Diagenetic modification of kerogens

Abstract

Kerogen is the part of sedimentary organic matter that is not petroleum. It can be isolated by using acid destruction of minerals, except in recent sediments where this procedure results in important hydrolysis reactions. Whether the scale of observation is microscopic or molecular, kerogens are heterogeneous mixtures whose compositions vary according to the sedimentation medium. However, they can be globally characterized by their H : C and O : C ratios. Diagenetic modification is a thermodynamically oriented process that consists of passing from complicated chemical structures inherited from living organisms or created during sedimentation to simple stable molecules. However, it is kinetically controlled by the thermal history of the sediment during burial. Three main steps are distinguished. (i) Diagenesis, where O :C decreases and CO ₂ and H ₂ O are released. (ii) Catagenesis, where H : C decreases and petroleum and then gas are produced. (iii) Metagenesis, which consists of structural reorganization of the carbonaceous residue with little change in the elemental analysis. Modification can be followed by observing characteristic changes in physical and chemical properties.