Position-specific carbon and hydrogen isotopic compositions of propane from natural gases with quantitative NMR

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Position-specific carbon and hydrogen isotopic compositions of propane from natural gases with quantitative NMR ⁎

Changjie Liua, , Gregory P. McGovernb, Peng Liua,c, Heng Zhaoa,c, Juske Horitaa,

⁎

a

Department of Geosciences, Texas Tech University, Lubbock, TX 79409, USA Department of Chemistry and Physics, West Texas A&M University, Canyon, TX 79016, USA c Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, CAS, Lanzhou, 730000, China b

A R T I C LE I N FO

A B S T R A C T

Editor: Michael E. Böttcher

Position-specific isotope compositions of light hydrocarbons are expected to provide valuable information on their formation and migration-degradation processes. Here we present a high-accuracy and high-precision (≤ ± 10 and ≤ ± 1‰ for 2H and 13C isotope compositions, respectively) method to determine position-specific hydrogen and carbon isotope compositions of propane from natural gases with quantitative NMR. Customized, light-weight high-pressure sapphire NMR cells were developed for liquefied propane samples. Precision and accuracy of our technique were demonstrated using 13C-labeled compounds, neat samples of C3-C5 with natural isotope abundances, and inter-laboratory comparison of a C7 sample. To determine position-specific isotope compositions of propane from natural gas samples, a method was developed to collect and purify large amount (~6.8 mmol) of propane, using a variable-temperature cold trap. A test with a synthetic sample of natural gas mixture indicates that little isotope fractionation occurred during the propane separation and purification from natural gas mixtures. For the first time, high-precision and high accuracy data are reported of position-specific carbon and hydrogen isotope compositions of propane from different sources, including conventional and unconventional petroleum reservoirs. Preliminary results show that position-specific isotope fractionations between the center and terminal sites of propane vary widely for the different sources. Position-specific isotope compositions of propane have the potential to improve our understanding on the origins, migration and degradation processes of natural gas and hydrocarbons in general.

Keywords: Position-specific isotope analysis Propane Natural gas Quantitative NMR

1. Introduction Light hydrocarbons (methane to pentane/hexane) constitute the largest fraction of petroleum hydrocarbons. Major formation mechanisms of these light hydrocarbons include microbial, thermogenic, and abiogenic pathways. It has also been increasingly recognized that hydrocarbons of various chain-lengths can be stable even under deep Earth conditions (McCollom, 2013; Spanu et al., 2011) and that their fluxes may play a key role in the carbon dynamics of the Earth. In the past decades, chemical and stable isotopic compositions (δ13C and δ2H) of light hydrocarbons have been extensively utilized as major tools to investigate their origins (microbial, thermogenic, and abiogenic), migration processes and degradation (Fuex, 1977; Galimov, 2006; Lu et al., 2015; Schoell, 1980, 1983). With wide-spread applications of the compound-specific isotope analysis of hydrocarbons using gas chromatography combustion/pyrolysis isotope ratio mass spectrometry (GC-IRMS), many systematic studies of δ13C and δ2H variations of hydrocarbon gases have reported new findings, including the “inverse” ⁎

relationship and “roll-over” phenomena in δ13C values of natural gases from crystalline rocks and deeply buried sedimentary basins (Burruss and Laughrey, 2010; Lollar et al., 2002; Zumberge et al., 2012). The incorporation of hydrogen from water to hydrocarbons is also recognized as an important mechanism for controlling δ2H values of kerogen and hydrocarbons (Gao et al., 2014; Killops, 2005; Reeves et al., 2012). Despite their proven utility, conventional approaches using the bulk stable isotopes (δ13C and δ2H) of light hydrocarbons are often not sufficient to distinguish their sources, maturation stages/ temperatures, and migration-degradation processes, because their bulk isotopic compositions strongly depend on the isotopic compositions of sources, in addition to production mechanisms and migration-degradation processes. Most natural compounds, including light hydrocarbons, are composed of a set of diverse isotopic molecules that differ in the number of isotopic substitutions and/or positions of isotopic substitutions within a given molecule. Many of light hydrocarbon molecules contain heavy hydrogen and carbon isotopes substituted in energetically non-

Corresponding authors. E-mail addresses: [email protected] (C. Liu), [email protected] (J. Horita).

https://doi.org/10.1016/j.chemgeo.2018.05.011 Received 7 November 2017; Received in revised form 7 May 2018; Accepted 8 May 2018 0009-2541/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Liu, C., Chemical Geology (2018), https://doi.org/10.1016/j.chemgeo.2018.05.011

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of modern NMR spectroscopy, quantitative NMR techniques for position-specific 2H and 13C isotopes have emerged and have been applied to food science and environmental science studies (Gilbert et al., 2013; Jamin and Martin, 2008; McKelvie et al., 2010; Rezzi et al., 2004). In contrast to the IRMS methods discussed above, the quantitative NMR technique does not require degradation or fragmentation of molecules and can be applied to a wide range of organic compounds for both position-specific 2H and 13C isotopes. In addition, the samples of organic molecules can be recovered after NMR measurements for multiple measurements or different types of analysis. Major shortcomings of quantitative NMR techniques include the requirement of large sample quantities (~6.8 mmol) due to low NMR sensitivity and relatively long experimental times (Gilbert et al., 2012; Silvestre et al., 2009).

