Construction of multidimensional structure model of the Mesoproterozoic Xiamaling shale kerogen, northern North China

Abstract

In the southeastern Xuanlong sag of Northern China, the organic-rich black shale of the Mesoproterozoic Xiamaling Formation holds significant potential as a source rock. Through advanced analytical techniques including elemental analysis, 13C NMR, XPS, FTIR, and XRD, the elemental composition and chemical structure were thoroughly characterized, and the molecular structure of the Xiamaling shale kerogen was established. The 13C NMR analysis revealed the carbon composition of kerogen, indicating 75.19 % aliphatic carbon, 22.06 % aromatic carbon, and 2.75 % carbonyl carbon, with quantitative support from FTIR and XRD spectra. The dominant methylene carbons exhibited a low average methylene chain length (Cn) of 3.87, suggesting that they are primarily short-branched or alicyclic compounds rather than long straight chains. This conclusion was verified by the absence of a sharp peak at a wavenumber of 719.95 cm−1 in the FTIR spectrum. Oxygen-containing functional groups observed in 13C NMR spectra were also identified in FTIR spectra at a wavenumber range of 3500–1000 cm−1. XPS analysis indicated that nitrogen-containing groups were amino, quaternary, pyridine, and primary pyrrole groups, and sulfur existed as sulfoxide, aromatic sulfur, aliphatic sulfur, and sulfone. Finally, a relatively rational 2D molecular structure with the chemical formula of C278H346N6O26S2 was established by comparing the predicted and experimental 13C NMR spectra. Using MD simulation, five optimized 2D molecules were combined to construct a robust 3D kerogen model with a physical density of 1.067 g/cm3. Comparatively, the aliphatic structure of the Mesoproterozoic Xiamaling kerogen model consists of some monocyclic alkanes and polycyclic saturated structures, resembling Paleozoic kerogen models but differing from Mesozoic counterparts. The constructed kerogen model addresses the gap in the Precambrian kerogen modeling and sets the stage for in-depth exploration of pyrolysis mechanisms in the future.