Crude Oil: Definition, API Gravity, and Global Production

What Is Crude Oil?

Crude oil is a naturally occurring liquid petroleum extracted from subsurface geological formations, composed primarily of hydrocarbon chains and varying concentrations of sulfur, nitrogen, oxygen, and trace metals including vanadium and nickel. Refineries process crude oil into transportation fuels, petrochemical feedstocks, lubricants, and asphalt for infrastructure across every major economy on Earth.

How Crude Oil Forms and Is Classified

Crude oil originates from the burial and thermal transformation of organic matter, principally Type II kerogen derived from marine algae and plankton deposited in ancient sea beds. As sedimentary layers accumulate over millions of years, increasing burial depth raises temperature to the so-called "oil window," generally between 60°C and 120°C (140°F and 248°F), where the kerogen cracks into liquid hydrocarbons and associated natural gas. Above approximately 150°C (302°F), cracking continues to produce predominantly dry gas rather than oil. The generated petroleum migrates upward through porous and permeable rock until it reaches a structural or stratigraphic trap, where an impermeable cap rock halts further migration and allows accumulation in the reservoir.

Once produced, crude oil is characterized by its reservoir properties and surface measurements. The American Petroleum Institute gravity scale, expressed in degrees API, quantifies density relative to water using the formula: API gravity = (141.5 / specific gravity at 60°F) - 131.5. Water has an API gravity of 10°, and hydrocarbons lighter than water carry API gravities above 10°. Light crudes above 35° API flow freely at surface conditions, contain abundant low-boiling-point fractions such as naphtha and kerosene, and command premium prices because they yield the highest proportion of transportation fuels per barrel. Heavy crudes below 25° API are viscous, rich in high-molecular-weight compounds, resins, and asphaltenes, and require more intensive refinery processing including thermal cracking or hydrocracking to convert the heavy residue into marketable products.

The second major classification axis is sulfur content. Sweet crudes (below 0.5 wt% sulfur) such as WTI, Brent, and Nigerian Bonny Light require less refinery hydrogen and produce lower sulfur dioxide emissions during combustion, making them the preferred feedstock for simple or hydroskimming refineries. Sour crudes such as Venezuelan Merey (API approximately 16°), Mexican Maya (API approximately 22°), and Canadian Access Western Blend (AWB, API approximately 20°) contain above 1.0 wt% sulfur and require deep-conversion refinery configurations with coking or residual hydrocracking units and significant hydrotreating capacity to meet finished product specifications such as Euro VI diesel sulfur limits of 10 parts per million.

Crude Oil Composition: SARA Analysis and Distillation Fractions

Petroleum chemists describe crude oil composition using SARA analysis, which partitions the crude into four chemical families: saturates (straight-chain and branched alkanes, plus cycloalkanes), aromatics (single-ring benzene derivatives and multi-ring polycyclic aromatics), resins (polar compounds of intermediate molecular weight containing nitrogen, oxygen, and sulfur heteroatoms), and asphaltenes (the heaviest, most polar fraction that precipitates in n-heptane and creates deposition and flow assurance challenges). Light paraffinic crudes such as Libyan Es Sider contain predominantly saturates and light aromatics, while heavy Venezuelan Merey and Athabasca bitumen carry high asphaltene fractions that complicate transport and refinery inlet operations.

Atmospheric distillation separates crude oil by boiling range into distinct product cuts. Straight-run LPG (boiling below approximately 35°C or 95°F) is recovered as field gas and refinery off-gas. Naphtha (35°C to 175°C or 95°F to 347°F) serves as catalytic reformer feedstock for high-octane gasoline blending and as ethylene cracker feedstock for petrochemicals. Kerosene and jet fuel (175°C to 250°C or 347°F to 482°F) powers commercial aviation. Diesel and gas oil (250°C to 370°C or 482°F to 698°F) fuels compression-ignition engines. Atmospheric residue, the fraction boiling above approximately 370°C (698°F), is further processed in a vacuum distillation unit to recover vacuum gas oil (VGO) for fluid catalytic cracking (FCC) or hydrocracking, leaving a vacuum residue that is either thermally cracked in a coker, processed in a deasphalter, or sold as fuel oil or asphalt.

