Simplified criteria and main stages for the generation process:

Criteria:

• Sediments with a large content of carbon and hydrogen of organic material from plants and small organisms such as plankton.
• Little or no oxygen such that the sediments do not oxidize (brake down as they would if oxygen were present) but are conserved and buried by new layers of sediments.
• Increased pressure and temperature due to new layers of sediments.

Main stages:

1. The generation of gas (natural gas) starts soon.
2. As the temperature increases kerogen is generated. Kerogen is a wax like materiel composed of complex hydrocarbon chains.
3. As the sediment layers becomes deeper and the temperature exceeds 75° C the hydrocarbon chains of kerogen will start to break into lighter hydrocarbons as oil and gas.
4. Exceeding 150° C the oil will start to break into even lighter hydrocarbons as natural gas. At temperatures above 230° C the gas transforms into rock graphite.

The generation of fossil oil and gas is a continuous process taking millions of years from sedimetaion to the development of hydrocarbons.

Due to the time span from sedimentation to hydrocarbon generation, fossil oil and gas normally is regarded as a non-renewable resource.

Geology
Determining what the temperature has been is key when evaluating the possibility of finding oil and gas in the sub surface.

Conditions required for forming hydrocarbons (oil and gas):

• There has to be a source rock that contains a large amount of organic material that when heated generates oil and gas. Primarily shale are found to be source rocks
• Reservoir rock is the porous rock similar to a sponge that can contain the oil and gas migrating from the source rock. Sand stone, but also limestone and chalk are typical reservoir rocks.
• To avoid the oil and gas from escaping there has to be a cap or seal rock, this would typically be shale or any other non permeable rock.