Oil and gas are made of carbon and hydrogen compounds. They occur naturally in the earth’s crust at shallow to moderate depths (1–12 kilometres). The formation and trapping of oil and gas requires the combination of a number of significant events, involving the presence of a suitable source rock, its burial, and the preservation of hydrocarbons.
Source rock
Oil and gas formed when organic material (land plants and marine plankton) was laid down in peat swamps, estuaries and shallow seas. Sediments buried this material, preserving it from decay.
Burial
The next stage involved burial to just the right depth (3–5 kilometres). The heat (over 130º C) and pressure at this depth slowly cook the material, converting it to oil and gas.
Preservation
As oil and gas are buoyant, they move upwards through gaps in surrounding rock and eventually seep out at the earth’s surface, unless their rise is blocked. Underground geological features, such as domes or different sedimentary rock layers (impermeable rocks on top of porous reservoir rocks), trap and seal in the oil and gas.
Exploration
Oil and gas exploration focuses on finding these trap features (known as prospects). The first step is to examine the surface geology to decide if the right types of rock are present. The next stage is usually to undertake seismic surveys, which bounce sound waves off underground rock layers. This tells geologists what the rock structures below the ground are like – and whether there are places where oil and gas may be trapped.
Once a suitable structure is found, drilling to depths of many kilometres is needed to test whether any oil or gas is present. A good reservoir rock must be both porous (with gaps to store oil and gas) and permeable (the spaces must be connected). This allows oil and gas to move through the reservoir rock and be extracted.
