How Oil and Gas Deposits Are Formed

When a living organism dies, it is generally recycled in one of two ways:

• It is eaten by predators, scavengers or bacteriaTiny living organisms that are made up of a single cell that does not have a distinct nucleus, such as a prokaryotic cell....
• Through exposure to ambient air or oxygen-rich water, it oxidizes. That means that the hydrogenThe simplest and lightest atom, the most abundant element in the universe. , carbon, nitrogen, sulfur and phosphorus contained in the matter combine with oxygen atoms present in the air. The organic matter breaks down into water (H₂O), carbon dioxide (CO₂See Carbon Dioxid), nitrates, sulfates and phosphates that nourish new plants.

The Slow Formation of Source Rock

A tiny proportion of this organic matter — about 0.1% — escapes this fate. Transported by water, it sometimes sinks to the bottom of the sea or large continental lakes. It is partly preserved in these poorly oxygenated environments, well away from tidal currents. It mixes with inorganic matter, such as clay particles and very fine sand, and with dead marine plankton (microscopic organisms). This mixture is transformed into dark, foul-smelling mud by anaerobicDescribes an organism or microorganism that requires an environment without air and... bacteria.

Over time, this mud accumulates and hardens. Mud that contains at least 1 to 2% organic matter may be transformed into source rock, which eventually produces oil and gas deposits. This percentage may seem low, but that is because one or more specific requirements are necessary to enable the process to take place:

• A hot climate that is conducive to the growth of large quantities of plankton.
• A location near the mouth of a major river carrying a lot of plant debris.
• No nearby mountains that could limit the volume of inorganic sediment within the rock. 

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Source Rock Subsidence

The weight of accumulating sediment very slowly pushes the source rock further under the Earth's crust, by a few meters to a few hundred meters every million years or so. This gradual sinking is called subsidenceThe gradual sinking of a sedimentary basin under the weight of sediments accumulated over millions of years... and leads to the formation of sedimentary basins.

As it sinks below ground, the source rock is subjected to increasingly high temperatures, the organic matter that makes up the rock is crushed by the weight of the accumulating sediments, and the pressure increases by 25 bar every 100 meters on average. At one kilometer underground, the temperature is 50°C and pressure is 250 bar.

Under these physical conditions, the nitrogen, sulfur and phosphorus atoms are gradually converted into kerogenThe intermediate state of organic matter undergoing conversion during the sedimentation process..., an intermediate material made up of water, carbon dioxide, carbon and hydrogen, which is then transformed into oil or gas.

How Oil and Gas Forms

At a depth of 2,000 meters, when the temperature reaches 100°C, kerogen starts to release preservation (hydrocarbons)The final phase in petroleum system formation, after a deposit has accumulated...:

• Between 2,000 and 3,800 meters, it turns into oil. This depth interval is known as the oil window.
• When the source rock sinks further, to between 3,800 and 5,000 meters, production of liquid hydrocarbons peaks. The liquids produced become increasingly lighter and gradually turn into methane (ch4)The main component of natural gas deposits and oil deposit gas caps. Methane is produced naturally by landfills... gas, the lightest hydrocarbonOrganic compound consisting of carbon and hydrogen. Hydrocarbons are the principal constituents of crude oil, natural gas and petroleum products.. This depth interval is known as the gas window.
• There are no hydrocarbons below a depth of 8 to 10 kilometers, because they are destroyed by the high temperature.

The proportion of liquids and gas generated in this way depends on the type of source rock. If the organic debris is composed mostly of animal origin, it will produce more oil than gas. If it is composed mainly of plant debris, the source rock will produce mostly gas. 

With an estimated average sedimentation of 50 meters every million years, it takes 60 million years for dead animals to become liquid hydrocarbons. It is hardly surprising, therefore, that oil is classified as a non-renewable energyNon-renewable energy or fossil fuels are forms of primary energy that cannot be replenished after use on a human time scale... source.