Characterization of Kerogen and Source Rock Maturation Using Solid

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Characterization of Kerogen and Source Rock Maturation Using Solid-State NMR Spectroscopy Andrew Clough, Jessica L. Sigle, David Jacobi, Jeff Sheremata, and Jeffery L White Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01669 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on September 20, 2015

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Characterization of Kerogen and Source Rock Maturation Using Solid-State NMR Spectroscopy Andrew Clough, Jessica L. Sigle, David Jacobi,± Jeff Sheremata, ± and Jeffery L. White* Department of Chemistry, Oklahoma State University, Stillwater, OK 74078

KEYWORDS: kerogen, solid state NMR, bitumen

Abstract. Solid-state NMR methods common to the analysis of polymers and other rigid solids are utilized for the study of kerogen, bitumen, and the organic content in source rocks. The use of straightforward non-destructive techniques, primarily employing solid-state NMR, is shown to provide useful information about both individual samples and changes between samples that cover a range of thermal maturities of type II kerogen. In addition to aromatic fraction and chemical structure, one of the most striking changes to isolated kerogen with maturity is the distribution of pore sizes, studied with both

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Xe NMR and complementary nitrogen

physisorption, that may help to understand the process of bitumen generation. Ultimately, direct in situ analysis of source rock samples that allow kerogen and bitumen to be distinguished is desirable, as it would eliminate the time and effort to isolate and prepare kerogen samples. By proper consideration and removal of the background, we find that a clear 13C NMR signal can be obtained from source rock with total organic carbon weight as low as 2 %. Simple 1H NMR methods are shown to quickly provide a qualitative measurement of the bitumen in source rocks, while 13C cross-polarization is found to be an easy method to distinguish kerogen from bitumen.

*author to whom all correspondence should be addressed at [email protected] ±

ConocoPhillips, Houston, TX

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Introduction Fundamental knowledge of the chemical structure and the material properties of kerogen and the total organic matter in source rock through a range of maturation is critical for understanding the process of oil and gas generation. Organic material in the source rock matrix can be divided into two categories depending on its solubility in organic solvents. Bitumen, or extractable organic matter (EOM), is soluble in organic solvents and can easily be removed from the rock matrix through the use of such solvents. The remaining organic content, which is insoluble in organic solvents, is called kerogen. The kerogen can further be classified into different types, which depend on the location of the kerogen on a Van Krevelen diagram based on the hydrogen to carbon (H:C) and oxygen to carbon (O:C) ratios, and is also related to the source of the organic material and type of EOM output (oil and/or gas).1–3 Type I kerogens come from marine environments and tend to be mostly aliphatic (H:C > 1.25), type III kerogens are derived from terrestrial organic material and are predominantly aromatic (H:C < 1), while type II kerogens are an intermediate type which may come from either marine or terrestrial environments. In each case, kerogen thermal maturation results in a more aromatic kerogen with H:C and O:C decreasing along paths similar to that of alginite, exinite, and vitrinite carbonization paths of coal macerals.1 Many of the common analysis techniques employed by the energy industry are destructive in nature, probe only larger macroscopic properties, or rely on complicated models. Core samples are expensive to obtain and are often limited in size, thus there is a major incentive to develop non-destructive kerogen characterization methods that will work with limited amounts of sample. Solid-state NMR (ssNMR) is a powerful tool to study solid materials (such as kerogen and source rock) in a non-destructive way. Until recently most measurements of kerogen with ssNMR were performed using

13

C cross-polarization magic angle spinning (CP-


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MAS).4–6 A major drawback of CP-MAS is that the results are generally not quantitative. For kerogens, polarization transfer times between 1H and

13

C (i.e. the contact time) of 1 - 2 ms

usually result in aromatic to aliphatic carbon intensity ratios similar to that obtained using quantitative direct-polarization magic angle spinning (DP-MAS), though the signal intensity with CP-MAS suffers because the kerogens tend to have very short 1H spin-lattice relaxation times in the rotating frame (T1ρH).7 There is no guarantee that the ratio of individual components observed using CP-MAS, particularly the protonated and non-protonated aromatic carbon intensities, would be quantitative. The use of DP-MAS to provide more quantitative results for kerogens has only recently been addressed by the Mao group, who have adapted various ssNMR techniques for the study of kerogens.8–10 In this work, non-destructive methods are applied to study type II kerogen covering a range of thermal maturities, along with the corresponding bitumen and source rock. The primary focus is on utilizing simple 13C and 1H ssNMR techniques to study both isolated kerogen and the organic material in intact source rock. Along with determining the aromatic carbon fraction (fAr) using

