Thermal Evolution of Organic Matter in Low-Maturity Shale: A Multimodal Nanoscale Investigation

Abstract

Systematic characterization of the nanoscale geomechanical and geochemical evolution of organic matter at elevated temperatures is critical for assessing the technical feasibility of in situ thermal methods in the development of low-maturity shale oil and gas. This study investigates the pyrolysis process of low-maturity, organic-rich shale from Yanchang Formation, focusing on thermal evolution in morphology, geochemistry, and geomechanical properties. The comprehensive analysis is performed through a series of sophisticated techniques, including thermogravimetric analysis coupled with thermogravimetric-Fourier transform infrared-gas chromatography/mass spectrometry (TG-FTIR-GC/MS), backscattered electron of the scanning electron microscopy (BSE-SEM), micro-Raman spectroscopy, atomic force microscopy-infrared spectroscopy (AFM-IR), and AFM PeakForce quantitative nanomechanics (PFQNM). Pyrolysis products evolve across three stages: water vapor dominates below 200 °C; hydrocarbons, CO₂, and sulfur compounds release in the range of 200–650 °C; and carbonate decomposition drives CO₂ emissions above 650 °C. Heating induces significant morphological alterations, including surface shrinkage, pore collapse, and thermal cracks (notably above 400 °C). Geochemical analyses show that the differences in structure among solid bitumen, vitrinite, and inertinite decrease as the temperature increases, alongside detaching aliphatic side chains and oxygenated functional groups and increasing the degree of aromatization. Geomechanical properties, measured via AFM-PFQNM, demonstrate an initial decrease in Young’s modulus (25–250 °C) due to pore water loss, followed by modulus increase (250–600 °C) attributed to the aromaticity enhancement and matrix shrinkage. These insights advance the understanding of in situ thermal conversion processes, offering practical guidelines for enhancing hydrocarbon recovery from low-maturity shale reservoirs. The multidisciplinary approaches resolve the interplay among thermal, chemical, and mechanical dynamics in shale pyrolysis.