Solid bitumen formation and resulting differential porosity development in the Mesoproterozoic Xiamaling shale during artificial thermal maturation

Abstract

Solid bitumen constitutes a predominant organic component in thermally mature source rocks, yet its role in shale reservoirs remains poorly constrained. This study integrated hydrocarbon expulsion simulation via sequential solvent extraction with isothermal pyrolysis experiments conducted on an early mature bituminite-rich shale sample from the Mesoproterozoic Xiamaling Formation in North China. Organic petrography and gas adsorption were applied to characterize the solid bitumen and pore structure in pyrolysis residues, respectively. The primary objective was to advance the understanding of the solid bitumen formation and its influence on the porosity development of shales within the dry gas window. Quantitative petrographic evidence confirms that the composition of retained extractable organic matter exerts fundamental control over the yield and properties of solid bitumen. Notably, the polar fraction (e.g., asphaltene component) exhibits superior solid bitumen-generating capacity, indicating that expulsion-driven compositional fractionation of residual hydrocarbons serves as the primary determinant of intraformational solid bitumen heterogeneity. Furthermore, empirical data demonstrate an inverse relationship between solid bitumen content and porosity development of shales across the investigated maturity spectrum (vitrinite reflectance 1.02 %–3.62 %), manifested through depressed specific surface area and diminished pore volume metrics. Our findings suggest that provenance-controlled variations in solid bitumen occurrence and subsequent pore architecture may induce complex reservoir quality modifications, potentially accounting for divergent porosity trends observed in natural shale systems. While this investigation provides fundamental insights into organo-petrophysical relationships, the general applicability of these conclusions warrants further validation through subsequent research.