13

C DP-MAS, we investigate whether isolated kerogens exhibit other differences

associated with maturity that are observable with relatively simple 1-dimensional ssNMR methods. In addition, the pore size distribution of the kerogen samples are probed using

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Xe

NMR, which is a common method used to study free volume dimensions in polymers11,12 and pore sizes in zeolites13 and coals14,15 due to the high sensitivity of the 129Xe chemical shift to the local environment. The 129Xe NMR results are compared with pore size distribution results from nitrogen physisorption experiments, which also provide surface area measurements for the kerogen samples. Along with isolated kerogen, information about the chemical structure and the aromatic carbon fraction of isolated bitumen is determined from static NMR before analyzing the source rock.

We show that relatively quick 1H ssNMR experiments can provide useful


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information about the organic content in the source rock, allowing for a fast qualitative determination of the mobility of the bitumen compared to the rigid kerogen in the source rock. Using 13C DP-MAS, the organic carbon in the source rock is found to be observable even for an organic carbon weight of 2% in the source rock, which combined with information from the isolated kerogen and bitumen, allows for an estimation of the relative amounts of kerogen and bitumen in the source rock. Finally, we demonstrate that 13C CP-MAS will provide a signal only from the kerogen in the source rock, allowing for a possible means of distinguishing the kerogen from the bitumen without any a priori knowledge from the isolated components.

Experimental A series of isolated kerogen samples, along with corresponding rock samples, was provided by ConocoPhillips. As the focus of this work is on the development of experimental methods of non-destructively characterizing kerogen and the organic content in source rock, no information was provided about the source of the kerogen samples. The samples come from the same reservoir, are type II kerogens, and cover a range of thermal maturities. In addition to kerogen and source rock, bitumen samples corresponding to three different maturities were also provided for this study. The samples are labeled alphabetically A through F for this work, with the isolated kerogen, isolated bitumen, and source rock of sample A being identified as Ker-A, Bit-A, and Rock-A, respectively. Samples were analyzed as provided, with no special treatment or handling after receiving the isolated kerogen, isolated bitumen, and source rock. Solid-state

13

C and 1H MAS NMR measurements were collected on a Bruker DSX-300

spectrometer operating at a magnetic field strength of 7.05 T (13C frequency = 75 MHz and 1H frequency = 300 MHz). A Bruker 4.0 mm triple resonance MAS probe was used to study isolated kerogen and bitumen samples, while a Bruker 7.0 mm triple resonance MAS probe was


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used for the source rock in order to provide a larger amount of sample to partially offset the reduced sensitivity resulting from the organic content being only a fraction of the total mass of the source rock. Due to the larger sample rotor and the risk of arcing in the probe as a result of the higher power consumption, measurements in the 7.0 mm probe were performed with spinning speeds of 5 kHz and 90° pulse widths of 5 µs, while measurements of the kerogen samples in the 4.0 mm probe were performed at spinning speeds of both 5 kHz and 10 kHz and 90° pulse widths of 3.5 µs. The bitumen samples were measured in a static 4.0 mm rotor, as their low viscosity prevented the need or desire to use magic angle spinning. All measurements were performed at room temperature. Single pulse 1H measurements with 64 transients were made using the pulse widths stated above and a receiver dead time of 4 µs. The repetition delay was determined by measuring the spin-lattice relaxation time (T1H) using an inversion recovery method and requiring a delay of > 5 T1H to ensure full signal recovery between scans. For the kerogen samples, a repetition delay of 1 s was found to be sufficient, while longer delays of 4 s and 2 s were used for the bitumen and source rocks, respectively.

Background was removed by subtracting from the sample

spectrum an empty rotor spectrum obtained under the same experimental conditions. Quantitative 13C DP-MAS measurements (i.e. Bloch decay) were made with a 90° pulse width as stated above and a total receiver dead time of 30 µs. Acquisition of the 13C signal was made directly after the pulse with high power 1H decoupling at 72 kHz (4.0 mm rotor) or 50 kHz (7.0 mm rotor) to remove the effect of heteronuclear dipolar interactions.

