Natural and laboratory-induced maturation of kerogen from the Vaca Muerta Formation: A comparison study

Abstract

Artificial maturation of kerogen is a widely used technique to assess the hydrocarbon potential of shale rocks and to observe thermal transformations of organic matter in the laboratory. However, the degree of reproducibility of natural geological transformation at the molecular level is still not fully established. In the present work, a set of kerogens isolated from Vaca Muerta Formation core samples at varying levels of thermal maturity were studied. Another set of samples was obtained by anhydrous pyrolysis of kerogen in a closed system. Molecular structures measured by solid-state techniques (13C NMR, XPS and FTIR) were compared in natural and artificially matured samples at equivalent levels of thermal maturation. We observed that heating in an anhydrous closed system accurately reproduces most of the molecular structural changes observed during natural thermal maturation, with exceptions related to the branching degree and oxygen containing groups. The evolution of some parameters, such as N and S content, are highly variable in the natural samples because of differences in their deposition environments. In these cases, artificial thermal maturation changes are smaller and follow more clearly defined trends because variables other than thermal changes are not present. This is the first work comparing natural and artificial thermal maturation in the Vaca Muerta Formation and add valuable data regarding the limitation of artificial maturation that can be extrapolated to other formations.