Work

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

Year: 2025

Type: article

Abstract: The absence of humic vitrinite complicates the determination of thermal maturity in pre-Devonian sediments. Lower Paleozoic shales contain structured zooclasts, solid bitumen, vitrinite-like materials, and their reflectance has been found to be valid maturity proxies. Yet there remains much controversy as to the degree of their reflectance. The current work aims to investigate the transformation degree and morphology evolution for marine-derived macerals, as well as the correlations between their reflectances. 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 dispersed organic matter (DOM) and terrigenous vitrinite. These two original samples were cut into small blocks and placed in the identical vacuum stainless vessel simultaneously. The pyrolysis was carried out at isothermal temperatures of 250~550°C for 48 hours. The morphologic and reflectance changes of DOM in pyrolysis residues were studied and compared with previous pyrolysis results.Based on the combined use of optical reflectance and scanning electron microscope (SEM), petrographic analysis shows that DOM in the shale components are solid bitumen (SB), bituminite, chitinozoans, minor graptolites, vitrinite-like particles (VLP), alginate and liptodetrinite. The reflectance sequence from high to low is zooclasts, VLP and SB. Their reflectance gradually increases as the pyrolysis temperature rises, although the rate of growth is slower than that of co-heating coal vitrinite. Zooclasts yield significantly higher reflectance values than those of VLP and SB at each pyrolysis temperature. Following 350°C of pyrolysis, however, the examined VLP starts to exhibit fine-grained mosaic texture, gas evacuation vacuoles, and contraction cracks in optical properties and morphological characteristics, and the reflectance of VLP and SB are close to each other. At temperatures of 500°C and above, the reflectance value of SB is higher than that of VLP. Laboratory simulated maturation results demonstrate that zooclasts have the same thermal stress as vitrinite, and the reflectance of graptolite and chitinozoan has the same growth rate. However, VLP undergoes two distinct phases in reflectance development, despite gradual morphology changes similar to vitrinite. Based on optical evidence of texture evolution, we argue that VLP in Upper Ordovician shale is not a graptolite fragment. Consequently, VLP serves as an organic matter maturity assessment for Ordovician shales with a reflectance value of less than 2.0%. Future investigations for the VLP thermal evolution should include shales from diverse pre-Devonian ages and stratigraphic sections to enhance our knowledge of differences in chemical composition and aromatization.

Source: Petroleum Science

DOI: 10.1016/j.petsci.2025.03.028