Molecular Structure Evaluation and Image-Guided Atomistic Representation of Marine Kerogen from Longmaxi Shale

Abstract

Atomistic representations of kerogen, which appropriately capture chemical and physical properties, will aid in unconventional gas extraction and in following maturation transformations. Here, the structure of an overmature kerogen (from Longmaxi shale) was investigated using HRTEM, 13C NMR, XPS, and CO2 sorption evaluations. The fringe property distributions of length, angle, tortuosity, and stacking were quantified from seven HRTEM micrographs. The fringe lengths were estimated and were equivalent in size to benzene (31%), naphthalene (22%), phenanthrene (10%), 2 × 2 rings (22%), and 3 × 3 rings (9%). The stacking was limited with 10% of fringes being within a stack. However, the fringes were well aligned with 44% (range of 33–54%) of the total fringe length within a 45° angle in the major direction. The aromaticity (fa ′) was 82.2%, with aliphatic and carbonyl carbons accounting for 13.9 and 3.9%, respectively. The kerogen micropore volume was 0.06 mL/g. An image-guided construction strategy, Fringe3D, and Packmol were used to generate an atomistic structural representation that was geometry-optimized (C853H601O45N13S4). The structure incorporates the elemental compositions, carbon forms, heteroatom functionality, and aromatic molecule alignment, stacking, and curvature to generate a chemically and physically appropriate macromolecule (the model contains six macromolecules) within a small unit volume (23.5 × 23.5 × 23.5 Å).

Kerogen Differential Swelling during CO2-CH4 Adsorption: Mechanism and Significance

“…Creation process for kerogen macromolecules: (a) structural characterization from HRTEM micrographs, NMR, XPS, and microporous analysis; (b) slice model containing linear, curved, and single PAH molecules and stacks added to a user-defined cube with Packmol; (c) phosphorus atoms (marked as pink) arranged in cubes (6 Å × 6 Å × 6 Å) were added within or on the surface of the model. Reproduced with permission from ref . Copyright 2021 American Chemical Society.…”

“…The challenges remain the same for kerogen and coal, such as capturing orientation issues, stacking diversity, curvature development, and geometric scale differentiation. 34 Nevertheless, these representations could not efficiently characterize the above structural properties of polycyclic aromatic hydrocarbons (PAHs). In our previous work, we have achieved a molecular structure of marine kerogen from Longmaxi shale based on aromatic fringe extraction and image analysis of high-resolution transmission electron microscopy (HRTEM) through quantitative image analysis (QIA, Reindeer Graphics software) within Adobe Photoshop, successfully quantifying the fringe length, orientation, stacking, and curvature extent.…”

“…The advantage of this image-guided construction approach is the rapid, direct capture and recreation of structures from the fringe length, preferential orientation, stacking, and curved fringes. 34 Then, the aliphatic chains, oxygen functional groups, and heteroatom groups were incorporated using Perl construction and evaluation scripts with Packmol. This HRTEM method goes beyond the spectrum analysis by direct extraction from the micrograph and could reflect more structural features than the latter.…”

Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review

“…A last interpretative model worth mentioning�that actually sits at the boundary between interpretative and reconstructed (see below) models�is the mature Longmaxi kerogen (type I−II) model constructed by Wang et al using an Image Guided (IG) approach. 59 In this work, a 3D fully cross-linked and periodic kerogen model is constructed by assembling aromatic fragments�including Copyright 2014 American Chemical Society). The bottom row shows kerogen models developed using the HRMC method for immature Eagle Ford kerogen samples containing C, H, and O atoms, with a porosity of (d) 0.8 g/cm 3 or (e) 1.2 g/cm 3 .…”

“…While this may be a reasonable model of some coal tar pitches, this obviously cannot be one of overmature kerogen. That being said, strategies to turn molecular models into cross-linked ones, as was done with the image guided method, 59 remain promising, provided that the building blocks are diverse and numerous enough to provide a good statistical description of the kerogen.…”