What Is Crude Oil? The Storage & Infrastructure Guide
Crude oil is a naturally occurring liquid hydrocarbon mixture extracted from underground geological formations and used as the primary feedstock for global fuel production, petrochemicals, and energy infrastructure. For terminal operators and storage professionals, understanding what crude oil is — specifically its density, sulphur content, and viscosity — directly determines tank specifications, liner requirements, heating systems, and product compatibility within a terminal.
The term petroleum (from the Latin petra oleum — rock oil) covers both unrefined crude and the refined products derived from it. Global consumption runs at approximately 97 million barrels per day, making crude oil the world’s most heavily traded physical commodity and the primary driver of tank terminal capacity planning.
What Is Crude Oil Made Of
Crude oil is a complex mixture of hydrocarbons — compounds containing only carbon and hydrogen — with trace quantities of nitrogen, oxygen, sulphur, and metals including vanadium and nickel. The carbon content runs consistently at 83–85% by weight, with hydrogen at 10–14%.
Four hydrocarbon classes make up the bulk of crude composition. Alkanes (also called paraffins) account for roughly 30% of the average crude barrel and are the primary feedstock for gasoline, diesel, and jet fuel production. Naphthenes (cycloalkanes) make up around 49% and are key inputs for petrochemical production. Aromatics — including benzene-ring compounds — contribute approximately 15%, while asphaltic compounds form the remainder, producing bitumen and heavy fuel oil after refining.
The molecular composition varies significantly between producing regions, which is why crude assay data — a detailed laboratory breakdown of a crude stream’s properties — is essential for terminal operators blending or handling multiple grades in shared tankage.
How Crude Oil Forms
Crude oil is a fossil fuel derived from the remains of marine microorganisms — primarily zooplankton and algae — that accumulated on ancient seabeds over millions of years. Buried under sediment and subjected to intense heat and pressure, this organic matter underwent catagenesis, converting first into a waxy intermediate material (kerogen) and then into liquid hydrocarbons.
This process occurs within a specific temperature range known as the oil window — roughly 60°C to 200°C at depth. Below the minimum temperature, oil remains trapped as kerogen. Above the maximum, liquid hydrocarbons convert to natural gas through thermal cracking. The geothermal gradient — typically 10–30°C per kilometre of depth — determines how quickly formations reach these conditions.
Approximately 70% of the world’s oil deposits formed during the Mesozoic era. The Middle East holds around 80% of the world’s readily accessible conventional reserves, with Saudi Arabia, the UAE, Iraq, Qatar, and Kuwait collectively accounting for 62.5% of known global reserves.
Crude Oil Classification: API Gravity and Sulphur Content
When asking what is crude oil in practical terms, classification by grade is the most operationally relevant answer. The petroleum industry uses two primary criteria that directly affect refinery yield, transportation requirements, and storage specifications.
API gravity measures crude density using an industry-standard scale developed by the American Petroleum Institute. Light crude (API above 31.1°) flows freely, yields higher proportions of gasoline on distillation, and commands a price premium. Heavy crude (API below 22.3°) is viscous, requires heating to flow through pipelines and into tanks, and produces more residual fuel oil per barrel. Medium crude sits between these two classifications.
Sulphur content determines the sweet/sour distinction. Sweet crude contains less than 0.5% sulphur by weight and requires minimal processing to meet fuel specifications. Sour crude — common in Middle Eastern and Latin American production — carries higher sulphur levels, demands more complex refining, and typically trades at a discount. For terminal operators handling multiple crude streams, sulphur content is a critical product compatibility variable: sour crude requires corrosion-resistant tank linings and more rigorous vapour management.
Crude Oil Benchmarks: WTI, Brent, and Dubai
Global crude trading references a small number of benchmark crudes — specific grades with well-defined quality characteristics used as pricing anchors for physical and futures contracts.
West Texas Intermediate (WTI) is a sweet, light crude (API ~39.6°, sulphur ~0.24%) produced in the Permian Basin and priced at Cushing, Oklahoma — the largest crude oil storage and pipeline hub in North America. Cushing’s storage capacity, estimated at around 90 million barrels, makes it a globally watched inventory indicator and a benchmark for what is crude oil worth in the North American market.
Brent Blend is a light, sweet crude (API ~38.3°) drawn from multiple North Sea fields and landed at Sullom Voe terminal in Shetland. Brent underpins ICE Futures contracts in London and serves as the reference price for approximately two-thirds of global crude trade.
Dubai-Oman is a medium sour benchmark used for Middle Eastern crude flowing to Asia-Pacific refineries — the reference grade for Fujairah and other regional terminal hubs pricing eastward cargoes.
See current pricing data on our crude oil price page, Brent crude price page, and WTI crude price page.
