Occurrence of organic matter in argillaceous sediments and rocks and its geological significance: A review

The positive correlation between total organic carbon and specific surface area in sediments and mudstones implies the association between sedimentary organic matter (OM) and clay minerals. However, the traditional classification of OM, including insoluble kerogen and soluble OM (SOM), ignores the role of minerals, significantly limiting the understanding of the role of mineral-OM interactions in OM preservation and evolution. This paper reviews the mineral and OM characteristics and their relationship in argillaceous sediments and rocks and classifies sedimentary OM into three forms of occurrence, namely free SOM (FSOM), mineral-bound OM (MOM), and particulate OM (POM). FSOM, dominated by non-polar compounds, is physically adsorbed on the external surface of minerals or free in pores of different sizes, so it is susceptible to oxidation. MOM includes polar SOM and insoluble amorphous OM. The former is chemically adsorbed on the external surface, sheet edge and intercalated into interlayer space, and the latter is encapsulated in the clay microfabric. MOM is preserved by mineral adsorption, physical encapsulation, and degradation-recondensation, so it is very stable. POM is predominantly insoluble structured OM in the form of discrete particles and is preserved by selective preservation at the expense of chemical reactivity. The three forms of OM occurrence result from the synergistic operation of multiple preservation mechanisms, in which mineral-OM interactions dominate long-term OM preservation. Depending on environmental conditions, the change of mineral-OM interaction leads to the OM release or cracking, thereby promoting carbon cycling. Compared to traditional OM classifications, the new model of OM occurrence incorporates mineral protection mechanisms and provides a unifying principle linking inorganic and organic processes in OM preservation and evolution. OM preservation and evolution is essentially the process of mutual transformation of OM between different forms of occurrence, so this model is very useful for understanding carbon cycling and hydrocarbon generation and should be valued.