Advanced micromechanical and pore structural characterization of organic matter-rich rocks: Toward a better understanding of dual porosity and permeability

Sanja Vranjes-Wessely

Research output: ThesisDoctoral Thesis

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In recent years, nano-scale material characterization in many geoscience disciplines and particularly in petroleum-related fields is becoming increasingly important. The necessity to understand transport properties of fine-grained, organic matter-rich rocks (including coals) for the purpose of unconventional hydrocarbon production (e.g., shale oil/gas, coal bed methane) represents a main driving force for method development in the field of high-resolution imaging and micromechanical characterization. A profound characterization of the nano-scale matrix pore system and the mechanical behavior of individual constituents from such low-permeability rocks helps to predict producibility of oil and gas. However, analysis at the nanometer scale still represents a major challenge in source rock studies, as many well-established, spatially-resolving characterization techniques have not been fully transferred from traditional material science to petroleum geoscience yet. This thesis aims at filling this gap by providing a profound nano-scale characterization workflow for organic matter-rich rocks. Furthermore, it gives insights on the interdependency between nano-mechanical and micro-structural properties, as well as on the geological controlling factors of these. To do so, nanoindentation, a broad spectrum of high-resolution imaging techniques, and gas adsorption methods were applied on i) a well-investigated set of Carboniferous coal samples from the Ukrainian Donets Basin (0.62 � 1.47 %Rr vitrinite reflectance) and ii) Cretaceous shales from the Chinese Songliao Basin (1.33 � 2.23 %Rr). The nanoindentation study on Donets coals revealed different impact factors on material parameters (hardness H and reduced elastic modulus Er) for each maceral group. Mechanical parameters of liptinite seem to be mainly impacted by transformational processes, linked to thermal maturation and depositional environment, while H and Er of inertinite are mainly controlled by the prevailing temperature during paleo-wildfires. In comparison, vitrinite macerals revealed a more complex evolution of H and Er with thermal maturity. This might be due to the pore-structural evolution of vitrinite with increasing maturity, as observed during the subsequent high-resolution transmission electron microscopy (HRTEM) study. It was demonstrated that Er of vitrinite is controlled by pore sizes, as smaller pores (? 5 nm) likely facilitate a more efficient load-sharing between individual pores. HRTEM imaging revealed further nano-structural heterogeneities in vitrinite, such as domains of higher ordering observed already at low-coal rank (0.69 and 0.81 %Rr). Low-pressure CO2 and N2 adsorption, Raman spectroscopy and high-pressure CH4 sorption experiments collectively indicated severe structural changes at around 1.10 %Rr as a result of thermally induced processes marking the transition from peak oil to gas window. In comparison to coal macerals, the micromechanical characterization of dispersed organic matter within shale rocks is further complicated due to mineral matrix effects and the small sizes of individual particles. The high-speed nanoindentation mapping and correlative imaging study on organic matter particles in fine-grained rocks of the Cretaceous Shahezi Formation (Songliao Basin), facilitated by femtosecond laser grids, revealed complex influencing factors on phase-specific micromechanical parameters. Various impacting effects were identified by correlative imaging (optical microscopy, scanning electron microscopy, and helium ion microscopy). Furthermore, the micromechanical raw data was processed by an unsupervised machine learning algorithm (k-means clustering). Future characterization and micromechanical modelling studies will benefit from the established, fast and reliable micromechanical and pore structural assessment workflows, including the presented sampl
Translated title of the contributionFortgeschrittene mikromechanische und porenstrukturelle Charakterisierung organisch-reicher Gesteine
Original languageEnglish
Awarding Institution
  • Montanuniversität
  • Sachsenhofer, Reinhard, Supervisor (internal)
  • Misch, David, Assessor A (internal)
  • Kiener, Daniel, Co-Supervisor (internal)
  • Littke, Ralf, Assessor A (external), External person
Publication statusPublished - 2021

Bibliographical note

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  • Organic Matter
  • Coal
  • Shale rocks
  • Nanoindentation
  • High Resolution Imaging
  • Pore Structure
  • TEM
  • HIM
  • HP CH4 sorption
  • CO2 and N2 Physisorption
  • Raman spectroscopy
  • Unsupervised Machine Learning

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