Nanostructural investigations on organic matter-rich shales and coals

This project is funded by the FWF (P29310) and is conducted in cooperation with the Chair of Physical Metallurgy and Metallic Materials, the Erich Schmid Institute of Materials Science and RWTH Aachen.

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.

(a) Segmentation of total porosity for a wet gas mature Rudov sample from a high-resolution BIB-SEM mosaic. Artificial cracks and gypsum from core alteration can be separated from true porosity by an overlay of EDX and BSE/SEI maps combined with characteristic geometry factors. (b) High resolution BSE image of an oil-window mature, oil-prone sample, hosting nanoporous bitumen. Isolated segmentation of OM-hosted porosity revealed a higher amount of porous OM in the oil-prone sample at lower maturity. This coincides with differences in extract composition and composition of hydrocarbons generated during pyrolysis.