XPS direct analysis on shale rocks: Correlation with kerogen type and maturity

Abstract

A method to study kerogen type and maturity of organic matter in source rocks using XPS was developed. Samples from three Argentinian shale rocks containing different kerogen types were analyzed by XPS directly, without kerogen isolation. Analysis focused on characterizing the chemical structure of kerogen, in particular of carbon species. Two new parameters were proposed: (%Cox/OCxps) that measures the oxidized organic carbon content, and ArP related to aromaticity ratio. A relationship was found between (%Cox/OCxps) and kerogen origin, allowing changes in kerogen type (from Type II to Type III) to be recognized. The aromaticity was estimated from Csp2 XPS peak and was compared to traditional maturity parameters such as vitrinite reflectance or pyrolysis Rock-Eval Tmax. A correlation between the methods was observed, suggesting the direct XPS analysis of shale rock as an estimation of kerogen maturity. Both maturity and kerogen type can be determined in the same measurement, performed directly on the sample, without organic matter isolation.