 5,000), which completed the unwinding of the 2023 voluntary cuts. October required levels: Saudi Arabia 10.478 million barrels per day, Russia 9.949, Iraq 4.431, Kuwait 2.676, Kazakhstan 1.628, Algeria 1.007, Oman 0.841. Delegates expect quotas to hold through year-end while a 2027 baseline review runs; the next review is October 4.
  Decided Sep 6, 2026; accessed 2026-09-24 NewsGoLive (quoting OPEC statement); September increase detail via World Oil
• A record 3.5 million barrels per day of US crude was scheduled to arrive in Asia-Pacific for June 2026 delivery, with Japan alone taking more than 530,000 barrels per day versus a prior monthly high of 290,000 in December 2025, as Asian complex refiners substituted Dubai-basket sour grades with Atlantic Basin barrels. India's July arrivals included 2.8 million barrels per day of Russian Urals (over 55 percent of imports), while Reliance's Jamnagar complex took record volumes of Latin American, US and Canadian barrels and blended them to approximate a medium-sour feedstock. The flow shows Asian sour demand leaking out of the Dubai pricing system entirely, which caps how much support the sour complex can draw from Asian buying.
  Aug/Sep 2026 issue (June 2026 delivery data); accessed 2026-09-24 Petroleum Economist (citing S&P Global Commodity Insights)

Refining economics: max runs, deferred cokers, record diesel cracks

• US refinery utilization hit 97.4 percent in late August 2026, the highest since 2018, and shipbroker Gibson reports very little autumn maintenance is confirmed because robust margins keep refiners from taking units offline. The only confirmed turnaround on the Petroleum Administration for Defense District 3 (PADD 3) slate is ExxonMobil's Beaumont fluid catalytic cracking (FCC) unit plus at least two hydrotreaters, running about 45 days from early December to mid-January. PBF pushed Chalmette's crude distillation unit (CDU) and coker into 2027 and CITGO did the same with the Lake Charles coker, while Valero guided third-quarter Gulf Coast throughput at 1.78 to 1.83 million barrels per day. The International Energy Agency (IEA) still expects October North America runs of 18.8 million barrels per day versus 20.5 in August. Sustained near-max runs keep US Gulf Coast (USGC) coker demand for Mars, Western Canadian Select (WCS) and Merey-type barrels elevated, but deferred maintenance raises unplanned-outage risk.
  Reported Aug 31, 2026; accessed 2026-09-24 Hellenic Shipping News (citing Gibson)
• Asian refining margins for 10 parts per million (ppm) sulfur diesel surged to a record just above $87 per barrel in mid-September 2026, versus pre-war levels around $22 per barrel and a prior record of $85.60 set in late March 2026, while the front-month diesel spread stood slightly above $11 per barrel, near a five-month high. Because medium-sour grades carry higher distillate yields than light sweets, record diesel cracks directly support bids for Mars-type barrels and widen the incentive for complex refiners to run heavy sour slates.
  ~Sep 18, 2026; accessed 2026-09-24 IDNFinancials (citing Reuters and LSEG data)

Mexican Maya: the disappearing USGC heavy barrel

• Pemex crude exports fell to a 35-year low of about 368,000 barrels per day in December 2025, down 54 percent year over year, as the Olmeca refinery (Dos Bocas) ramped to over 260,000 barrels per day of heavy crude processing and absorbed barrels once destined for export. The export collapse removed roughly 440,000 barrels per day of heavy sour supply year over year, and US imports from Mexico fell to 224,500 barrels per day in December (down 22 percent month over month). With crude production capped at 1.8 million barrels per day under Mexico's Energy Sovereignty 2030 roadmap, the industry may lean more heavily on Western Canadian Select (WCS) or Mars as the Americas heavy benchmark as Maya liquidity dries up.
  Data: Dec 2025; published Jan 28, 2026; accessed 2026-09-24 Daily Courier / MarketMinute (via FinancialContent)
• The Maya export collapse partially reversed as Pemex's refineries stumbled: the Olmeca (Dos Bocas) refinery, with 340,000 barrels per day of nameplate capacity, averaged only 144,000 barrels per day in the second quarter of 2026 (about 42 percent utilization) after a January electrical failure knocked out its coker, catalytic and hydrodesulfurization units, deferring around 150,000 barrels of crude processing. Mexican crude exports averaged about 550,000 barrels per day in March through May 2026, down from roughly 780,000 a year earlier, while fuel imports climbed to 700,000 barrels per day in June. If domestic plants keep underperforming, Maya barrels could swing back onto the US Gulf Coast (USGC) export market, so the heavy-sour shortfall is not one-directional.
  Published ~mid-Aug 2026 (Q2 2026 data); accessed 2026-09-24 OilPrice.com

