Introduction: What is oil and where does it come from?

Introducción: ¿Qué es y de dónde proviene el petróleo?

The history of oil begins more than 200 million years ago, when most of the planet Earth was still covered in water. Geological processes and the slow bacterial action on the organic matter accumulated at the bottom of the sea gave rise to this mixture of hydrocarbons on which the development of current Western society has been based.

The nature of oil and its origin

Petroleum is a viscous liquid of green, yellow, brown or black color, and which is made up of different hydrocarbons, that is, by compounds formed by carbon and hydrogen atoms in variable quantities. No two oil deposits have ever been found to have exactly the same composition, since, along with hydrocarbons, there are often other oxygenated, nitrogenous, and other organic compounds with elements such as sulfur, nickel, or vanadium.

Black gold, as oil is metaphorically called, has its origin in the decomposition of the tiny aquatic organisms that lived in the ancient seas of the Earth millions of years ago, when humans had not yet appeared.

At that time, the planet's surface did not have the same characteristics as it does today. Pangea is the name by which the only large terrestrial plate that existed is known, in which all the continents were gathered. When these animal and plant microorganisms died and fell to the bottom of large bodies of water, successive layers of inorganic sediments -sand and clay- were deposited on top, burying them ever deeper. The high pressure of the earth layers, the high temperatures and the action of bacteria in the absence of oxygen -that is, in an anaerobic environment- slowly transformed the organic remains into what we know today as crude oil. The process of decomposition of organic matter and the formation of oil takes between 10 and 100 million years.

A characteristic property of petroleum is the miscibility of all its fractions, which is why it forms a continuous organic phase. On the other hand, hydrocarbons are slightly miscible with water, and since they are lighter, they always form a layer on its surface.

Oil does not form large underground lakes, but fills the pores and holes in rocks of sedimentary origin, like water in aquifers or in a sponge. Its liquid nature means that it has a tendency to migrate vertically or horizontally, taking advantage of the permeability of the rocky layers that it finds in its path. When that happens, the oil advances until it reaches the surface -the light products that make it up evaporate and the rest oxidize, giving rise to asphalt-, or it forms a reservoir when it is trapped in an impermeable layer that it cannot penetrate.

This great mobility makes it often difficult to know where oil has formed, since we can find it in all geological strata. The most frequent correspond to the Cenozoic (65-0.01 million years), followed by the Paleozoic (590-248 million years), the Mesozoic (248-65 million) and the Preconian (more than 500 million years ago). ), although all have originated during the course of a long evolutionary history, in which petrographic, sedimentological, structural, paleontological factors, etc. have acted.

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