What Is Crude Oil?
A scientific and technical overview of crude oil: its geological origins, chemical composition and the physical properties that matter for exploration, production and refining.
By the editorial teamPetrole.Africa
🎯 Learning Objective
Provide a scientific and technical understanding of crude oil, from its geological origins to its chemical composition, including the essential physical properties that matter for exploration, production and refining.
I. 🔍 Definition of Petroleum
Crude oil is a complex mixture of hydrocarbons, composed mainly of carbon (C) and hydrogen (H), formed from buried organic matter that has been transformed by pressure and temperature over millions of years.
It is a liquid fossil resource extracted from the subsurface, and it may also contain nitrogen, sulfur and oxygen compounds as well as heavy metals.
II. 🧬 Chemical Composition of Petroleum
Petroleum consists mostly of organic compounds:
| Type of compound | Description | Percentage |
|---|---|---|
| Paraffins (alkenes/alkanes) | Straight or branched chains | 30–60% |
| Naphthenes (cycloalkanes) | Saturated rings (e.g., cyclohexane) | 30–40% |
| Aromatics | Hydrocarbons with a benzene ring | 3–30% |
| Asphaltenes & resins | Heavy, polar, highly complex compounds | 1–10% |
| Heteroatoms | S, O, N, metals (Ni, V, Fe) | 0.1–3% |
🔬 Technical focus:
The proportion of these families determines how the oil behaves (viscosity, density, refining yield).
III. 🧪 Essential Physical Properties
1. Density (°API)
- Measured in API degrees (American Petroleum Institute).
- The lighter the crude, the higher its °API.
- Classification:
- > 31° API → light
- 22–31° API → medium
- < 22° API → heavy
- < 10° API → extra-heavy (e.g., bitumen from Venezuela)
2. Viscosity
- Resistance to flow
- Depends on temperature and composition
- Heavy oils, or oils rich in asphaltenes, have high viscosity and require special extraction techniques
3. Boiling point
- Light components (methane, propane) → low boiling point
- Heavy components (asphaltenes) → very high boiling point
- Used in the fractional distillation of petroleum
4. Sulfur content
- Sweet crude: < 0.5% sulfur
- Sour crude: > 0.5%
- Sulfur is a pollutant and requires treatment in refineries
IV. 🌋 Origin and Geological Formation
1. Biological origin
- Organic matter (algae, plankton, bacteria) deposited in marine/anoxic environments
- Burial under sediments → thermal transformation
- Formation of kerogen, followed by hydrocarbon generation
2. Generation windows
| Temperature | Product | Approximate depth |
|---|---|---|
| 50–90°C | Biogas | 1,000 – 2,000 m |
| 90–150°C | Oil | 2,000 – 4,000 m |
| > 150°C | Thermal gas | > 4,000 m |
3. Required geological conditions
- Source rock rich in organic matter (shales, carbonates)
- Reservoir rock that is porous and permeable (sandstones, carbonates)
- Impermeable cap rock (clays, salt)
- Trap structure (anticline, fault, salt dome…)
V. 🧭 Classification of Crude Oils
According to:
- Geographic origin
- Brent (North Sea), WTI (Texas), Dubai, Bonny Light (Nigeria), Doba (Chad), Zafiro (Equatorial Guinea)…
- Physicochemical profile
- Light/sweet → most sought after
- Heavy/sour → require more processing
- Industrial use
- Lubricants, fuels, bitumen, petrochemicals, etc.
VI. 🔎 Examples of African Crudes
| Crude | Country | °API | Sulfur content | Main use |
|---|---|---|---|---|
| Bonny Light | Nigeria | 35–37° | 0.14% | International refining |
| Doba | Chad | 25° | 1.9% | Heavy crude export |
| Zafiro | Equatorial Guinea | 34° | 0.13% | Fairly sweet |
| Cabinda | Angola | 32–34° | 0.4% | Medium/sweet crude |
VII. 🛠️ Practical Relevance of These Properties
- Density & viscosity → Choice of extraction techniques (pumping, EOR)
- Chemical profile → Compatibility with local refining capacity
- Sulfur → Removal required before marketing
- Boiling point → Yield of light versus heavy products
📚 Sidebar: What Is Kerogen?
Kerogen is the solid organic matter found in source rock.
Under the effect of temperature and pressure, it breaks down and generates oil and gas.
There are several types:
- Type I (lacustrine) → oil-rich
- Type II (marine) → oil and gas
- Type III (continental/terrestrial) → dry gas
🧾 Specialized Glossary
- Hydrocarbon: A molecule composed solely of carbon and hydrogen.
- API: Oil density scale, inversely proportional to actual density.
- Kerogen: Precursor of oil; solid organic matter in source rock.
- Bitumen: An extremely heavy, highly viscous form of petroleum.
- Sweet/sour crude: Low or high sulfur content.
🧠 Key Takeaways
- Oil is a complex fluid resulting from the slow transformation of organic matter.
- Its composition determines its market value, its processability and the technology needed to produce it.
- Understanding its properties is essential for any decision in exploration, production or refining.
