Assessing mechanical properties of organic matter in shales: Results from coupled nanoindentation/SEM-EDX and micromechanical modeling

Abstract

Mechanical properties of organic matter in fine grained source rocks has been the subject of extensive research. Most studies on kerogen have been performed on kerogen samples isolated by dissolving the rock matrix. However, recent studies have shown significant microstructural alteration of kerogen samples after demineralization leading to alteration of their mechanical properties. In this work, in order to study the role of organic matter on mechanical properties of the rock, both organic-rich and organicfree carbonate-rich shale rocks are investigated. A recently developed nano-chemomechanical characterization method is used to directly correlate mechanical properties of both organic-rich and organic-free samples with their chemical composition and mineralogy at micrometer length scales using coupled nanoindentation and Energy Dispersive X-ray Spectroscopy (EDX) technique. The experimental results are used in multiscale structure thought models in which the role of organic matter is modeled as a matrix surrounding inclusions in the composite. Application of these models to the interpretation of nanoindentation results allows us to obtain consistent mechanical properties for the main constituents of these samples, i.e. organic matter and calcite, which are in excellent agreement with the results in the literature. Moreover, adequate agreement is observed between model predictions and experimental measurements of elastic properties of carbonate-rich shale rocks at macroscale.