Model representations of kerogen structures: An insight from density functional theory calculations and spectroscopic measurements

Philippe F Weck,Jessica N Kruichak,Melissa M Mills,Roland J.-M Pellenq,Yifeng Wang,Eunja Kim,Edward N Matteo

doi:10.1038/s41598-017-07310-9

Abstract

Molecular structures of kerogen control hydrocarbon production in unconventional reservoirs. Significant progress has been made in developing model representations of various kerogen structures. These models have been widely used for the prediction of gas adsorption and migration in shale matrix. However, using density functional perturbation theory (DFPT) calculations and vibrational spectroscopic measurements, we here show that a large gap may still remain between the existing model representations and actual kerogen structures, therefore calling for new model development. Using DFPT, we calculated Fourier transform infrared (FTIR) spectra for six most widely used kerogen structure models. The computed spectra were then systematically compared to the FTIR absorption spectra collected for kerogen samples isolated from Mancos, Woodford and Marcellus formations representing a wide range of kerogen origin and maturation conditions. Limited agreement between the model predictions and the measurements highlights that the existing kerogen models may still miss some key features in structural representation. A combination of DFPT calculations with spectroscopic measurements may provide a useful diagnostic tool for assessing the adequacy of a proposed structural model as well as for future model development. This approach may eventually help develop comprehensive infrared (IR)-fingerprints for tracing kerogen evolution.

Highlights

Kerogen is a high-molecular weight, carbonaceous polymer material resulting from the condensation of organic residues in sedimentary rocks; such organic constituent is insoluble either in aqueous solvents or in common organic solvents[1]
Existing model structures of kerogen were generally constrained from elemental analyses and functional group data obtained from X-ray photoelectron spectroscopy (XPS) and 13C nuclear magnetic resonance analyses (NMR)[4]
In this study, infrared signatures for a variety of generic kerogen models proposed recently[4, 5] were calculated within the framework of density functional perturbation theory (DFPT) and compared to Fourier transform infrared (FTIR) spectra collected from kerogen samples with different types and maturity

Summary

Results and Discussion

The 3D-periodic kerogen structures initially considered in this study were based on generic molecular fragments proposed by Ungerer et al.[4] to represent kerogen structures with lacustrine (type I) and marine (type II) depositional origin. 5, the GDOS measured by neutron spectroscopy (cf Fig. 6) are dominated by vibrational modes of hydrogen atoms forming bonds with carbon by s-sp[2] or s-sp[3] overlap While such neutron spectroscopy measurements probing C–H bonding are useful to estimate maturation in terms of sp2/sp[3] or H/C ratios, IR spectroscopy can provide valuable information in terms of C–O, C– S, C–N, C–C and complex carbon-backbone vibrational modes, as discussed above for the analysis of type-I and -II models of Ungerer et al.[4]. Such information is crucial for constraining functional group distributions and their chemical bonding environments in kerogen. This approach may lead us to develop a comprehensive library of infrared fingerprints for tracing kerogen structural evolution

Methods

Author Contributions

Additional Information