Crude Oil Across International Jurisdictions

Canada. The Alberta Energy Regulator (AER) governs crude oil production under the Oil Sands Conservation Act (OSCA) and the Oil and Gas Conservation Act (OGCA). The AER publishes monthly crude oil production data in Statistical Report ST-3 and annual reserve assessments in ST-98, which tracks Alberta's proved reserves of both conventional crude and oil sands bitumen. Alberta's three oil sands regions, Athabasca, Cold Lake, and Peace River, collectively hold the majority of Canada's non-conventional reserves. Production from oil sands occurs through two main methods: open-pit mining for shallow deposits within approximately 75 metres (246 feet) of the surface, and steam-assisted gravity drainage (SAGD) for deeper pay zones, where pairs of horizontal wells inject steam to reduce bitumen viscosity and allow gravity drainage to the lower producer well. The horizontal drilling component of SAGD pairs typically extends 500 metres to 1,000 metres (1,640 feet to 3,281 feet) within the McMurray Formation. Western Canadian Select (WCS), blended at Hardisty, Alberta, is the primary Canadian heavy crude benchmark and trades at a persistent discount to WTI, with the differential driven by pipeline capacity constraints, quality differences, and transportation economics.

United States. The Bureau of Ocean Energy Management (BOEM) and Bureau of Safety and Environmental Enforcement (BSEE) regulate offshore crude oil production on the US Outer Continental Shelf, primarily in the Gulf of Mexico (GOM) where deepwater fields such as Thunder Horse, Appomattox, and Anchor operate at water depths exceeding 1,500 metres (4,921 feet) and reservoir pressures above 138 MPa (20,000 psi) in the ultra-high-pressure Wilcox and Lower Tertiary formations. The Energy Information Administration (EIA) reports domestic crude production in its Weekly Petroleum Status Report. WTI, the US benchmark crude, is a light sweet crude (approximately 39.6° API, 0.24% sulfur) delivered at Cushing, Oklahoma, the largest crude oil storage hub in North America with a working storage capacity of approximately 76 million barrels (12 million cubic metres). The WTI-Brent spread reflects differential supply, infrastructure, and quality factors between the North American and international markets.

Middle East. Saudi Aramco produces the world's largest single-country crude oil supply and publishes its official selling prices (OSPs) monthly for Arab Light (approximately 33° API), Arab Medium (approximately 31° API), Arab Heavy (approximately 28° API), and Arab Extra Light (approximately 38° API). The Abu Dhabi National Oil Company (ADNOC) produces Murban crude (approximately 40° API, 0.7% sulfur), which in 2021 became the underlying commodity for the Abu Dhabi Crude Oil Index (IFAD) exchange-traded contract. OPEC+ production quotas, jointly administered through the OPEC Secretariat in Vienna, directly constrain production volumes from member states including Saudi Arabia, UAE, Iraq, Kuwait, and others, making OPEC+ decisions the single most influential policy lever in global crude oil pricing.

Norway and the North Sea. Equinor operates the Johan Sverdrup field on the Norwegian Continental Shelf (NCS), which produces a heavy crude grading approximately 26° API at peak production of approximately 755,000 barrels per day (120,000 cubic metres per day). The Norwegian Offshore Directorate (formerly Sodir) regulates NCS production and publishes monthly field-by-field production statistics. Dated Brent, the global benchmark that underpins pricing of approximately 70% of the world's internationally traded crude oil, is a blend of crudes from the Brent, Forties, Oseberg, Ekofisk, and Troll fields (the BFOET basket) loaded from North Sea terminals. The Forties blend, produced onshore at Hound Point terminal on the Firth of Forth, frequently sets the dated Brent price as it is the most sulfurous and often cheapest of the five streams.

Australia. The National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) regulates offshore crude production under the Offshore Petroleum and Greenhouse Gas Storage Act 2006. The Carnarvon Basin off Western Australia and the Bass Strait in southeastern Australia are the primary offshore producing regions. The North West Shelf (NWS) project, operated by Woodside, produces condensate alongside LNG. The Montara field in the Timor Sea, operated by PTTEP, produces light sweet crude at approximately 44° API. Australian domestic crude production has declined significantly from its Bass Strait peak in the 1980s and the country is now a net crude oil importer, relying heavily on imports from the Middle East, particularly for its east coast refineries.