Typically 2K

transients were used for kerogen samples, while 4K transients were usually required to obtain good spectra for the source rocks. The number of transients used for the bitumen varied from 1K to 5K depending on the viscosity of the sample. The repetition delay for the kerogen samples was found using a DP-MAS saturation recovery method with recovery times up to 60 s, which


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shows that 15 s is sufficient for complete or almost complete relaxation of the kerogen samples in this study, though it should be noted that other kerogen samples not covered in this work were found to require a longer repetition delay of 30 s. A 15 s repetition delay was also found for the static bitumen samples with a similar method. In lieu of performing time consuming

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C DP-

MAS saturation recovery experiments on the source rock samples, the single pulse DP-MAS measurements were made with a repetition delay of 15 s, followed by a second measurement with fewer transients and a 60 s repetition delay in order to verify that there was no observable increase in the organic carbon signal between the two measurements. The only increase seen was in an inorganic carbonate peak associated with the rock near 170 ppm, indicating that a repetition delay of 15 s is sufficient to study the organic carbon in the source rock. A method of background removal for

13

C DP-MAS based experiments recently

developed by Jaeger and Hemmann16 is used to obtain background free

13

C spectra. Their

Elimination of Artifacts in NMR SpectroscopY (EASY) method uses a double acquisition buffer to acquire one signal (AQ1) followed immediately by the same pulse sequence and second signal acquisition (AQ2). While AQ1 includes both the sample signal and background, the second pulse sequence starts before any significant

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C spin-lattice relaxation occurs, so AQ2 will only

contain signal from acoustic ringing that may extend into the acquisition time and signal from material outside of the coil region that still has most of its magnetization along the z-direction since it experiences a significantly smaller applied magnetic field (B1) from the coil during the pulse sequence. Because these two signals are acquired simultaneously, the conditions of the electronics (including the 0 and 1st order phase corrections to the spectra) and environment during the acquisitions are the same, allowing the two spectra to be directly compared. A simple subtraction of the two spectra results in a spectrum free of these sources of background. Any background originating from within or near the coil, such as from the rotor, will not appear in


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AQ2 and must be determined by running the same pulse sequence on an empty rotor. While this method of background removal is more involved than simply subtracting the background obtained with an empty rotor, it is very useful when dealing with cases where the background is large compared to the sample signal, which is the case for the source rocks. One modification to the EASY pulse sequence was made in the form of two additional 90° pulses after AQ1, as shown in figure 1, to suppress sample signal that may be present in AQ2 due to spin-lattice relaxation between the start of AQ1 and the start of the second pulse sequence. In our case, this addition reduced the signal from the sample present in AQ2 without affecting the background signal observed.

Figure 1. Pulse sequence incorporating the modified EASY method. A regular Bloch decay (single 90° 13 C pulse and acquisition AQ1 with 1H decoupling) is followed by a 10 ms hardware delay (th), then two more 90° pulses before the Bloch decay is repeated to acquire a signal free spectrum during AQ2, after which the sequence is repeated following the repetition time delay (trep). The delays t1 and t2 are 10 ms and 10 µs, respectively. Due to the very short duration of t2, spin-spin relaxation is not complete, but the remaining signal before the second Bloch decay is removed by appropriate phase cycling.

Kerogen and source rock

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C CP-MAS measurements were performed with the same

repetition time as reported for the 1H measurements above and typically 4K transients were acquired. A simple cross-polarization pulse sequence beginning with a 90° 1H pulse followed by a spin-locking contact pulse between 1H and

13

C was used at spinning speeds of 5 kHz. The

Hartmann-Hahn matching conditions were usually made on the centerband (γCB1C = γHB1H), with γB1 = 72 kHz and 50 kHz for the kerogen and rock, respectively. Some measurements made at spinning speeds of 10 kHz used ramped-amplitude CP-MAS (RACP-MAS), in which the


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power level was linearly ramped by 10 % to minimize the effect of mismatch in the HartmannHahn condition.17 Experiments involving interrupted decoupling, where the 1H decoupling is delayed for a short period of time, were performed for kerogen using both 13C DP-MAS and CP-MAS with 4K transients. The pulse sequence is similar to that described above for DP-MAS and CP-MAS, however a rotor synchronized τ-180°-τ sequence is employed before acquisition where τ is equal to one rotor period (e.g. 100 µs at 10 kHz, as in Harbison et. al. JACS, 1985, 107, p. 4809). The 1

H decoupling is turned on during the first τ, either immediately or after a delay of about 50 µs.

