The molecular model of Marcellus shale kerogen: Experimental characterization and structure reconstruction

Abstract

A realistic three-dimensional (3D) kerogen molecular model adds significant values in petrographical evaluation in shale, especially it is essential to conduct molecular simulation for gas storage and transport behaviors in shale. In this study, the 3D kerogen molecular structure of Marcellus shale was reconstructed based on molecular simulation, and eight experimental techniques, including 13C nuclear magnetic resonance (13C NMR) spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), elemental analysis, helium porosimetry, low-pressure CO2 adsorption, and radial distribution function (RDF) obtained from synchrotron X-ray. The XPS data provides elemental compositions and the contents of functional groups of carbon, oxygen, nitrogen and sulfur on the kerogen surface. FTIR and 13C NMR spectroscopy provide the contents of aliphatic chains and aromatic rings. The two-dimensional (2D) kerogen molecular structure was initially constructed, and the connection of kerogen fragments was verified by comparing the experimental and calculated 13C NMR spectra. Then, fourteen 2D kerogen molecules were used to reconstruct the 3D kerogen molecular structure. Experimental helium density and micropore volume were used to verify the density and micropore structure of the final 3D model. Experimental RDF data were also used to analyze the spatial distribution of the kerogen atoms. The reconstructed 3D kerogen molecular structure of Marcellus shale contains 5936 atoms (C2856H2492N56O532), and the unit cell dimension is 3.755 × 3.755 × 3.755 nm. This study provides a systematic approach to establish a realistic 3D kerogen molecular model by leveraging the advantages of various techniques. The established Marcellus kerogen model lays the foundation for future fluid transport and storage studies.