O-PTIR reveals the thermal evolution mechanism of ultra-deep light oil: Based on molecular structure and chemical composition analysis

Abstract

The natural cracking of light oil in deep reservoirs has attracted significant attention in the exploration of ultra-deep unconventional oil and gas. However, the mechanisms of microscopic oil cracking and bitumen conversion remain unclear. This study aims to investigate the molecular composition and chemical functional group distribution of organic matter from gold-tube pyrolysis of light oil, using submicron-resolved Optical Photothermal Infrared Spectroscopy (O-PTIR) for in-situ analysis. The results indicate that in the early stages of thermal evolution, asphaltene clusters in residual oil undergo polymerization reactions, leading to an increase in resin and asphaltene content. Point O-PTIR spectra of light oil and asphaltene exhibit similar features, with minimal aromatic (C=C) absorption and dominant CH₂ and CH₃ bending vibrations. At an EasyR_o of 1.81 %, thermal cracking converts some asphaltenes into solid bitumen, which increases the chemical heterogeneity in O-PTIR maps. As thermal maturity progresses, solid bitumen content rises, molecular composition shifts towards polycyclic aromatic hydrocarbons (PAHs), and the microstructure of solid bitumen changes from powdery to crumbly. At EasyR_o = 2.40 %, the chemical heterogeneity of solid bitumen peaks with vein-like distributions of aliphatic compounds. Further maturation leads to molecular homogeneity, characterized by pronounced C=C absorption and broad CH₂/CH₃ vibrational peaks. Branch ratio and pseudo-van Krevelen analyses further demonstrate that residual organic matter during oil cracking is not homogeneous but varies regionally. Understanding these molecular and chemical heterogeneities at the microscale is crucial for elucidating the migration, fractionation, and conversion processes of ultra-deep light oils.