Evolution of pore structure in the Upper Cretaceous Second White Speckled Shale during thermal maturation: Insights from artificial and naturally matured samples

Abstract

The evolution of pore structures in marine shale during thermal maturation was investigated using naturally matured samples (Ro 0.46–1.26 %) from the Upper Cretaceous Second White Speckled Shale and artificially matured samples in a semi-open system (200–450 °C). Analytical techniques, including Rock-Eval pyrolysis, FIB-FESEM, and nitrogen adsorption, revealed key trends in pore volume (PV) and specific surface area (SSA). Immature samples exhibited high PV and SSA, which decreased during early oil generation (Ro ∼0.98 % or 350 °C) due to compaction and oil infill. PV and SSA rose significantly between Ro 0.98 % and 1.26 % (350–400 °C), driven by mesopore development, and remained elevated at higher temperatures. Artificially matured samples showed higher PV and SSA compared to naturally evolved samples, reflecting the absence of compaction and cementation processes in laboratory conditions. Naturally evolved samples demonstrated greater heterogeneity due to expulsion dynamics and geological factors, developing complex pore networks during hydrocarbon generation. Organic matter (OM) composition, dominated by Type II kerogen with terrestrial inputs, played a critical role in pore evolution. Amorphous organic matter (AOM) and solid bitumen were the primary OM components, with liptinite macerals and terrigenous vitrinite and inertinite also contributing. Clay minerals dominated the rock matrix, while pyrite framboids contributed dissolution-induced secondary porosity. SEM imaging identified five pore types, with OM-hosted pores forming predominantly in bitumen rather than kerogen. Mesopores (2–50 nm) were the dominant pore type, while micropores (< 2 nm) were negligible. Fluorescence microscopy and pyrolysis experiments confirmed increasing maturity with depth, accompanied by significant intraparticle pore formation in migrated bitumen at higher temperatures. Artificial maturation studies highlight faster hydrocarbon generation and pore development compared to natural systems but fail to replicate long-term burial effects. PV correlated positively with expelled oil in artificial systems, while bitumen content negatively correlated with PV and SSA in both systems. Advanced imaging techniques and integrated natural and experimental models are essential to further understanding pore evolution, connectivity, and hydrocarbon generation mechanisms in shale reservoirs. This study emphasizes the interplay between OM composition, mineralogy, and thermal processes in shaping shale porosity during natural and artificial maturation.