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

Abstract

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 CO2 adsorption experiments. High-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), and 13C nuclear magnetic resonance (13C 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 CO2 indicate that kerogen samples in shale with strong deformation have larger micropore volume and surface area (<2 nm). 13C 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 × 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° range). In the most strongly deformed sample, LZ-2, distribution orientations of aromatic fringes are dispersed and primarily concentrated at 15–105°. 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.

Characterization of the evolution of thermal maturity and pore structure of continental organic-rich shales

“…These bands were also observed in the FTIR spectra of coals (Coates, 2000;Van Niekerk et al, 2008;Okolo et al, 2015;Phan et al, 2021;. The band at 1,030 cm −1 observed in Figure 5 indicates the intensity of C-C stretching vibrations within an aliphatic skeleton (Okolo et al, 2015;Jing et al, 2019;Razavian and Fatemi, 2021;Shang et al, 2021). The 1,280-1,000 cm −1 peak heights of all pyrolyzed samples showed a steady increase with increasing VR, and the narrowing peak width as VR increased indicated that some organic functional groups (such as C-O and O-H) were rapidly pyrolyzed and hence sharply decreased; the remaining functional groups (such as C-C) were slowly pyrolyzed and hence showed less of a decrease.…”

“…The out-of-plane aromatic C-H bending vibrations indicated the presence of substituted aromatic species in aromatic rings; and trans-and cis-CH 2 in long, saturated aromatic CH-CH chains were observed in the 900-700 cm −1 region of the spectra in the entirely pyrolyzed samples (Coates, 2000;Van Niekerk et al, 2008;Malumbazo et al, 2011;Okolo et al, 2015;Jing et al, 2019;Shang et al, 2021;. The peaks in the 900-700 cm −1 region of the spectra became more obvious with increasing VR (Figure 5), indicating an increase in aromatic groups.…”

Adsorption behavior and mechanism of CO2in the Longmaxi shale gas reservoir

“…The source rock reached the generation peak of gaseous hydrocarbon and formed a great amount of expansion micropores in organic matter. 38,39 Xie et al , Li et al , and Wang et al also suggested that micropores are the main contributor to the specific surface area in over mature shale gas reservoirs, even with a ratio higher than 90%. Therefore, the micropore filling theory can better characterize the adsorption behavior of CO 2 in shale.…”

Molecular Structural Characteristics and 3D Model Reconstruction of Organic Matter in Longmaxi Formation Shale

“…23−26 Moreover, HRTEM can also visualize the quantitative structural parameters of aromatic lamellae to support the molecular structure characterization results. 27 In addition to the above individual experimental analyses, some scholars have established an average molecular 2D structural model for kerogen based on kerogen elemental analysis and molecular structure characterization experimental data. 28,29 With the development of computer technology, several novel means of molecular structure simulation have emerged.…”