The Geological Rock Cycle and Petroleum Formation

The Rock Cycle

The rock cycle is the transformation of the set of rocks and minerals where minerals are transformed into others, giving rise to another rock in a cyclic manner. We begin the cycle with magma from the crust that consolidates either in the interior or exterior. In the interior, magmatic rocks originate. When these emerge to the surface, they are attacked by external geological agents, resulting in erosion, transport, and sedimentation, causing sediment that, through a process of diagenesis, results in sedimentary rocks. These can follow two ways:

• They can be attacked by external agents again to obtain more sediment through weathering, returning them to the start of the sedimentary process.
• If they are introduced into the terrestrial interior, they are subjected to high pressures and temperatures and become metamorphic rocks through metamorphism.

On the other hand, magmatic rocks convert through metamorphism into metamorphic rocks, which in turn can follow two ways:

• If they reach the surface, they suffer weathering to become sediments and, through diagenesis, turn into sedimentary rocks.
• If they increase in depth, they suffer a process of total fusion (anatexis) and a new magma originates, by which the cycle is activated again.

Since rocks do not always pass through all stages, several shortcuts exist, determining what we can call minor lithologic cycles.

Metamorphic Rocks

They come from the transformation of others due to high pressures and temperatures. The typical progression is: Clay → Slate → Schist → Mica Schist → Gneiss.

• Slate: It is a rock of black or gray color with a visible lamellar structure. The clay minerals are transformed into white mica.
• Schist: It is like slate but has a glossy shine. It contains more mica than slate.
• Mica Schist: All clay minerals are transformed into mica.
• Gneiss: Its composition is similar to granite. It is distinguished by its arrangement in bands.

Source of Oil

Oil is a mixture of hydrocarbons and some impurities, such as sodium chloride and sulfur. Conditions for formation:

• Shallow marine areas.
• It originates from plant and animal remains, particularly fats.
• Performance of anaerobic bacteria.
• Presence of clay because it advances the action of bacteria.
• Sedimentation of new materials over the remains of plants and animals so that it becomes liquid when it is deep enough.

Because oil is liquid, it rises through cracks or fractures. Two things can occur:

• If it rises to the surface, it becomes oil-impregnated rocks and tar is formed.
• If it climbs and encounters a porous rock that is restricted by an impermeable layer, the oil can be exploited by wells.