Oil formation: from organic matter to a deposit

Oil forms neither quickly nor in an empty cavity. It results from a succession of geological processes that transform a small share of organic matter buried in sediments, then move hydrocarbons towards rocks where they can accumulate. A deposit requires a sufficiently rich source rock, appropriate maturation, migration pathways, a reservoir rock and a sealed trap. This combination explains why hydrocarbons occur in some basins and not in others.

Key facts

• Origin: organic matter from living organisms, often aquatic, preserved in oxygen-poor sediments.
• Stages: burial, transformation into kerogen, thermal maturation, migration and accumulation.
• Timescale: millions to tens of millions of years, depending on the basin's history.
• Deposit: an accumulation forms only if the reservoir rock and trap are in place at the right time.

Where does oil come from?

Oil is a natural mixture of hydrocarbons, molecules mainly composed of carbon and hydrogen. Its formation begins in environments where microscopic organisms, plants or other organic matter accumulate. A small proportion escapes complete decomposition and settles at the bottom of a lake, delta or sea.

Preservation is favoured when sediments are oxygen-poor. Organic matter then mixes with mineral particles, such as clays, to form layers of mud. These deposits accumulate over time. New layers cover and bury them, progressively changing their properties.

From organic matter to kerogen

During early burial, bacteria living in low-oxygen environments transform the organic matter. The most readily degradable compounds disappear; a solid, carbon-rich substance called kerogen remains dispersed throughout the sediments.

As burial continues, mineral sediments compact and may form a fine-grained, often impermeable rock: the source rock. It contains kerogen that, under suitable conditions, will provide the raw material for oil and gas formation. The quantity and type of organic matter vary with the depositional environment, influencing a basin's petroleum potential.

Maturation: the role of heat and time

Temperature and pressure increase with depth. Over millions of years, this change causes kerogen to mature thermally. Organic molecules gradually break down into liquid and gaseous hydrocarbons. Oil therefore results from a slow transformation, sometimes called geological ‘cooking’, which depends on the basin's thermal history rather than a single universal depth.

Within a favourable maturation range, the source rock mainly generates oil. If temperatures continue rising and burial continues, liquid molecules can break down and generate more gas, particularly methane. Conversely, a rock that has not been heated sufficiently may retain untransformed kerogen. The limits vary with the type of organic matter, the duration of exposure to heat and the geological setting.

A source rock can therefore be immature, sufficiently mature to generate oil, or overmature and predominantly gas-generating. Geologists assess these conditions using rock analyses, drilling data and burial models.

Migration: how hydrocarbons leave the source rock

Source rock is generally compact and has low permeability. As oil and gas form within it, some may be expelled by pressure and enter more permeable layers. This first stage is called primary migration. Hydrocarbons can then move through pores, fractures or along geological structures: this is secondary migration.

Being lighter than the water in rocks, oil and gas tend to rise. Some reach the surface and may escape or be altered. Others accumulate in reservoir rock: sandstone or limestone whose connected pores can hold fluids. The reservoir rock is not necessarily the rock in which the oil formed; geological layers may separate the two.

The trap: a requirement for accumulation

To form a conventional deposit, hydrocarbons must be stopped during migration. An impermeable seal rock, such as clay or salt, can prevent them from rising further. If the geometry of the layers closes the escape route, the reservoir rock and seal form a trap.

Structural traps result from deformation of the layers: an arch-shaped fold, a fault or a structure associated with rising salt can create a retention zone. Stratigraphic traps instead form when rock properties or continuity change laterally. In both cases, the elements must be present at the right time. A trap formed after hydrocarbons have passed through may remain empty; a fractured seal may allow the accumulation to leak.

The presence of oil in rock does not automatically make it a commercially recoverable reserve. Volume, fluid quality, reservoir porosity and permeability, pressure, depth, costs and infrastructure determine whether extraction is technically and economically possible.

Conventional oil and source-rock oil

In a conventional deposit, hydrocarbons have left the source rock and become concentrated in a reservoir rock beneath a seal. In so-called unconventional accumulations, some oil or gas remains dispersed in the source rock or in very low-permeability rock. Extraction may then require specific techniques, such as horizontal drilling and hydraulic fracturing, which change the project's economic and environmental conditions.

Oil shale also differs from oil that has already been generated: it contains kerogen that has not reached sufficient maturity. Converting it requires additional industrial processing. These terms therefore describe different geological situations and should not be confused.

Petroleum systems specific to each basin

A deposit forms through several connected elements: source rock, maturation, migration, reservoir, seal and trap. Geologists use the concept of a petroleum system to study their compatibility and timing. In the Niger Delta and, more broadly, the Gulf of Guinea, the history of sediments, deltas and coastal basins has created favourable settings for varied petroleum systems. Each discovery nevertheless depends on local geology and specific subsurface data.

Seismic surveys map structures; drilling and samples provide information about rocks and fluids. However, a promising model does not replace confirmation through drilling.

Frequently asked questions

Does oil form from dinosaurs?

Most oil comes from microscopic organisms and organic matter accumulated in aquatic environments and then buried in sediments. Large dinosaurs are not its main source.

Why is gas found with oil?

Maturation of organic matter produces liquid and gaseous hydrocarbons. More advanced maturation can favour gas generation, and methane may also be present in the same reservoirs as oil.

Does every kerogen-rich source rock contain a deposit?

No. Appropriate maturation, migration, reservoir rock, an effective trap and an effective seal are also required. If an element is missing or forms at the wrong time, hydrocarbons may fail to accumulate or may escape.