Diagenesis

With regards to the Earth Sciences, diagenesis refers to all the chemical, physical, and biological changes undergone by a sediment after its initial deposition, and during and after its lithification, exclusive of surface alteration (weathering). Diagenesis is the lowest grade of metamorphism.

The study of diagenesis in rocks is used to understand the tectonic history they have undergone, the nature and type of fluids that have circulated through them and, from an economic standpoint, allows the assessment of the likelihood of finding various economic minerals and hydrocarbons.

The role of diagenesis in hydrocarbon generation

When animal or plant matter is buried during sedimentation, the constituent organic molecules (lipids, proteins, carbohydrates and lignin-humic[?] compounds) break down due to the increase in temperature and pressure. This transformation occurs in the first few hundred metres of burial and results in the creation of two primary products: kerogens and bitumens.

It is generally accepted that hydrocarbons are formed by the thermal alteration of these kerogens (the biogenic theory). In this way, given certain conditions (which are largely temperature-dependent) kerogens will break down to form hydrocarbons though a chemical process known as cracking, or catagenesis.