Coupled chemical and nanostructural evolution of solid bitumen derived from oils with heterogeneous composition

Solid bitumen is an important organic matter (OM) component in shale systems, and its chemical and resulting nanopore structure exert a strong control on unconventional reservoir properties. Solid bitumen is commonly regarded as a product of thermal evolution of primary kerogen or secondary transformation products such as retained oil. The nanoporous structure of post-oil solid bitumen is strongly influenced by the molecular composition of its organic precursors.

In this study, pyrolysis experiments on heterogeneous precursor oil samples were conducted to systematically investigate the coupled chemical and nanostructural evolution of solid bitumen under proceeding thermal maturation. A combination of Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, optical reflectance measurements, pore structural characterization, and scanning electron microscopy (SEM) was applied.

The results show that the size and arrangement of aromatic structural units and the abundance of functional groups vary for solid bitumen derived from different oil types at comparable thermal maturity level. These nanostructural variations control nanopore development, leading to systematic differences in pore types and pore size distributions among samples. Micropores and small mesopores are closely linked to the growth, stacking, and structural reorganization of aromatic clusters, whereas stress-related processes mainly control larger mesopores and therefore exhibit a weaker coupling with molecular-scale aromatic evolution.

This study suggests that nanopore development in post-oil solid bitumen is not solely governed by thermal maturity but is also strongly influenced by the composition of precursor oils. These findings are important for assessing the fluid storage and transport behavior of fine-grained OM-rich sedimentary rocks.