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Refiners compare heavy crude grades by matching current assays on a common basis: sulfur by weight, API gravity, and the yields of defined distillation cuts. They then test that feed profile against the refinery’s desulfurization and conversion capacity, product needs, and blending limits. An assay describes what is in the crude; it does not guarantee how much gasoline or diesel a refinery will ultimately produce.
What a crude assay tells you—and what it does not
A crude assay is a physical and chemical description of a sample. Its distillation results show the fractions recoverable within specified boiling ranges, such as naphtha, kerosene, diesel-range material, vacuum gas oil (VGO), and residue. Those are properties of the feed, not a final product forecast.
Refineries first separate crude by distillation, then may transform heavier fractions through processes such as cracking or coking and remove sulfur through hydrotreating. The U.S. Energy Information Administration (EIA) explains that simple distillation of denser, heavier crude produces lower-value products; upgrading capability affects what can be made from those fractions. See the EIA’s refining inputs-and-outputs explainer.
For a valid yield comparison, keep the assay’s cut-point definitions and measurement basis consistent. A percentage by volume is not interchangeable with a percentage by weight, and two figures labelled “diesel yield” cannot be compared reliably if their boiling ranges differ. A larger VGO fraction may suit a refinery with available conversion capacity, but it is not automatically a larger final diesel yield.
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Which measures belong in a heavy-crude comparison?
| Measure | How to compare it | Why it matters |
|---|---|---|
| Total sulfur | Record weight percent, sample or assay date, and whether the figure is a typical specification, market guide value, or measured cargo result. | Higher sulfur can require more removal capacity and affect processing economics. |
| API gravity | Compare on the same reference convention where available, and use the assay date. | It is a first-pass density screen: lower API generally points to heavier material, but does not replace the distillation profile. |
| Distillation cuts | Match boiling cut points and report whether yields are volume or weight percentages. | Shows the feed’s distribution across light and middle cuts, VGO, and residue. |
| Residue share | Use the same residue threshold and assay basis for both grades. | Indicates the bottom-of-barrel volume that may need upgrading or may remain a residual product. |
| Refinery conversion capacity | Assess site-specific vacuum distillation, cracking, and coking configuration and utilization. | Determines whether heavy fractions can be converted into lighter products. |
| Desulfurization capacity | Assess the site’s unit capability and available capacity. | Determines whether higher-sulfur feed and resulting streams can be processed economically. |
| Commercial and operating fit | Include delivered cost, transport, blending limits, desired product slate, and other assay properties such as metals and acidity. | A favorable sulfur or yield profile alone does not establish the most profitable feed choice. |
Published grade figures are useful context, not a yield ranking
Maya and Western Canadian Select specifications
PEMEX lists Maya crude at 21.0–22.0° API and 3.4% sulfur by weight on its page modified May 26, 2026, and identifies it as feedstock for fossil fuels: PEMEX Maya crude. S&P Global Platts’ April 2026 Americas specifications guide gives Maya as 21.5° API and 3.4% sulfur, and Western Canadian Select (Cushing/Nederland) as 21.8° API and 3.63% sulfur. The guide notes that specifications can vary over time: April 2026 guide.
These are published grade specifications or guide values, not guaranteed measurements for a cargo. They also do not provide a matched current set of distillation-cut yields, so API and sulfur alone cannot establish which grade will yield more diesel or less residue at a particular refinery. Procurement and planning decisions should use current cargo or grade assays.
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Historical residue comparisons
An EIA research paper published in 2019 compared assay-derived atmospheric residue shares using historical assay inputs: WTI 33.3%, Brent 34.2%, Mars about 47%, and Maya 61.2%. The figures illustrate the general association between heavier grades and greater residue; they are not current guaranteed yields for those grades. The paper notes cited assay inputs from 1994 and 2000: EIA Figure 2.1 and research paper.
How to turn the comparison into a refinery decision
- Obtain comparable assays. Use current assay data for the grades or cargoes under consideration. The U.S. Department of Energy’s Crude Oil Assay Manual (August 2024) is a reference for assay concepts and reporting.
- Normalize the comparison. Confirm dates, sulfur units, API reference convention, cut points, and whether each cut yield is expressed by volume or weight. Flag any value that is a typical specification rather than a measured sample.
- Map the fractions to available units. Estimate how the refinery can handle each grade’s light cuts, middle fractions, VGO, and residue in its actual operating configuration and at its available capacity.
- Check sulfur and other constraints. Compare sulfur-removal requirements alongside blending limits and other relevant assay properties. A technically processable crude may still be a poor fit if units are constrained or the resulting streams do not match product requirements.
- Evaluate commercial fit. Bring delivered economics and the refinery’s desired product slate into the assessment. Neither a lower sulfur number nor a higher nominal cut yield, by itself, identifies the better feed.
Keep national refinery averages separate from grade assays
National operating statistics can describe system context, but they cannot substitute for a grade-specific assay or a refinery-specific yield estimate. EIA reports that U.S. refinery crude input quality in July 2026 averaged 1.24% sulfur and 33.71° API; those monthly weighted averages were released September 30, 2026. They describe the U.S. refinery system, not a heavy-crude benchmark or an individual facility: EIA U.S. refinery crude oil input qualities.
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Likewise, EIA’s 2023 U.S. refinery figures are national averages, not outputs from a named heavy grade. In 2023, the reported output per 42-gallon barrel of crude input included 19.57 gallons of finished motor gasoline, 12.47 gallons of distillate fuel oil, 4.41 gallons of kerosene-type jet fuel, 2.06 gallons of petroleum coke, and 0.71 gallons of residual fuel oil. EIA also reports a U.S. refinery processing gain of about 6.3% for 2023, or around 45 gallons of refined product from a 42-gallon crude barrel. This volume expansion reflects the lower density of many refined products; it is not a crude-grade assay yield. See the EIA’s refinery inputs-and-outputs data and explanation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Can sulfur and API alone identify the better heavy crude?
No. Sulfur content and API gravity are useful screening measures, but they do not describe the full boiling distribution or the refinery’s ability to upgrade it. A defensible comparison needs matched cut data and a site-specific view of conversion, desulfurization, blending, and commercial constraints. The reviewed published examples do not establish a current, broad ranking of heavy grades by distillation yield.
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