Evaluation of Spatial Alignment of Kerogen in Shale Using High-Resolution Transmission Electron Microscopy, Raman Spectroscopy, and Fourier Transform Infrared

Abstract

As the three-dimensional (3D) molecular structure of kerogen plays important roles in further understanding of shale gas storage and transport, accurate characterization methods for 3D kerogen structures are attracting increasing attention. Spatial alignment is important information for 3D kerogen modeling, but was usually ignored in previous studies. In this work, seven kerogen samples with different maturities were isolated from organic-rich shale using a chemical method and highresolution transmission electron microscopy (HRTEM) was employed to quantitatively characterize the spatial alignment of these seven kerogen samples. Raman spectroscopy was used to investigate the overall structural disorder of the kerogen molecules and Fourier transform infrared (FT-IR) was conducted to study the chemical structure of these kerogen samples. The results show that immature, mature, and overmature kerogen samples all show an obvious alignment on the scale of 20 nm × 20 nm. In the immature kerogen sample Yl-1 with equivalent vitrinite reflectance (VR eqv ) = 0.4%, 60% of total aromatic fringes align in the major direction (with a 60°range), while 87% of the total aromatic fringes align in the major direction for the overmature kerogen sample Lmx-3 (VR eqv = 3.1%). However, unlike local alignment in the scale of 20 nm × 20 nm, the aromatic fringes in different regions may have different directions in larger scale. Meanwhile, based on FT-IR data, aliphatic carbons and oxygen containing functional groups contribute to a large proportion in immature, mature, and overmature kerogen samples. Thus, immature, mature and overmature kerogen samples all show overall disorder according to Raman data. In addition, the size of aromatic rings is also quantitatively characterized based on HRTEM images. In immature kerogen samples, the proportion of aromatic rings smaller than 3 × 3 is larger than 70%. In mature and overmature kerogen samples, 3 × 3 sized aromatic rings always occupy the largest proportion. This study provides the quantitative information on spatial alignment and the size of aromatic rings for kerogen samples, which contribute to an improved understanding of the 3D structure of kerogen.

Molecular Structure of Kerogen in the Longmaxi Shale: Insights from Solid State NMR, FT‐IR, XRD and HRTEM

“…Compared with these results, the proportions of aromatic rings in the major direction in this study (Fig. 11) agreed well with the values of the overmature kerogen samples reported by Liu et al (2018a).…”

“…However, the aromatic ring content smaller than 3×3 decreased from 81 to 18% in the same VR eqv interval. The layer size distribution was in good agreement with the results from the HRTEM analysis of overmature kerogen samples analysed by Liu et al (2018a). In addition to calculating the size distribution of the aromatic layers, the HRTEM images could provide information about the spatial alignment of the kerogen structures.…”

“…The dominant size of the aromatic rings, obtained from HRTEM images was consistent with that calculated through XRD. Recent research results indicate that the sizes of aromatic rings are continuously enlarged with increasing maturity (Mathews and Sharma, 2012;Liu et al, 2018a). Liu et al (2018a) found that the proportions of aromatic rings larger than 3×3 increased from 1% (VR eqv =0.60%) to 50% (VR eqv =3.2%).…”

“…Recent research results indicate that the sizes of aromatic rings are continuously enlarged with increasing maturity (Mathews and Sharma, 2012;Liu et al, 2018a). Liu et al (2018a) found that the proportions of aromatic rings larger than 3×3 increased from 1% (VR eqv =0.60%) to 50% (VR eqv =3.2%). However, the aromatic ring content smaller than 3×3 decreased from 81 to 18% in the same VR eqv interval.…”

“…The nanoscale organic pores, which contribute to the largest part of the total pore volume, are the main storage space for shale gas (Wang Min et al, 2016). Meanwhile, it is well documented that some nanoscale organic pores (<2 nm) are formed within the three-dimensional (3D) molecular structure, and their pore structure parameters are controlled by the molecular composition and spatial structure of the organic matter (Bousige et al, 2016;Huang et al, 2017;Vasileiadis et al, 2017;Liu et al, 2018a). Therefore, the evaluation of gas storage and transport is strengthened by a robust understanding of the molecular structure of the organic matter.…”