The former case results in a spectrum containing both protonated and non-protonated carbons, while only non-protonated aromatic carbons, methyl carbons, and terminal methylene carbons (CH2 attached to a methyl carbon) will remain in the latter. It must be noted that the results of interrupted decoupling suffer from spin-spin relaxation during the spin-echo, and therefore may not be fully quantitative due to different components of the sample having different spin-spin relaxation times (T2C). However, due to the heterogeneous nature of kerogen, this is the best way to distinguish different chemical species, as the significant overlap of peaks renders ordinary deconvolution methods unreliable. Spin-counting measurements using an internal standard were performed with the 7.0 mm probe to determine the amount of organic content in the source rock. Approximately 1 mg of polydimethylsiloxane (PDMS) was added to a known mass of sample confined in the center region of a rotor (ca. 20 % of total rotor volume), as described in previous work from our group.18,19 As both 1H and

13

C measurements were desired, boron nitride spacers were used to

confine the sample in the homogeneous field region of the coil since the Teflon spacers often used for 1H spin-counting would produce a large 13C signal relative to the sample signal at ~ 110 ppm, on the upfield shoulder of the aromatic peak. Static and 5 kHz MAS 1H spectra were ACS Paragon Plus Environment

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acquired using a 5 µs 90° pulse width, repetition delay of 10 s (due to the longer T1H of PDMS), and 128 transients. Quantitative

13

C DP-MAS were acquired with a 5 µs 90° pulse width,

repetition delay of 15 s, and 8K transients. A Varian Inova 400 spectrometer with a double resonance solutions probe was used for 129

Xe measurements in order to make qualitative measurements of free volume of select kerogen

samples. After placing a quantity of sample required to fill 2/3 of the coil region into a 5 mm valved NMR tube from New Era Enterprises (part # NE-CAV5-M-133), the NMR tube was pressurized with typically 6 atm of xenon by attaching to a pressurized xenon line as described in an earlier work.11 Single pulse measurements were performed at 298 K with 75K transients, a pulse width of 7 us, and a repetition delay of 0.5 s. By only partially filling the coil volume with kerogen, the free

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Xe peak, which defines 0 ppm, is readily seen with 256 or fewer transients.

This allows the sample to be checked both before and after the experiment to verify that there is no significant loss of xenon in the NMR tube during the measurement.

Complementary

physisorption measurements where performed using a Quantachrome NOVA 2200e to obtain pore size and surface area information by analyzing the nitrogen gas isotherm (at 77 K and pressure range of 0.05 to 1.0 atm) using the BJH model20 and BET theory,21 respectively.

Results and Discussion Isolated Kerogen Analysis. The 1H MAS spectra of the six isolated kerogen samples (Ker-A through Ker-F) at spinning speeds of 5 kHz are shown in Figure 2. The signal of each kerogen consists of a broad Gaussian peak with a full width at half maximum (FWHM) of ca. 140 ppm (42 kHz) and a smaller narrow peak, along with several spinning-sidebands (SSB) separated by 16.7 ppm (5 kHz). The broad component comes from 1H spins that are rigid with motional correlation times too slow to allow any averaging of the homonuclear dipolar


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couplings, while the 1H associated with the narrow peaks observed are less rigid, although not nearly as mobile as many mobile polymers where very narrow chemical shift peaks (FWHM < 1 ppm) can be distinguished in the isotropic region (0 – 10 ppm) at 5 kHz MAS. The data for each kerogen given in the first two columns of Table 1 indicates that the FWHM of the rigid component is similar for all of the kerogen samples, ranging from 36 to 48 kHz. The variation in the non-rigid signal fraction is larger, accounting for between 4 % and 14 % of the total kerogen 1

H signal depending on the sample. While it is possible that part of this non-rigid signal arises

from non-kerogen 1H sources in the sample (such as interstitial water, which would likely appear at a chemical shift of around 5 ppm), an aliphatic peak near 1.5 ppm is observed in all samples, which we attribute to either kerogen or residual bitumen that was not removed during the bitumen extraction process. Additionally a distinct smaller second peak is observed near 5 ppm for most samples, indicating a small portion of the signal is due to water. The aliphatic peak of Ker-F, though still visible upon inspection of the isotropic region, appears to be only a small contribution to a rather broad isotropic peak (center = 4 ppm, FWHM = 10 ppm), suggesting that the majority of this non-rigid component is from interstitial water.

