How Oil and Gas Reservoirs Develop

The modern world relies on fossil fuels, mostly on oil and gas. We extract these resources from underground reservoirs where they have been 'stored' for millions of years. This section looks at how the oil and gas were made, and at how it gets trapped.


What the Syllabus Says - and the Details

Oil and gas are hydrocarbon mixtures. These hydrocarbons come from the rotting soft tissues of millions upon millions of organisms. Hydrogen and Carbon from these rotting tissues remained together where other elements such as nitrogen and oxygen were mostly lost. The result was a series of compounds made of 'only hydrogen and carbon bonded together' - the definition of a hydrocarbon. 'Gas', as we call it, is actually a mixture of several gases; methane (pictured below), ethane, ethene, butane, butene... 

Methane - the dark atom is carbon, the white ones are hydrogen

The oil is a mixture of heavier hydrocarbons such as octane (shown below) and decane.

Octane - 8 black carbons, 18 white hydrogens

Oil and gas are most frequently made in mudstone deposits where the sediment includes very little oxygen, which is why the carbon and oxygen don't form carbon dioxide and water which is normal when organisms rot. 

The gas and oil then 'migrates' upwards through the rocks because it is lighter (less dense) than the water in the rocks so it floats up on top of it. The oil and gas can only migrate through porous or permeable rock, so when it rises up so far that it reaches an impermeable layer it can't carry on upwards. When this happens the oil and gas will try to migrate sideways. If there are impermeable rocks to the sides as well then it is trapped. This is good as it means it is now trapped underground and we can find it and drill a hole to put a pipe through so we can get it. 

If the oil and gas can migrate through the rock without meeting an impermeable barrier then it will escape from the rocks completely. Oil and gas will only be found trapped where there is a source rock, a porous reservoir rock for the oil and gas to be in (shown blue in the diagram), and an impermeable cap rock (shown yellow) to trap the oil and gas.

A typical scenario for trapping oil and gas.

The accumulation of oil and gas by migration from source beds to reservoir rocks depends upon the contrasting porosity and permeability of reservoir and cap rocks. The reservoir rock has to be porous to allow the oil and gas to collect in it. The cap rock has to be impermeable to stop the oil and gas continuing upwards. 

These structures that catch oil and gas in this was are called traps. Three of the main types of trap for oil and gas are: anticline traps (top left), fault traps (top right) and unconformity traps (bottom left). In each case the oil (shown in pink) and gas (shown in yellow) are held in the porous reservoir rock, such as sandstone, underneath an impermeable cap rock, such as shale.

The fourth type of trap you have to know about is a salt dome trap - shown below...

A salt dome trap

In this situation the oil (grey) is held below an impermeable cap rock (purplish-grey) and can't move sideways because it is against the impermeable barrier of the salt (white). You'll look at how salt domes form in A level Geology and beyond. Salt layers also make excellent cap rocks in anticline and fault traps.

Task - how has the oil got trapped in each of the scenarios shown in the diagram below?