Organic petrographic investigation for artificially matured marine shale: Insights from anhydrous pyrolysis of Upper Ordovician shale from the Baltic Basin, Lithuania

The absence of humic vitrinite complicates the determination of thermal maturity in pre-Devonian sediments. Lower Paleozoic shales contain structured zooclasts, solid bitumen, and vitrinite-like materials, and their reflectances have been found to be valid maturity proxies. Yet there remains much controversy as to the degree of evolution of reflectances. The current work aims to investigate the reflectance of dispersed organic matter (DOM), as well as their transformation degree and organic pore development. To this end, this study evaluated an Upper Ordovician zooclasts-bearing shale and a Middle Jurassic coal sample via anhydrous-pyrolysis to compare differences in the maturation pathways between marine-derived DOM and terrigenous vitrinite. These two original samples were cut into small blocks and placed in an identical vacuum stainless vessel simultaneously. The pyrolysis was carried out at isothermal temperatures ranging from 250 to 550 °C for 48 h. The morphologic and reflectance changes of DOM in pyrolysis residues were studied and compared with previous pyrolysis results.

Adopting the combined use of optical reflectance and scanning electron microscope (SEM) techniques, the study shows that DOM in the shale components consists of solid bitumen (SB), bituminite, chitinozoans, a few graptolites, vitrinite-like particles (VLP), alginate and liptodetrinite. The reflectance sequence, from high to low, is zooclasts, VLP and SB. Their reflectances gradually increases as the pyrolysis temperature rises, although the rate of growth is slower than that of co-heating coal vitrinite. Notably, zooclasts yield significantly higher reflectance values than those of VLP and SB at each pyrolysis temperature. However, the VLP undergoes two distinct phases in reflectance development, despite gradual morphology changes similar to vitrinite. Based on evidence of the optical texture and organic pore evolution, we argue that a fair amount of VLP in studied Upper Ordovician shale does not consist of graptolite-type fragments. Consequently, the VLPR_o serves as a reliable indicator for assessing organic matter maturity in Ordovician shales with a reflectance value of less than 2.0%. This investigation enhances scholarly understandings of marine-derived DOM evolution issues, providing clearer correlations among reflectances of different DOM and reducing uncertainties in thermal maturity determination.