equivalent positions, e.g., methyl (-CH3) and methylene (-CH2-) groups of propane (C3H8): isotopomers or position-specific (intramolecular) isotope fractionation. Isotope fractionation, introduced by chemical, physical, and/or biological processes, occurs primarily at reactive sites because of different rates of bond breaking and formation among the isotopomers involved. Therefore, studies on intramolecular or positionspecific isotope distributions can yield the most specific information about chemical origins and kinetic processes (Brenna, 2001; Martin et al., 2006). Conventional methods for determining isotopic compositions of light hydrocarbons with compound-specific IRMS convert molecules of interest to analyte gases (CO2 and H2), which are then introduced into an IRMS. Due to the inherent nature of the conventional compound-specific IRMS, information of intramolecular or positionspecific isotopic compositions within the molecules of interests is lost. To date, several methods have been developed to determine position-specific isotopic compositions within select organic molecules. The earliest studies of position-specific isotope fractionation were carried out on amino acids, which chemically cleave the analyte amino acids to fragments (the carboxylic acid fragments and the remains). Then, the fragments were separately combusted to CO2, and analyzed by dualinlet IRMS for position-specific carbon isotope compositions (Abelson and Hoering, 1961; Monson and Hayes, 1982a; Monson and Hayes, 1982b). Recently, Gao et al. (2016) analyzed position-specific carbon isotopic compositions of propane by converting propane to acetic acid, using enzyme-catalyzed and chemical reactions. An essential step in this conversion was the enzymatic reaction from propane to 2-propanol with high site-selectivity. Then, position-specific carbon isotopic composition of propane can be calculated from bulk carbon isotopic compositions of initial propane and acetic acid (chemical oxidation product of 2-propanol). The conventional GC-IRMS techniques have also been modified for intramolecular isotopic compositions of organic molecules (Brenna, 2001; Corso and Brenna, 1997, 1999; Yamada et al., 2002). A temperature-controlled thermal pyrolysis produces fragmental products of organic compounds of interest, which are then introduced to GCcombustion-IRMS for their isotopic analyses. Gilbert et al. (2016) measured position-specific 13C isotopic composition of propane by online propane pyrolysis coupled with GC-combustion-IRMS. δ13C values of fragments (CH4, C2H6, C2H4) from propane pyrolysis were used to calculate position-specific isotopic composition of the original propane. Recently, high resolution mass spectrometry (MAT 253 Ultra) has been developed to measure clumped and position-specific isotope compositions of molecules without converting to the common analyte gases (CO2 and H2, Eiler et al., 2014; Stolper et al., 2014a; Stolper et al., 2014b). Propane gas can be directly introduced to the mass spectrometer and δ13C values of one‑carbon and two‑carbon fragmented ions can be determined that are produced within the ion-source by electron impact (Piasecki et al., 2016b). These MS-based techniques for positionspecific isotope analysis require relatively small sample quantities (sub μmol to 100's of μmol). However, possible isotope fractionations have not been successfully addressed that are associated with the breakingdown of the bulk molecules to smaller moieties or ionized fragments. The accuracy of these methods needs to be assessed by other proven methods. In addition, the applications of the MS-based techniques are likely limited to small organic molecules, because complex and unpredictable nature of chemical-enzymatic reactions, thermal pyrolysis, and electron impact fragmentation would be very hard to control for large organic molecules. Nuclear magnetic resonance (NMR) spectroscopy can distinguish non-equivalent sites of NMR-active isotopes (2H and 13C) within a given molecule. In an NMR spectrum, nuclei of the same isotope at nonequivalent sites have different electromagnetic environments, which results in different resonance frequencies (chemical shifts). The NMR signals are theoretically proportional to the numbers of nuclei in resonance. Therefore, NMR can in principle provide information on the relative abundances of the non-zero spin isotopes (2H and 13C) at nonequivalent sites within a given, intact molecule. With the development

2. Bulk and position-specific isotope compositions A stable isotope ratio of bulk molecules is defined as the ratio of the number of a heavy isotope to that of a light isotope within the entire molecule:

R bulk = Z∗/Z

(1)

where Z is the light isotope of an element of interest (carbon, hydrogen and others) and Z⁎ is the heavy isotope. A position-specific isotope ratio within a given molecule can similarly be defined as:

Ri = (Z∗/Z)i = Z∗i /Zi

(2)

where the subscript i denote a specific, non-equivalent position of an element of interest within a given molecule. Thus, the conventional δ values for bulk molecule and its specific position i can be expressed:

R δbulk = ⎛ bulk − 1⎞ ∙1000 (‰) ⎝ Rstd ⎠

(3)