Thus, except in Ker-F, a

significant portion of this non-rigid component is believed to come from the organic material, which indicates that these kerogens are not simply all rigid, with a portion being more mobile than the rest of the kerogen.


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Figure 2. 5 kHz 1H MAS NMR spectra of the kerogen samples in this study, indicating that the kerogens are mostly rigid, but have a small fraction (< 15 %) of semi-rigid or mobile protons. The spectrum of Ker-A is divided by 2 for ease of comparison with the other samples.

Table 1. Percent of mobile (non-rigid) kerogen and the FWHM (in kHz) of the rigid component as determined from 5 kHz 1H MAS NMR, aromatic carbon fraction (fArKer) from 10 kHz 13C DP-MAS, and the H:C ratio observed with ssNMR.

Sample Ker-A Ker-B Ker-C Ker-D Ker-E Ker-F

1

H Mobile % 5.8 4.0 8.5 11.0 6.4 13.8

1

H FWHM (kHz) 38 48 42 43 39 36

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C fArKer 0.32 0.66 0.63 0.75 0.87 0.89

H:C 1.34 0.54 0.99 0.67 0.60 0.59

Due to the very broad 1H spectrum of the kerogen, the aromatic and aliphatic components of the samples cannot be distinguished using 1H MAS at the rotor spinning speeds available in the probes used here. We turn to

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C MAS, where the larger chemical shift separation and


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smaller line widths can provide more useful information. As seen in Figure 2, MAS results in a portion of the signal intensity appearing in SSBs which are a separated from the main centerband by an integer multiple of the rotor spinning frequency. While the SSBs in 1H MAS are far away from the isotropic region, this is not the case for 13C MAS. For the magnetic field strength used here, the chemical shift separation between the 13C aromatic and aliphatic regions (~100 ppm) is 7.5 kHz. In order to prevent significant overlap of the aromatic SSB with the aliphatic peak, MAS speeds must be limited to either ≤ 5 kHz or ≥ 10 kHz. Faster spinning has the advantage of smaller SSBs due to more of the intensity appearing in the centerband. Figure 3 shows the 13C spectra (normalized by the aromatic peak height) of the kerogen samples acquired using DPMAS at 10 kHz. The aromatic fraction of carbons is known to correlate well with other techniques, such as vitrinite reflectance, that track kerogen maturity.6 The aromatic fraction in our series of kerogens (fArKer), as determined by numerical integration of the spectra, including the intensity in the aromatic SSBs, vary from fArKer = 0.32 for Ker-A (low maturity) to fArKer = 0.89 for Ker-F (high maturity), as indicated in the third column of Table 1. It is apparent in Figure 3 that the SSBs of the aromatic signal, denoted by *, are small at 10 kHz MAS compared to the main aromatic peak around 130 ppm and that the SSB near -6 ppm is upfield of the aliphatic region, minimizing any misidentification of signal between the aliphatic and aromatic components.


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Figure 3. Comparison of the 13C DP-MAS spectra of the kerogen samples at 10 kHz spinning speeds. The * denotes the locations of the aromatic SSBs. Kerogen thermal maturity increases from top to bottom, with the spectra normalized to the same aromatic peak height. The dashed lines indicate the location of the aliphatic CH2 chemical shift and the center of the high maturity kerogen aromatic peak.

