Effects of illite-smectite clay minerals on the thermal evolution of aliphatic organic matter-clay complexes: a study with thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR)

Illite-smectite (I-Sm) clay minerals widely existed in clay-rich rocks or sediments. Understanding the effects of I-Sm clay minerals on the thermal evolution of organic matters (OMs) may facilitate interpreting hydrocarbon generation levels in petroleum basins and the global organic carbon cycle. In this study, mixed-layer I-Sm mineral rectorite (Rec), end-members montmorillonite (Mnt), and illite (Ilt) were chosen as the typical clay minerals and aliphatic organic compounds with different functional groups were selected as model OMs. Two kinds of OM-clay complexes, including OM-clay mixtures and OM-clay interlayer composite, were prepared. Pyrolysis experiment of OM-clay complexes were conducted via thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR). TG-FTIR analysis showed that I-Sm clay minerals influenced the main thermal decomposition temperature (T-M) and gas products of pyrolyzed OMs. Both Rec and Mnt decreased the T-M of OMs and promoted decarboxylation, and the effect of Rec on T-M was more obvious. Ilt had no significant effect on the T-M and decarboxylation. Physicochemical property of the minerals showed that the solid acidity of the clay mineral was the key factor for the T-M and types of thermal decomposition reaction (i.e., decarboxylation and C-C cleavage). Further analysis indicated that the functional groups of OMs affected the pyrolytic behaviors of OMs. OMs with cationic groups can be intercalated into the interlayers of clay minerals, which thus exhibited a high thermal stability. Functional groups such as alkyl chain and carboxyl groups had different binding effect with clay minerals, which leading to a difference of T-M. Our results provide new insights into the role of I-Sm clay minerals in hydrocarbon generation in fine-grained sedimentary rocks as well as the geochemical behavior of organic carbon in response to inorganic minerals.

Automated summary

In this study, Illite-smectite (I-Sm) clay minerals were investigated for their effects on the thermal evolution of organic matters (OMs). It was found that the I-Sm clay minerals influenced the main thermal decomposition temperature and gas products of pyrolyzed OMs. The physicochemical properties of the minerals and the functional groups of OMs played key roles in these effects.