Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How can organic matter be trapped and transformed into an oil reserve? (D)

Short answer

Organic matter accumulates on the bottom of water bodies and gets compacted under increasing pressure and temperature to form a source rock containing kerogen. As the rock gets buried deeper, thermal maturation breaks kerogen into hydrocarbon molecules. These hydrocarbons migrate upwards through porous rocks until they encounter a cap rock. The trapped hydrocarbons accumulate in reservoir rocks, forming an oil reserve within suitable geological structures. The process takes millions of years and depends on specific geological conditions.

Step-by-step solution

Organic matter accumulation

Begin by explaining that organic matter, mainly composed of dead plants and animals, accumulates on the bottom of water bodies such as oceans, lakes, and rivers. This accumulation process takes place in areas with low oxygen conditions and slow sedimentation rates, which help preserve the organic matter from decomposition by microorganisms. Eutrophic environments with high nutrient concentrations tend to produce more organic matter, leading to a higher potential for oil reserve formation.

Formation of a source rock

Over time, the layers of organic matter, along with sediments such as clay particles, accumulate and get compacted under the increasing weight of the overlaying layers. As the layers continue to build up, pressure and temperature increase, transforming the organic matter and sediments into a source rock, primarily composed of shale. This rock contains the initial organic material, which is known as kerogen.

Generation of hydrocarbons

As the source rock continues to get buried deeper under the ground by the addition of more layers, it experiences even higher pressure and temperature. Once the temperature reaches around 60-120 degrees Celsius, a process called thermal maturation takes place, in which kerogen breaks down into smaller hydrocarbon molecules that constitute crude oil and natural gas. The time required for this process depends on the geological conditions of the region and may take millions of years.

Migration of hydrocarbons

Once hydrocarbons are generated, they start migrating upwards through porous rocks, due to their lower density compared to the surrounding water and rock materials. During this migration, hydrocarbons may travel long distances through permeable rocks, such as sandstones, until they encounter an impermeable layer called a cap rock, which blocks their further upward movement.

Formation of an oil reserve

The trapped hydrocarbons accumulate in porous and permeable rock layers known as reservoir rocks, forming an oil or gas reserve. An essential condition for the formation of an oil reserve is the presence of a suitable geological structure which can trap the hydrocarbons, like an anticline or a fault trap. Over time, due to geological movements, the oil reserve may move closer to the Earth's surface, allowing for easier extraction of hydrocarbons by drilling wells. In conclusion, the transformation of organic matter into an oil reserve involves several steps, including the accumulation of organic matter, the formation of a source rock, generation of hydrocarbons through thermal maturation, migration of hydrocarbons, and the trapping of hydrocarbons in suitable geological structures leading to the formation of an oil reserve. These processes occur over millions of years and require specific geological conditions to take place.

Key concepts

Organic Matter

Organic matter is the starting point in the oil formation process. It consists mainly of the remains of dead plants and animals, which collect at the bottom of oceans, lakes, and rivers.
This process happens in places where there is not much oxygen, because less oxygen helps to keep the organic matter from being broken down by microorganisms.
In environments rich in nutrients, more organic matter tends to form, setting the scene for potential oil reserves in the future. These nutrient-rich areas are known as eutrophic zones, which can create a good starting point for what becomes oil.

Source Rock

As layers of organic matter pile up, they mix with sediments like clay particles. Over time, these accumulations get compressed under the weight of all the layers on top of them.
This pressure and heat help transform the matter into a source rock. Shale is a typical example of a source rock, which contains the initial organic material known as kerogen.
Kerogen is an essential ingredient in the future creation of oil because it holds all the organic material within the rock.

Hydrocarbon Generation

The transformation of kerogen into hydrocarbons is influenced by pressure, temperature, and time. As the source rock gets buried deeper, it experiences higher temperatures and pressures.
Once the temperature hits between 60 and 120 degrees Celsius, thermal maturation occurs. This process breaks the kerogen down into smaller hydrocarbon molecules like crude oil and natural gas.
Hydrocarbon generation can take millions of years, and varies based on the particular geological scenarios of the region where the source rock is located.

Oil Migration

After hydrocarbons are formed, they begin to move. Migration starts because hydrocarbons, which are less dense than surrounding water and sediments, naturally move upwards.
They travel through rocks with gaps (porous rocks), such as sandstone. During this movement, the hydrocarbons can cover long distances until they meet a cap rock, an impermeable barrier that stops them from moving further.
Cap rocks are crucial because they prevent hydrocarbons from escaping, leading them towards potential traps where they can accumulate.

Reservoir Rock

The final destination for migrating hydrocarbons is a reservoir rock. This type of rock is permeable, allowing hydrocarbons to collect and form an oil or gas reserve.
Anticlines and fault traps are common geological structures that effectively trap hydrocarbons in reservoir rocks. These formations create pockets where oil can gather, ready for extraction.
Over time, tectonic activities might bring reservoir rocks closer to the Earth's surface, making it simpler to access oil reserves through drilling. The entire process from organic matter to reservoir rock takes millions of years and specific geological conditions.