Where does oil come from?

According to historians, oil has been known since ancient times, especially in Mesopotamia, Egypt and China. However, the history of oil in the modern era began in 1859. Indeed, in that year, the first oil extraction by drilling was carried out by the American Edwin L. Drake, who was inspired by the techniques of drilling salt wells that were already widely in vogue. This drilling at a depth of 20 meters launches the implementation of innovative technologies for oil extraction around the world. But where does oil come from and what is the process of its formation?

Kerogen

Oil, like natural gas, comes from the decomposition of plants and micro-organisms living in aquatic environments (oceans, deltas, lakes, etc.) millions of years ago. These are plants, animals and plankton which first form a biomass, part of which is not destroyed by bacteria, but is deposited at the bottom of these environments. Preserved by the lack of oxygen in the environment, this material mixes with mineral materials to create sedimentation sludge. Over time, this sludge accumulates in successive layers. With heat and pressure, the layers expand and become more and more compact. This process results in the formation of kerogen, a solid compound disseminated in the form of threads within the sediments and composed of carbon and hydrogen.

From kerogen to oil

Over the years, the sedimentary layers sink deeper and deeper into the Earth’s crust. Beyond 1000 meters below the ocean floor, the sludge residues solidify into a rock called bedrock that traps kerogen. Buried in turn at between 2500 and 5000 meters, the bedrock undergoes thermal cracking, technically called pyrolysis, under the action of the high temperatures that prevail in this environment. This gives oil which is accompanied by gas. It is a process that lasts tens of millions of years. At more than 5000 meters, the oil in turn cracks and becomes gas.

Oil shale

If the bedrock is not sufficiently buried, the kerogen does not undergo pyrolysis.  The fossil fuel thus blocked in the pre-oil stage is then called shale oil. Industrially, it is possible to pyrolysis at 500°C of this oil shale to make oil. If the oil formation process comes to an end, the crude oil initially contained in the source rock migrates. This is said to be primary when water, oil and gas from kerogen are expelled and migrate to a future reservoir rock. Migration is said to be secondary when the mixture, slowly escaping through the permeable sedimentary layers adjacent to the source rock, reaches the first few meters of the soil where it is degraded into bitumen by the action of bacteria. The oil produced is then said to be heavy or extra-heavy. It is accompanied by oil sands. This secondary migration may also be stopped by a cover rock, which induces the formation of a reservoir rock under the cover rock.

Criteria for classifying oil

Apart from geographical origin, which is an important classification criterion, there are many other criteria used to categorize oil. Thus, the density, expressed in APIs, makes it possible to classify crude oil from light to extra-heavy. The lighter an oil is, the lower its density and the higher its API index. This is the case for Brent, the benchmark crude oil in the North Sea, whose API is above 30°API. On the other hand, with a score of 10°API, Venezuela’s oil is qualified as extra-heavy. The viscosity of the oil is also measured (the more viscous the oil, the heavier it is). Light oil has the appearance of diesel: this is the case of the Saharan fields. The medium oil deposits are found in the Middle East. As for heavy or extra-heavy oil, it is found in South America. This oil hardly flows at room temperature. Finally, there are bitumen deposits, the main reserves of which are in Canada. Finally, we will mention a final variant of measurement, namely the sulfur content, which makes it possible to distinguish between sweet oil (with a low sulfur content) and sulfurized oil.