Molecular Structure of Kerogen in the Longmaxi Shale: Insights from Solid State NMR,FT‐IR,XRD and HRTEM |
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| Authors: | WANG Xiaoqi ZHU Yanming LIU Yu and LI Wu |
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| Institution: | 1 Key Laboratory of Coalbed Methane Resources and Reservoir Formation on Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China
2 School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China,1 Key Laboratory of Coalbed Methane Resources and Reservoir Formation on Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China
2 School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China,1 Key Laboratory of Coalbed Methane Resources and Reservoir Formation on Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China
2 School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China and 1 Key Laboratory of Coalbed Methane Resources and Reservoir Formation on Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China
2 School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China |
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| Abstract: | Kerogen plays an important role in shale gas adsorption, desorption and diffusion. Therefore, it is necessary to characterize the molecular structure of kerogen. In this study, four kerogen samples were isolated from the organic‐rich shale of the Longmaxi Formation. Raman spectroscopy was used to determine the maturity of these kerogen samples. High‐resolution transmission electron microscopy (HRTEM), 13C nuclear magnetic resonance (13C NMR), X‐ray diffraction (XRD) and Fourier transform infrared (FT‐IR) spectroscopy were conducted to characterize the molecular structure of the shale samples. The results demonstrate that VReqv of these kerogen samples vary from 2.3% to 2.8%, suggesting that all the kerogen samples are in the dry gas window. The macromolecular carbon skeleton of the Longmaxi Formation kerogen is mainly aromatic (fa'=0.56). In addition, the aromatic structural units are mainly composed of naphthalene (23%), anthracene (23%) and phenanthrene (29%). However, the aliphatic structure of the kerogen macromolecules is relatively low (fal*+falH=0.08), which is presumed to be distributed in the form of methyl and short aliphatic chains at the edge of the aromatic units. The oxygen‐containing functional groups in the macromolecules are mainly present in the form of carbonyl groups (fac=0.23) and hydroxyl groups or ether groups (falO=0.13). The crystallite structural parameters of kerogen, including the stacking height (Lc=22.84 Å), average lateral size (La=29.29 Å) and interlayer spacing (d002=3.43 Å), are close to the aromatic structural parameters of anthracite or overmature kerogen. High‐resolution transmission electron microscopy reveals that the aromatic structure is well oriented, and more than 65% of the diffractive aromatic layers are concentrated in the main direction. Due to the continuous deep burial, the longer aliphatic chains and oxygen‐containing functional groups in the kerogen are substantially depleted. However, the ductility and stacking degree of the aromatic structure increases during thermal evolution. This study provides quantitative information on the molecular structure of kerogen samples based on multiple research methods, which may contribute to an improved understanding of the organic pores in black shale. |
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| Keywords: | organic-rich shale molecular structure kerogen HRTEM Sichuan Basin |
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