Chemical Structure Transformations in Kerogen from Longmaxi Shales in Response to Tectonic Stress as Investigated by HRTEM, FTIR, and 13C NMR

Kerogen molecular structure controls the micropore (<2 nm diameter) evolution in shale and exerts a significant impact on shale gas adsorption, desorption, and diffusion. However, the influence of tectonic deformation on kerogen structure and organic micropores needs intensive study. Our study mainly focuses on the transformations in the extracted kerogen structure and organic micropores of Longmaxi shale during tectonic deformation. Vitrinite reflectance measurement (VRM) and Raman spectroscopy (RS) were utilized to determine the maturity of organic matter. Organic pore network attributes of kerogen samples were estimated using CO 2 adsorption experiments. High-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), and C-13 nuclear magnetic resonance (C-13 NMR) were utilized to interpret the kerogen molecular structures of these samples. VRM and RS results show that Longmaxi shale has reached the wet gas window. Adsorption analyses of CO, indicate that kerogen samples in shale with strong deformation have larger micropore volume and surface area (<2 nm). C-13 NMR and FTIR results reveal that the tectonic stress may promote the shedding of aliphatic carbon. HRTEM images exhibit that aromatic rings with a size of 3 X 3 account for the largest proportion in all kerogen samples, which indicates that the kerogen samples have reached a high thermal evolution level, consistent with the Raman experiment. For weakly deformed shale samples, 66.5 and 88.8% of the total number of aromatic rings in LZ-1 and LZ-4 kerogen belong to the major direction (within a 45 degrees range). In the most strongly deformed sample, LZ-2, distribution orientations of aromatic fringes are dispersed and primarily concentrated at 15-105 degrees. Such disorientation of nanometer-sized polyaromatic layers in the strongly deformed sample creates a nanopore network in organic matter (OM) that increases the micropore volume and surface area. These findings from integrated approaches can provide a new perspective of adsorbed gas enrichment in the complex tectonic areas.

Automated summary

This study focuses on the impact of tectonic deformation on kerogen structure and organic micropores in Longmaxi shale, revealing that tectonic stress promotes shedding of aliphatic carbon and increases micropore volume and surface area in shale samples with strong deformation. Integrated approaches provide a new perspective on gas enrichment in complex tectonic areas.