Evolution characteristics and mechanisms of molecular structures of shale Kerogen during structural deformation

Revealing the impact of tectonic stresses on shale kerogen molecules sheds light on the evolution mechanisms of shale organic matter. In this study, we first extracted shale kerogen samples from the undeformed and tectonically deformed shale (TDS) and then investigated how shale kerogen molecular structures respond to structural deformation by combining Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The result showed that structural deformation reduced the complexity and branching degree of molecular structures by promoting the breaking of aliphatic side chains and increased the aromaticity and structural order of kerogen molecules by enhancing condensation of aromatic rings. Additionally, shale kerogen displayed a relative enrichment of carbon by removing oxygen-, nitrogen-, and sulfur-containing functional groups. The removal of oxygen-containing functional groups was the most significant, showing a trend of “C-O bonds within aliphatic functional groups > C-O bonds within aromatic functional groups > C=O bonds within functional groups”. In general, the evolution of kerogen molecular structures caused by brittle and ductile deformation followed largely consistent patterns. However, the influence of ductile deformation was much more significant and pronounced than that of brittle deformation, and weak brittle deformation had little impact on kerogen molecular structures. • Molecular structures of kerogen extracted from deformed shale were characterized. • Spectral techniques were employed to study kerogen molecular structure evolutions. • Kerogen molecular structure evolutions induced by shale deformation were revealed.