diagenesis

thumb|upright=1.35|A form of diagenesis is permineralization, in which buried organisms are replaced by minerals. These [[trilobites (Lloydolithus) were replaced by pyrite during a specific type of permineralization called pyritization.]] thumb|upright=1.35|Permineralization in vertebra from Valgipes|Valgipes bucklandi

Diagenesis () is the process of physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a role as sediments become buried much deeper in the Earth's crust. In the early stages, the transformation of poorly consolidated sediments into sedimentary rock (lithification) is simply accompanied by a reduction in porosity and water expulsion (clay sediments), while their main mineralogical assemblages remain unaltered. As the rock is carried deeper by further deposition above, its organic content is progressively transformed into kerogens and bitumens.