Three important points can be made from a comparison of the spectra in Figure 3. First, the methylene (CH2) peak at 30 ppm that dominates the low maturity aliphatic signal is reduced significantly as maturity increases. Second, this methylene signal is only a small component of the high maturity aliphatic signal, which has a maximum near 20 ppm, indicating that for the high maturity kerogens, the methyl groups become more prominent than CH2. Third, the center of the aromatic peak appears to shift upfield with maturity, as indicated by the dashed line at 125 ppm. The aromatic peak can in general be considered to consist of protonated carbons (aromatic C-H), non-protonated carbons that are part of two connected aromatic rings (bridgehead carbons), and non-protonated carbons that connect to a carbon in either an aliphatic segment or a separate aromatic ring (branched carbons). While each of these types of carbons can have a range of chemical shifts, the protonated carbons tend to be further upfield, while the branched ACS Paragon Plus Environment

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carbons tend to be the most downfield. Thus the observed upfield shift of the aromatic peak with maturity indicates a reduction in the average number of branched carbons relative to the other aromatic carbons. This is consistent with the significant reduction of aliphatic chains, but also suggests that the aromatic composition of the mature kerogen is unlikely to consist of large amounts of single aromatic rings connected to other single rings through branched carbons. By using an external standard, hexamethylbenzene (HMB), to properly normalize the intensities between the 1H and

13

C spectra, the H:C ratios of the kerogen samples can be

measured, as indicated in the last column of Table 1. Except for Ker-B, the general trend of a reduction in H:C with maturity is seen in these kerogens. One possible explanation for the lower than expected H:C in Ker-B is fast relaxation from paramagnetic materials or free electrons that may be in the sample, which would reduce the signal observed in the 1H spectrum, and to a much smaller extent the 13C spectrum. While this effect is obvious in Ker-B, it is likely to be present in all of the samples to some degree. If we consider Ker-A, and assume that for each aliphatic carbon there are 2 protons (the majority of the aliphatic component appearing to be CH2), then the observed H:C of 1.34 is also lower than expected. Depending on the fraction of nonprotonated aromatic carbons, the H:C ratio is expected to be in the range of 1.36 (no protonated aromatic carbons) to 1.68 (all protonated aromatic carbons). Variable contact time 13C CP-MAS at 5 kHz spinning speed is used to determine whether there is any observable difference in some of the characteristic time constants related to crosspolarization, namely T1ρH and the cross-polarization time constant (TCH), between kerogens of different maturities.

Four samples were selected, one low maturity (Ker-A), one medium

maturity (Ker-C), and two high maturity (Ker-E and F) samples.

A series of CP-MAS

measurements were performed with contact times ranging from 5 µs to 4 ms. After numerical integration, the intensity of the aromatic centerband and aliphatic regions were fit to the relation


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 −t    −t  I ( t ) = I 0 1− exp  exp  H   TCH    T1ρ 

(1)

where I(t) is the intensity observed with contact time t, and it is assumed that TCH is small compared to T1ρH and T1ρC. After first determining I0 and T1ρH by fitting the long contact time data, where the term in brackets is ~ 1, TCH is found by fitting the very short contact time data. The intensities, along with the best fit to eqn. (1), for Ker-C and Ker-E are shown in Figure 4. The maximum signal intensity for the aromatic and aliphatic peaks occurs relatively fast (ca. 450 µs and 200 µs, respectively), much smaller than the 1 – 2 ms contact times that are reported to give fArKer values similar to that obtained from DP-MAS.7 The values obtained for TCH, T1ρH, and the contact time that provides maximum intensity for both the aromatic and aliphatic peaks are given in Table 2. As the kerogen maturity increases, a decrease is observed in the aromatic TCH, which is typically associated with the sample being more rigid. A similar decrease is observed in the aliphatic TCH if we exclude the low maturity sample, though a larger set of samples would be required to verify whether this systematically varies with thermal maturity. Similarly, understanding whether any differences observed in T1ρH between samples is the result of sample maturity would require a larger sample set, though we note that in all samples the aromatic T1ρH is longer than the aliphatic T1ρH. This difference between aromatic and aliphatic components is due, at least in part, to the long TCH of the non-protonated aromatic carbons, which results in a further increase to the aromatic peak intensity at longer contact times. The addition of interrupted decoupling to the variable contact CP-MAS experiment (not shown) indicates that the non-protonated aromatic signal does not begin to appear until 100 - 200 µs, around the time that the aliphatic peak is reaching its maximum intensity. Since the measured TCH comes from fitting the intensity at very short contact times, the measured aromatic TCH is actually only for the protonated aromatic carbons, while the measured aromatic T1ρH is not the


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true aromatic T1ρH, but an effective value that includes the additional effects of the TCH for the non-protonated aromatic carbons.

