Effects of montmorillonite charge reduction on the high-temperature/high-pressure pyrolysis of organic matter

Abstract

The diagenesis of clay minerals and thermal evolution of organic matter (OM) are consistent in space and time in most petroliferous basins. Within these processes, the synchronism of smectite illitization with peak hydrocarbon generation is one of the most important phenomena and the illitization process is believed to play an important role in catalytic effects on the conversion of OM to oil and gas. The variation in the layer charge of montmorillonite (Mt) is a key indicator of illitization. However, the effects of layer charge on OM pyrolysis have not been investigated. In this study, a series of homologous reduced-charge Mt (LiMt) samples were prepared by changing the temperature of heat treatment based on the Hofmann–Klemen effect. The 12-aminolauric acid and Mt were mechanically mixed to prepare clay–OM complexes. Pyrolysis experiments were conducted in a confined gold capsule system, and the gas components were evaluated by gas chromatography. The experimental results show charge reducing changed the solid acidity of Mt and further influence the catalytic activity of Mt on the OM pyrolysis. The solid acidity of Mt first increased and then decreased as the layer charge decreased, while the water content decreased. These sites, especially Brønsted (B) acid sites, catalyzed OM pyrolysis via the carbocation mechanism. The content of iso‐alkanes was controlled by the content of water in Mt because excess water molecules hindered the isomerization reactions occurring at the B acid sites. The CO2 content was affected by both the amount of Lewis (L) acid sites and the water content. These fundamental results suggest that changes of the layer charge in Mt which occurs during the processes of smectite illitization may play an important role in hydrocarbon generation. These findings are important for understanding the organic-inorganic interactions between clays and OM during sedimentation and diagenesis.