In geology, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering). These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. Diagenesis is the lowest grade of metamorphism.

After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization.

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 meters 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.