Figure 4. The aliphatic (●) and aromatic centerband (▲) intensities as a function of contact time in 13C CP-MAS at 5 kHz for a) the medium maturity Ker-C and b) the high maturity Ker-E samples. The larger than expected aliphatic component of Ker-E at long contact times is likely due to some overlap between the aromatic SSB at 5 kHz MAS and the aliphatic peak, leading to an increase in the observed aliphatic intensity.

Table 2. Measurements of TCH, T1ρH, and the contact time (tmax) producing the maximum signal intensity from variable contact CP-MAS at 5 kHz for both the aliphatic and aromatic components of four different kerogen samples.

Sample

Ker-A

Ker-C

Ker-E

Ker-F

Ar. TCH (µs)

180

160

110

92

Ar. T1ρH (ms)

3.7

5.4

3.5

3.6

Ar. tmax (µs)

550

570

385

340

Al. TCH (µs)

48

75

54

36

Al. T1ρH (ms)

2.2

2.3

1.1

1.3

Al. tmax (µs)

185

260

165

130


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In order to distinguish protonated from non-protonated aromatic carbons, interrupted decoupling with a rotor synchronized spin echo is used. Testing for optimal decoupling delay times using both glycine and one of the kerogen samples indicates that a decoupling delay of 50 µs is ideal for differentiating between protonated and non-protonated carbons (for further information, see the Supporting Information). In the aliphatic peak, both methyl carbons and terminal methylene, the methylene carbons directly attached to a methyl group, should remain after a 50 µs delay as a result of their rapid motions.5 Even though

13

C CP-MAS can give the

same values of fArKer as DP-MAS by appropriate choice of contact time, the ability of CP-MAS to also give quantitative information about the composition of the aromatic and aliphatic peaks is uncertain. The use of interrupted decoupling with 13C DP-MAS will give the most quantitative results, though deviations from a full quantitative result will still be present, such as from differences in T2C of the various components, which may affect the apparent composition of the spectrum observed after a two rotor period wait time compared to one measured directly after applying a single DP-MAS pulse. As performing a series of decoupling delays is time prohibited due to the long repetition delays required for DP-MAS, interrupted decoupling experiments are made at 10 kHz with decoupling delays of 0 µs and 50 µs for each kerogen sample. Figure 5a shows representative spectra for Ker-E, which can be compared to similar spectra obtained for Ker-E using RACP-MAS with a 1 ms contact time (Figure 5b).

A

comparison of the spectra in Figure 5 clearly shows that more signal remains in both the aromatic and aliphatic components of the DP-MAS acquired spectrum after a decoupling delay of 50 µs. Even though the two spectra with a 0 µs decoupling delay are very similar, with fArKer = 0.82 ± 0.01 for both (and only slightly lower than the fArKer = 0.87 found with quantitative DPMAS), the two techniques provide different results for the component specific information. The measured non-protonated aromatic fraction (fNPAr) and fast-motion (methyl and terminal


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Energy & Fuels

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methylene) aliphatic fraction (ffastAl) for each kerogen sample are given in Table 3. Even though the samples cover a range of maturities, there are no significant variations in either fNPAr or ffastAl between samples. The average fNPAr is 0.76, while the average ffastAl is 0.53, noticeably larger than what was measured for Ker-E using RACP-MAS with 1 ms contact time (0.53 and 0.29, respectively). Further measurements on four kerogen samples using CP-MAS based interrupted decoupling at 5 kHz MAS and variable contact times are provided in the Supporting Information.

Figure 5. Comparison of 13C spectra at 10 kHz MAS from interrupted decoupling with decoupling delays of 0 µs (black) and 50 µs (gray) for Ker-E using a) DP-MAS and b) RACP-MAS with a 1 ms contact time. Compared to the more quantitative DP-MAS based method, both the non-protonated aromatic signal and the aliphatic fast-motion methyl and terminal CH2 are underrepresented with CP-MAS. Table 3. Fraction of aromatic carbons that are non-protonated (fNPAr) and aliphatic carbons that have fastmotion aliphatic carbon (ffastAl) as determined by interrupted decoupling using DP-MAS at 10 kHz spinning speed.

