All crude oil is
not the same. In fact, each source is chemically variable and has different
flow characteristics. RBN Energy’s Liz Dicken explains that the main variations
distinguished are density and sulfur content. These are shown in the table
below. Quality is typically expressed by these two factors. Light, medium, and
heavy crude are distinguished by density, which is expressed in the API gravity
number. Sulfur content below 0.5% is sweet crude, and sulfur content above 0.5%
is sour crude.
She explains below why density matters.
“A crude’s density matters because it affects what
refiners can make from each barrel. Lighter crudes produce higher yields of
valuable refined products such as gasoline, diesel and jet fuel with relatively
less processing. Heavier crudes have larger hydrocarbon molecules that need
more complex refining units, such as cokers and hydrocrackers, to produce high
yields of gasoline, jet fuel and diesel. That extra processing adds cost.”
Regarding sweet vs. sour, she
notes that the standards have tightened in recent years.
“…the spec for Domestic Sweet, the standard grade
deliverable against the NYMEX WTI futures contract, is less than or equal to
0.42% sulfur.”
Other factors in determining crude
oil quality are given below:
Distillation
Density and sulfur are not
the whole story, however. Other factors, like paraffin content or waxiness, are
variable. These factors can be determined when the crude oil is distilled.
Refiners use a distillation curve to determine how much oil is vaporized
(recovered as liquid). Waxy crudes can yield more middle distillates than their
API gravity number suggests.
Molecule Types
These can determine how a
crude will behave when refined. Paraffin, isoparaffin, naphthene, and aromatics
content can determine what products the crude is best suited to make.
“The molecule types prevalent in crude oil
(paraffins/isoparaffins, naphthenes and aromatics), play a big role in how that
oil behaves in a refinery and how easy (or difficult) it is to turn into
finished products. For example, in the jet/diesel range, aromatic and
naphthenic crudes produce the material with the best cold-flow properties, but
perform poorly in terms of cetane (a diesel fuel’s ignition quality) and smoke
point (a measure of a jet fuel’s tendency to produce soot or smoke during
combustion).”
Conradson carbon (Concarbon), or Micro Carbon Residue (MCR)
This determines how a crude
will behave in a coker unit and what the residue at the vacuum tower bottom
will be like.
Total Acid Number (TAN)
This is a measure of
naphthenic acids in the crude. Crudes with higher TAN are more corrosive,
especially at high temperatures.
Metals Content
The presence of trace metals
in the crude, such as nickel and vanadium, can affect the catalysts used in
refining, effectively shortening their productive lifespan. The presence of
iron particles can be abrasive and can contribute to plugging, equipment wear,
and corrosion. This happens over time. Iron is also associated with increased
sediment and contaminants in crude. The biggest problem with metals is catalyst
poisoning in fluid catalytic cracking (FCC) units.
Mercaptans
Mercaptans, or thiols, are
sulfur-containing compounds that can present corrosion, safety, and operational
challenges. They are highly reactive and can damage metal surfaces, "leading
to sulfide stress cracking, hydrogen-induced cracking and accelerated corrosion
in pipelines, storage tanks and refining equipment. They also pose serious
health and safety risks due to their toxicity.”
West Texas Intermediate (WTI)
Next, she explains the
development of the West Texas Intermediate (WTI) grade of crude oil. This is
the oil from the shale and tight rock plays of the Permian Basin, which is a
major source of U.S. oil. WTI is a light, sweet crude, around 42° API, with sulfur
levels near or below 0.2%. Before the advent of Permian Basin unconventional
production, the oil quality from the region was about 39° API and 0.35% sulfur.
Thus, WTI is a little heavier and significantly sweeter.
The U.S. began exporting
crude oil in 2015 and now exports quite a lot of it. She notes that beginning
in June 2023, WTI Midland began being incorporated into Brent’s international
designation. Brent is a North Sea grade. However, North Sea oil production has
been declining in recent years, although new exploration is hoping to stave off
that decline. She explains:
“The purpose of adding WTI was largely to add liquidity
and physical volume to the Brent complex as North Sea production declined, and
came with explicit quality standards. The move put a brighter spotlight on WTI,
and Midland has set the Dated Brent benchmark more than half the time since
late 2023, putting greater emphasis on potential quality issues with WTI.”
“Elevated iron, nickel, vanadium, sulfur, basic sediment
and water (BS&W), or other contaminants can affect refinery operations,
catalyst performance and crude-unit reliability — all of which affect the value
of the crude relative to what the buyer expected. That makes terminal
segregation, blending controls, pipeline-origin requirements, testing and
documentation critically important.”
The required specs for WTI
Midland are given below:
“Midland WTI deliverable into the Brent complex, calls
for crude oil with 0.2% max sulfur and 40°-44° API, along with limits of 10
mg/kg iron, 2 mg/kg nickel and 2 mg/kg vanadium.”
She concludes with a
question:
“A question that has become a hot topic recently still
remains: Have quality concerns become more prevalent naturally, or are we only
seeing more chinks in the armor due to added scrutiny caused by WTI’s inclusion
in the Brent benchmark? As changing market dynamics, evolving blending
practices and stricter quality standards reshape crude flows, the answer is
more nuanced than it appears.”
References:
A
Matter of Trust – Variations in Crude Oil Quality Make On-Spec Delivery
Critical for Global Refiners. Liz Dicken. RBN Energy. July 30, 2026. A
Matter of Trust – Variations in Crude Oil Quality Make On-Spec Delivery
Critical for Global Refiners | RBN Energy
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