Nanostructural investigations on organic matter of organic matter-rich shales and coals

Recently, great progress was achieved in the development of high resolution imaging techniques, also leading to their increased application in geosciences. Many open questions are related to the chemical composition and structure of organic matter (OM) in fine-grained sediments (shales) and coals, as well as their transformation under thermal stress (thermal maturation). It is suggested that processes in the nanoscale largely influence (1) gas storage in pores generated during thermal maturation and (2) incorporation of inorganic elements in OM (kerogen) as well as minerals formed within the kerogen. However, it is unclear how the different kerogen types behave. Aim of the proposed project is to study the development of nanopores within kerogen, as well as their shape and connectivity. Furthermore, the study aims to correlate these aspects with the origin of the kerogen, the mineralogical composition of the OM-rich rocks and the thermal stress that they have been subjected to. Apart from that, the incorporation of partly toxic trace elements in OM or nanoscale mineral phases will be examined. Addressing the above-mentioned issues will help to evaluate economically highly relevant aspects for a sample set from the Ukrainian Dniepr-Donets Basin, consisting of OM-rich shales as well as coals with high trace element contents at different thermal maturity: i)Storage of natural gas in shales ii) Influence of nanopores on the sealing capacity of shales iii)Methane-storage in coals as economic potential/mining hazard iv) Modes of occurrence of toxic heavy elements in coals For pore space characterization, highly innovative electron-optical techniques, initially applied in material sciences, will be combined. First attempts to use this approach revealed a great potential. However, studies focused on samples with a type II kerogen (marine), which is preferentially transformed to nanoporous solid bitumen during thermal maturation. It is not yet clear, when pore growth reaches its maximum or if nanopores also exist in primary kerogen (e.g. algae). Within the frame of the project, high resolution electron microscopy will be combined with organic- geochemical proxies (molecular composition, isotope ratios) and conventional coal microscopy, to clearly identify primary organic particles (macerals). The selected sample set allows to investigate rocks with different mixtures of kerogen types II and III (higher land plants), which shows less tendency to form nanoporous bitumen. Investigating coals enables a clear discrimination of macerals and their assignment to organic precursors. As changes in the chemical composition of kerogen with increasing thermal stress most likely also influence trace element incorporation, their abundance within different coals will be determined in-situ by using distinct techniques that allow the acquisition of spatially resolved chemical information down to the nanoscale. Integration of all results will enhance understanding of interactions between chemical composition and nanostructure of OM in geological materials.

Structural properties of shales and coals play a major role in understanding storage and retention/expulsion properties with respect to both energy carriers and pollutants carried by the pore fluid. The main aim of this project was to characterize the pore space development in the organic matter fraction of shales and coals using innovative, high-resolution imaging methods in combination with additional, complementary techniques (e.g. adsorption tests, advanced organic-geochemical analyses). A comprehensive understanding of the numerous factors controlling pore space development in organic matter could be established. Not only does the type of organic matter or the mineralogy of shales influence the development of pores, but also the temperature and pressure conditions (pore geometries, orientation) during burial. Basically, it has been shown that secondary pores in the visualizable range of highest-resolution imaging techniques (> 5 nm) occur almost exclusively in solid bitumen - a transformation product of primary organic matter. Primary pores could be visualized in liptinite macerals (e.g. alginite), but not in terrestrial organic matter. Nevertheless, a correlation of the inner surface area with the abundance of a bituminous high-molecular weight fraction, which is mainly contributed by such terrestrial organic compounds, could be shown by adsorption tests. Hence, the combination of both imaging and physisorption allowed for a comprehensive understanding of the micro- and mesopore classes associated with organic matter. Phase-specific, micromechanical properties were determined on organic components (= macerals) in coals and on mineral phases in shales, in order to understand the fracturing behaviour which has to be considered essential to continuum-scale fluid transport. Thereby, it was possible to correlate maceral-specific properties with depositional conditions and to prove the importance of diagenetic reactions due to changing mechanical properties between detrital and authigenic minerals. In the course of this project, the combination of high-resolution imaging and image processing techniques was further improved methodically and the implementation of micromechanical tests on geological samples has proven successful. The combination of advanced geochemical techniques and a comprehensive porosity characterization based on image analysis and sorption measurements is novel and will be expanded and further established in upcoming research projects.