Sample Ker-A Ker-B Ker-C Ker-D Ker-E Ker-F

DP-MAS 10 kHz fNPAr ffastAl 0.73 0.57 0.77 0.48 0.76 0.49 0.74 0.48 0.79 0.63 0.77 0.55


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Energy & Fuels

We see from Table 3 that one in four aromatic carbons are protonated, regardless of kerogen maturity, suggesting that the kerogen is comprised of either large aromatic ring clusters or single aromatic rings with multiple connections to other aromatic or aliphatic structures. If we assume that most of the aliphatic components make up aliphatic chains connected to an aromatic ring at only one end (and a terminating methyl group at the other), then a reasonable estimate of the average aliphatic chain length ( L ) in units of carbon atoms can be made and is given by

(1− f ) L = 2+ Al fast

fmethyl Al

(2)

where the first term accounts for the aliphatic methyl and terminal methylene, and the second term is the number of remaining aliphatic carbons divided by the number of aliphatic methyl groups (fmethylAl). Aromatic methyl, which is attached directly to an aromatic ring, is part of the aliphatic peak (appearing at 20 ppm) and ffastAl, but is not considered as an aliphatic chain for the determination of L . Eqn. (2) can be rewritten in terms of the ratio of aliphatic methyl to aromatic methyl (RCH3):

1  1− ffast Al  L= +  . ffast Al RCH3  ffast Al  2

(3)

Though the exact value of RCH3 is not known, a reasonable assumption is that it is close to 1, and eqn. (3) gives L = 4.7 for ffastAl = 0.53. Fewer aromatic methyl groups will decrease L slightly, down to a minimum of 3.8, while aliphatic chain components bridging aromatic rings or clusters will further reduce L . Kerogen Pore System Analysis. Having studied the organic material of the kerogen with ssNMR, we now turn to the kerogen pore systems. In unconventional shale plays, kerogen has significant contributions to both the overall pore network and to reservoir fluid adsorption. Intrakerogen porosity impacts the overall reservoir pore connectivity and porosity. The highly


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aromatic nature of kerogen makes it strongly oil wet, thus kerogen will strongly adsorb oil within the reservoir and have a significant impact on reservoir wettability. It is also expected that aromatic components of frac fluids will interact with the kerogen. Thus, knowledge of kerogen structure and properties provides insights that can aid in unconventional shale exploration, reservoir characterization, and production improvement. The

129

Xe chemical shift is very sensitive to the nuclei’s surroundings, and is thus a

useful tool for probing the smallest spatial dimension of the volume in which it is located, whether it is free volume in a polymer, or larger pore structures. A relation between the

129

Xe

chemical shift resulting from the pore structure (δs) and mean free path ( l ) for l > 0.25 nm was empirically determined (from zeolite data) by Demarquay and Fraissard13 to be

δ −δ  l = a a s   δs 

(3)

where δa = 243 ppm is the chemical shift of adsorbed

129

Xe, and a = 0.2054 nm. Modeling the

pores as cylinders with diameters small relative to their length, the pore diameter is equal to l plus the diameter of the xenon atom (0.44 nm). While eqn. (3) works well in practice for several types of zeolites, where the measured

129

Xe chemical shift (δ) is very close to δs, in more

complex materials there can be additional contributions to δ due to local electric field gradients within the sample. Since δ rather than δs is measured, there will be some uncertainty in l from these additional contributions.

129

Xe NMR measurements were performed with 6 atm of xenon

for four kerogen samples (Ker-A, Ker-B, Ker-E, and Ker-F) covering a range of maturities, the spectra of which are shown in Figure 6a. While a large chemical shift range was used during the measurements, there was no signal distinguishable at chemical shifts beyond 30 ppm. A peak just downfield (1 - 2 ppm) of the free

129

Xe peak is observed for each kerogen. As thermal


20

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maturity increases, the existence of a second peak downfield from the first peak becomes apparent, with the chemical shift increasing as maturity increases, indicating that the xenon atoms are probing pores with smaller average pore sizes in higher thermal maturity samples. As the typical

129

Xe chemical shift is ~ 3 - 10 ppm, eqn. (3) suggests that l , and thus the typical

pore diameter, is ~ 5 - 16 nm, indicating that the kerogen contains mesopores (2 nm < d

Characterization of Kerogen and Source Rock Maturation Using Solid

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