Petroleum evolves from organic matter through heat, pressure, and time. Initially, buried material transforms into kerogen (diagenesis). With deeper burial, kerogen breaks down into oil and gas (catagenesis). In the final stage, extreme heat converts hydrocarbons into methane and graphite (metagenesis). This natural process shapes the world’s energy resources.
Organic matter into petroleum:
Three major phases in the evolution of hydrocarbon from organic matter through burial.
Diagenesis:
As the organic matter is buried, it is converted to a complex insoluble material called kerogen under shallow burial and low temperatures in the zone of diagenesis. No thermal hydrocarbon gas of any significance is generated in this tone, and it is referred to as a thermally immature one. Temperature generally reaches up to 50° C at the bottom of the zone.
Catagenesis:
With further increase of temperature and depth the sediments enter into the zone of catagenesis where oil and gas are generated from kerogen. This zone generally lies within a temperature range of 60° C to 200° C (Hunt +998). Catagenesis is a thermally mature zone of moderate to deep burial, and it includes oil windows and gas windows. This is also referred to as Hydrocarbon Kitchen.
Metagenesis:
With further burial and an increase in temperature, the zone of catagenesis gives way to the zone of metagenesis, where only a small amount of gas is produced as methane generation diminishes and graphitic structures begin to form (Tissot and Wette 1984).
This is a zone of deep burial and temperature generally ranges from over 200 to 250° C. This is referred to as over mature zone. Beyond this zone further below, the higher burial temperature of the metamorphic zone will destroy oil gas and oil to produce only carbonate residual graphite.
Fig: Position of oil and gas window in an evolutionary path of organic matter.
