Deformation and kerogen macromolecular structure transformations of the Wufeng–Longmaxi shale under the control of the detachment layer

Abstract

Tectonism caused strong deformation of the organic-rich shale in the detachment layer of the Wufeng–Longmaxi Formation. In this study, the deformation of the samples collected from the detachment layers were observed using an optical microscope and field emission scanning electron microscopy. Fourier transform infrared spectroscopy (FTIR), 13C nuclear magnetic resonance, X-ray diffraction, and high-resolution transmission electron microscopy were employed to characterize the macromolecular structure of extracted kerogen samples. The research shows that tectonically deformed shale (TDS) is commonly developed in the detachment layer of the Wufeng–Longmaxi Formation, dominated by brittle–ductile and ductile deformation. In the brittle–ductile deformation stage, the aliphatic carbon (fal) decreases, manifested as the decrease of –CH or –CH2 (falH), from 6.77% to 3.12%. The Al/–OX (defined by FTIR A3000–2800/A1800–1520) gradually decreases, indicating that the brittle–ductile deformation has a stronger effect on the aliphatic structure. The aromaticity I and aromatic carbon (fa) show an increasing trend due to aromatization. In the ductile deformation stage, the fal and aliphatic carbon bonded to oxygen increase first and then decrease, falH decreases gradually, and the aliphatic side chain becomes shorter. The Al/–OX shows an increasing trend, and carbonyl decreases from 7.82% to 5.12%, indicating that the effect of ductile deformation on oxygen-containing functional groups is stronger. The fa shows an overall increasing trend, and the aromatic ring carbon and protonated aromatic carbon increase more significantly due to aromatic condensation. The change trend of the interlayer spacing is opposite in these two stages. However, both brittle–ductile and ductile deformation can improve the size and orientation of aromatic fringes to varying degrees, and the latter is more significant. The rigid mineral intrusion and organic matter (OM) chemical structure evolution are the two main reasons for the low development of OM pores in TDS.