How Crude Oil Is Refined
Raw crude cannot be used as a fuel directly. Refineries separate it into commercially useful fractions through fractional distillation — heating crude oil and exploiting the different boiling points of hydrocarbon chains to separate them in a distillation column.
Lighter fractions condense as gases (LPG, propane, butane), gasoline (C5–C8 alkanes), and naphtha. Mid-range fractions produce kerosene and jet fuel (C9–C16). Heavier fractions condense as diesel and heating oil. The heaviest residuals — fuel oil, lubricants, bitumen, and paraffin wax — require further cracking to yield higher-value products.
By volume, approximately 84% of hydrocarbons in a crude barrel are converted to combustion fuels. The remaining 16% feeds petrochemical manufacturing — plastics, fertilisers, pharmaceuticals, synthetic rubber, and solvents. Price and volume data for refined crude derivatives is tracked by the U.S. Energy Information Administration.
Crude oil price and inventory data is published monthly by the U.S. Energy Information Administration (EIA).
Crude Oil Storage: What It Means for Terminal Infrastructure
What is crude oil in infrastructure terms? It is the primary product class driving aboveground storage tank (AST) design, sizing, and specification at export terminals, import terminals, refinery tank farms, and pipeline injection points worldwide.
Light sweet crudes — including WTI and Brent grades — can be stored in standard fixed-roof or floating-roof tanks at ambient temperature with no heating requirement. Heavy sour crudes, including Venezuelan extra-heavy grades and Canadian bitumen blends, require heated tanks with steam coil or hot-oil heating systems to maintain product fluidity, raising both operating costs and energy consumption per barrel stored.
High sulphur content in sour crude streams drives additional infrastructure costs: tanks require corrosion-resistant internal linings, enhanced vapour recovery units to manage hydrogen sulphide (Hâ‚‚S) emissions, and more rigorous inspection regimes under API 653 and equivalent standards.
For a broader look at crude storage infrastructure, see our oil storage terminals guide and our ranking of the top 10 crude oil storage companies.
Crude Oil’s Role in the Global Supply Chain
Crude oil moves through a three-stage supply chain divided into upstream, midstream, and downstream sectors. Upstream covers exploration and production. Midstream encompasses transportation and storage — pipelines, tanker shipping, and tank terminals. Downstream covers refining and distribution of finished petroleum products.
Terminal operators occupy a critical midstream position, providing the storage and logistics infrastructure that bridges production and refining. A crude terminal may serve as an export loading hub adjacent to an oilfield, a receiving terminal at a refinery gate, or an independent commercial storage facility providing ullage to traders managing physical crude positions.
The tank terminals global guide covers the infrastructure layer in detail, including capacity by region and the key independent operators active in crude storage globally.
What is crude oil?
Crude oil is a naturally occurring liquid hydrocarbon mixture extracted from underground rock formations. It consists primarily of carbon and hydrogen compounds formed over millions of years from the remains of ancient marine organisms. It is the primary feedstock for gasoline, diesel, jet fuel, and a wide range of petrochemicals.
What is the difference between light and heavy crude oil?
The distinction is based on API gravity — a density measure developed by the American Petroleum Institute. Light crude (API above 31.1°) is less dense, flows freely, and yields more gasoline per barrel during refining. Heavy crude (API below 22.3°) is viscous, often requires heated storage and pipelines, and produces more residual fuel oil. Light crude typically trades at a price premium over heavy grades.
What is the difference between sweet and sour crude oil?
Sweet crude contains less than 0.5% sulphur by weight and is easier and cheaper to refine to meet fuel sulphur specifications. Sour crude has a higher sulphur content, requires more complex refinery processing, and typically trades at a discount. For terminal operators, sour crude handling requires corrosion-resistant tank linings and enhanced vapour management systems.
How does crude oil quality affect tank terminal storage requirements?
Crude grade directly determines storage infrastructure specifications. Light sweet crudes can be stored at ambient temperature in standard floating-roof tanks. Heavy sour crudes require heated tankage with steam or hot-oil coil systems to maintain product fluidity, plus enhanced vapour recovery units and corrosion-resistant linings to manage hydrogen sulphide emissions. Terminals handling multiple crude grades must also manage product compatibility carefully to prevent contamination between streams.
What are the main crude oil benchmarks?
The three primary international benchmarks are West Texas Intermediate (WTI), priced at Cushing, Oklahoma; Brent Blend, drawn from North Sea fields and priced via ICE Futures in London; and Dubai-Oman, used as the Middle Eastern sour crude reference for Asia-Pacific flows. WTI and Brent are both light sweet grades; Dubai-Oman is a medium sour crude. Most physical crude cargoes worldwide are priced as a differential to one of these three benchmarks.