Molecular changes in the organic geochemistry of the Vaca Muerta Shale Formation as a consequence of natural maturation

Abstract

The Vaca Muerta Formation, located in the Neuquén. Basin (Argentina), is one of the most important shale plays in the world. A detailed structural characterization of the formation and evolution of the organic matter during maturation is a must for the correct evaluation of the petroleum potential. The present work studied the molecular characteristics of the organic matter of samples from several wells within the formation. Speciation of organic carbon, oxygen, nitrogen and sulfur present in the rocks was analyzed by XPS and 13C solid-state NMR spectroscopy and compared to data from more typical geochemical analyses, including pyrolysis and solid bitumen reflectance. Samples present with high TOC and cover a wide range of maturity levels (%BRo between 0.7 and 1.6%). Differences in carbon aromaticity measured by 13C NMR and XPS are discussed. Carbon, nitrogen, and sulfur present in organic isolates were studied and compared to reported data for several other shale formations (Duvernay, Bakken, Woodford and from the Bohai Bay Basin). The results are a novel contribution to the understanding and modeling of organic matter in the Vaca Muerta Formation and can be used to support and improve the classification of kerogens based on their molecular structures.