Maceral-specific evolution of molecular structure and porosity from the early oil to dry gas window in black shales

The chemical composition and structural evolution of organic matter (OM) controls pore development in shale reservoirs, but their intrinsic relationship during thermal maturation remains insufficiently understood. This study integrates light microscopy, scanning electron microscopy (SEM), and Raman spectroscopy to investigate microscale variations in petrological features, molecular structures, and pore characteristics across diverse maceral types in the Upper Devonian Duvernay shales, spanning a thermal maturity range of 0.5 %–3.0 % reflectance (R_o) in the West Canada Sedimentary Basin. Results show that alginite and solid bitumen undergo more pronounced compositional transformation with thermal maturation than inertinite and vitrinite, as reflected in broad ranges of Raman spectral parameters. While inertinite consistently displays the highest aromaticity, vitrinite shows a similar aromaticity with pore-filling solid bitumen after R_o >1.0 %. The divergent evolutionary pathways among diverse OM macerals are attributed to the differences in biological origin and hydrocarbon generation kinetics. OM-hosted pores mainly develop in solid bitumen after the late oil window, coincident with a sharp increase in both OM aromaticity and oil expulsion. Moreover, pore-filling solid bitumen shows a higher apparent transformation ratio (15 %–25 %) and larger mean pore size (40–45 nm) than alginite-derived solid bitumen, likely due to chromatographic fractionation. Throughout the gas window, pore-filling solid bitumen accounts for >90 % of the total OM-hosted porosity. Those findings advance our understanding of pore generation and evolution mechanisms across maceral types, and provide a chemical framework for predicting shale reservoir quality over a range of thermal maturities.