WCS against the sour comp set

• Western Canadian Select (WCS) at Hardisty, Alberta traded at an $8.50 per barrel discount to West Texas Intermediate (WTI) on June 2, 2026, tightening from $9.75 on May 26, as wildfires shut in nearly 350,000 barrels per day of Canadian output (Cenovus's Christina Lake alone lost about 238,000 barrels per day). S&P Global Commodity Insights analysts noted that lower volumes from Mexico and Venezuela to the US Gulf Coast (USGC) have also limited heavy sour supply, and that Chevron's exit from Venezuela was expected to pull up to 200,000 barrels per day of heavy crude from the USGC. The episode shows WCS pricing off the same shrinking USGC heavy-sour pool that Mars and Merey serve.
  Jun 2, 2026; accessed 2026-09-24 Hellenic Shipping News (citing Platts)

For the WCSB book, this is the relative-value page: WCS against Mars, Dubai, and Merey tells you whether the differential is about Canadian barrels or the world sour barrel. Watch the Brent-Dubai spread and the Saudi and Iraqi official selling prices as the marginal read on Gulf sour demand. When the comp set firms together, the WCS differential usually follows.

Standing watch

What the daily feed updates on this page.

• Mars premium/discount to WTI (Platts assessments)
• WCS differential to WTI at Hardisty (broker quotes)
• Merey formula price reported to OPEC and traded discounts to Brent
• Platts Dubai assessments and Brent–Dubai spread
• VLCC and Aframax freight rates on key sour routes (MEG–Asia, USGC–Asia, Jose–USGC)
• Chinese teapot buying of ESPO/Iranian/Venezuelan grades
• SPR exchange delivery composition and return schedule
• USGC refinery coker utilization and heavy crude slate shifts
• Saudi Aramco and Iraqi SOMO monthly official selling prices to Asia (Arab Light versus Oman/Dubai; Basrah Medium and Basrah Heavy) as the marginal read on Gulf medium-sour demand.
• Mexican crude export volumes and Olmeca (Dos Bocas) refinery runs from Pemex monthly reports, to track whether Maya barrels return to the USGC heavy-sour pool.

Gaps

• No public price history series for Dubai, Mars, Merey or WCS was found in open sources; differentials cited are point-in-time reported values, not a time series.
• Current (September 2026) assessed values for Mars and Dubai premiums/discounts are behind paywalls (S&P Global Platts, Argus).
• The exact composition of the SPR's 2026 exchange deliveries (sweet vs sour split) is not published.
• No public source for current WCS forward-month differential values beyond reported broker quotes.
• No public time series exists for the Brent-Dubai Exchange of Futures for Swaps (EFS); the 2026 crisis arc (sub-$2 to over $15 per barrel) is stitched from point-in-time figures in the Petroleum Economist, not a continuous series.
• Iraqi SOMO November 2026 official selling prices for Basrah grades could not be corroborated: a real-time wire reported Basrah Medium to Asia at plus $0.85 per barrel to Oman/Dubai while an Argus sample table showed plus $2.10, so no Iraqi OSP bullet was drafted.
• No public monthly series confirms whether Mexico's crude export collapse (368,000 barrels per day in December 2025) persisted through 2026; the March-May 2026 average of about 550,000 barrels per day comes from a single OilPrice analysis, not Pemex data.

Sources