Combined High-Resolution Solid-State ¹H/¹³C NMR Spectroscopy and ¹H NMR Relaxometry for the Characterization of Kerogen Thermal Maturation

Abstract

A key factor for the petroleum potential of source rock is the degree of chemical and physical structure evolution of its kerogen fraction through a range of maturation processes. In this study, various high-field, solid-state NMR methods have been applied to a series of kerogen isolates (type I) over a defined maturity range (vitrinite reflectance R 0 from 0.98 to 1.86%). Results obtained from 13C MAS NMR show that the sp2/sp3-hybridized carbon ratio of kerogen, here defined as the aromatic/aliphatic ratio, increases with increasing maturity. 1H MAS NMR spectra contain partly overlapping aliphatic and aromatic resonances with distinct transverse relaxation behavior. In Hahn-echo experiments, the aromatic signal decays more slowly than the aliphatic signal, indicating that for these systems, transverse 1H relaxation is rather controlled by local distances between hydrogen atoms than by molecular mobility. Similar relaxation differences are also found in static (nonspinning) 1H Hahn-echo NMR experiments, here used to discriminate between phases with different proton mobilities and/or densities in the kerogen samples and, ultimately, between aromatic and aliphatic fractions. The distributions of the static transverse relaxation time (T 2), extracted from the Hahn-echo decays, are characterized by a short-T 2 peak (∼10 μs) and a long-T 2 peak (∼100 μs). The ratio between these two peaks correlates well with the aliphatic-to-aromatic signal intensity ratios in MAS NMR spectra of the corresponding kerogen samples, suggesting that a net decrease in kerogen proton densityoccurring during maturationis also reflected by 1H NMR relaxation. For the investigated kerogen isolates, the long-T 2 peak in the T 2 distribution can be considered an indicator of aromatic content, which can be directly detected by measuring 1H T 2 relaxation.

Critical Practices for the Preparation and Analysis of Kerogen

“…34,45,60,63,65,70,73,112,113,129−131 Previous studies of the kerogen molecular structure by 13 C SS-NMR have consistently shown that the aromatic content increases during thermal maturation, ranging in type II kerogen from aromatic carbon fractions as low as 20−80% or more. 60,65,112 However, aspects of 13 C SS-NMR spectral acquisition and analysis can lead to erroneous quantification of even basic structural group abundances in kerogen and other macromolecular OM, as demonstrated by the example here.…”

“…110,112 However, rapid magnetization decay of 1 H limits energy transfer and detection efficiency of 13 C, 69,132 and reliance on cross-polarization from 1 H favors excitation of protonated carbons and possibly neglects carbon structures not bonded to hydrogen. 65,69,110,112 This bias can underestimate the aromatic carbon content in carbonaceous OM, 69 especially in thermally mature kerogens with lower hydrogen concentrations. 23 Direct polarization of 13 C in magic angle spinning (DP/MAS) avoids these limitations but suffers from lower sensitivity and long recycle delays, generally requiring acquisition times of multiple hours (ca.…”

“…Quantitatively similar evolutionary trends obtained by 13 C DP/MAS have been reported for other type II kerogens as well as for type I kerogen. 63,65 The impact of SSBs on the 13 C CP/MAS NMR spectrum was evaluated by recording spectra at different rotor spin rates (sample R o = 1.4%; Figure 8). As described by Mao et al, the SSB associated with sp 2 carbons (labeled here as SSB1 and SSB1′) shifts to a position from ±50 ppm (at 5 kHz) to ±100 ppm (at 10 kHz) about the centerband.…”

“…However, neither method directly probes molecular chemical or structural properties of organic matter. Certain aspects of molecular structures (e.g., functional groups) of kerogen can be studied using several spectroscopic techniques, including solid-state nuclear magnetic resonance (SS-NMR), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge structure (XANES). ,,− …”

Quantification of Imbibed Heptane in Shale Rocks Determined by Edited T1–T2 Nuclear Magnetic Resonance Relaxation Experiments at High Magnetic Field

“…Even when small amounts of samples are required (about 150 mg), the signal-to-noise ratio in general increases for high magnetic fields, resulting in shorter experimental times. Another benefit is that 1 H NMR data from a single sample may be correlated to that obtained from other nuclei, such as 13 C NMR, either under static conditions or with magic angle spinning (MAS) techniques from which determination of maturity among other information is obtained. − The drawback is that transverse relaxation times are significantly reduced as the magnetic field is increased, and in some cases, signals from solid components overlap with fluids complicating the interpretation of the data. Recently, Zamiri et al introduced the use of T 1 – T 2 * correlation maps, that is, acquiring transverse relaxation data during the free induction decay (FID) of the signal rather than using a train of refocusing pulses.…”