Pore space characteristics of the Upper Visean ‘Rudov Beds’: insights from broad ion beam scanning electron microscopy and organic geochemical investigations

This study gives valuable insights into the microstructure and pore space characteristics of 17 compositionally variable Visean shale samples from the Ukrainian Dniepr-Donets Basin (the ‘Rudov Beds’). The representative imaging area varies considerably (from 10 000 to >300 000 µm2) as a function of the mineralogy and diagenetic overprinting. The pores hosted in organic matter (OM) are restricted to secondary solid bitumen. Based on high-resolution maps from broad ion beam scanning electron microscopy combined with organic geochemical and bulk mineralogical data, we propose that the amount of OM-hosted porosity responds to the availability of pore space, enabling the accumulation of an early oil phase, which is then progressively transformed to a porous solid bitumen residue. The type of OM porosity (pendular/interface v. spongy) is reflected in the individual pore size distributions: the spongy pores are usually smaller (<50 nm) than the pendular or OM–mineral interface pores. The OM-hosted porosity coincides with differences in the composition of the extract, with high amounts of extractable OM and saturated/aromatic compound ratios indicative of abundant porous solid bitumen. The average circularity and aspect ratio of the mineral matrix pores correlate with the corresponding values for the OM-hosted pores, which show a preferred bedding-parallel orientation, suggesting that compaction influenced both types of pore.