R δi = ⎛ i − 1⎞ ∙1000 (‰) ⎝ Rstd ⎠

(4)

⎜

⎟

⎜

⎟

A stoichiometric number of an element of interest Z within a given molecule (P) and a statistical molar fraction of a heavy isotope at the site i (Fi) can be expressed as: n

P=

∑ Pi

(5)

i=1

(6)

Fi = Pi /P

The numbers of heavy (Z⁎t ) and light isotopes (Zt) within a given molecule are given by: n

Zt∗ =

Zi∗

∑

(7)

i=1

n

Zt =

∑

Zi

(8)

i=1

Thus, a real molar fraction of a heavy isotope that occupy the position i (fi) is:

fi = Z∗i /Z∗t

(9)

Then, the position-specific isotope deviations relative to the stochastic, random distribution have been defined as Δi (Caytan et al., 2007a):

Δi = (fi /Fi − 1)·1000‰

(10)

For elements with two stable isotopes, (e.g., C and H), a positionspecific isotope ratio can be expressed as:

(Z ∗/ Z )i =

fi Zt∗ fi Zt∗ Z∗i = = ∗ ∗ Zi Fi (Zt + Zt ) − fi Zt (Fi − fi ) Zt∗ + Fi Zt

(11)

For carbon isotopes of natural abundances ( C/ C ~ 1.1 × 10−2), 13

2

12

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the term (Fi − fi)Zt∗ is at least 2000 times smaller than the term FiZt, if position-specific isotope effects are smaller than 50‰. For hydrogen isotopes of natural abundances (2H/1H ~ 1.5 × 10−4), this term is at least 10,000 times smaller than the term FiZt. Therefore, Eq. (11) can be reduced to:

f f Z∗ (Z ∗/ Z )i = Ri = ⎛ i ⎞ ⎛ t ⎞ = ⎛ i ⎞ R bulk F Z F i t ⎝ ⎠⎝ ⎠ ⎝ i ⎠ ⎜

⎟⎜

⎟

⎜

⎟

(12)

From Eq. (3), (4), (10), and (12), the three isotopic values (δi, Δi and δbulk) are related by:

(1 + δi) = (1 + ∆i )(1 + δbulk )

(13)

An intramolecular isotope fractionation factor between the two positions i and j within a given molecule (αi−j) can be expressed from Eqs. (10) and (12):

( ) = 1+∆ fi

αi − j

R = i = Rj

Fi

() fj

i

Fig. 1. NMR sample tubes used in this project: a sapphire tube with assembly valve for propane samples, LPV tube and flame sealed normal NMR tube.

1 + ∆j

Fj

(14)

Then, we obtain:

lnαi − j ≅ ∆i − ∆j = ∆i − j

preparation line to fill ~5 cm length as liquid inside the tubes. Due to long longitudinal relaxation time (T1) of 13C nuclei, a relaxation agent (Cr(Acac)3, Sigma-Aldrich, 99.99%) was added to samples to reduce T1 time. In most cases, 0.1 M Cr(Acac)3 in deuteratedsolvents (all purchased from Cambridge Isotope Laboratories) were prepared and dried beforehand (Caytan et al., 2007b). Propane and labeled ethanol used acetone‑d6 as solvent, while DMSO‑d6 and dioxane‑d8 were used for labeled acetic acid and n-heptane, respectively. Samples and Cr(Acac)3 solutions need to be thoroughly mixed in NMR tubes. In some cases, lower concentrations of Cr(Acac)3 in the solvent had to be used to keep Cr(Acac)3 from precipitating after mixing with samples.

(15)

3. Quantitative NMR of light hydrocarbons 3.1. Materials and sample preparation Samples of several commercial, neat light hydrocarbons with natural isotope abundances (C3-C5, C7) were used to test the sensitivity and precision of the quantitative 2H and 13C NMR techniques developed in this study. These samples were propane (Matheson Tri-gas, 99.9%), nbutane (Sigma-Aldrich, 99%), and n-pentane (Avantor Performance Materials Inc., ≥98%). A sample of natural abundance n-heptane was also tested, which has previously been analyzed at University of Nantes, France (Julien et al., 2016). Bi-labeled 13C ethanol and acetic acid ([1,2–13C2] ethanol and [1,2–13C2] acetic acid, 99%, Cambridge Isotope Laboratories) were used to test the precision and accuracy of the 13C quantitative NMR technique. NMR measurements of all light hydrocarbons have to be conducted in a liquefied form because of low densities of gaseous hydrocarbons. Regular 5 mm OD NMR tubes (4.9635 ± 0.0065 mm OD, 4.2065 ± 0.0065 mm ID) and low pressure/vacuum (LPV) NMR tubes (4.9365 ± 0.0065 mm OD, 4.2065 ± 0.0065 mm ID, all supplied from Wilmad-Labglass) were used to contain liquid samples for all these compounds other than propane. For propane, which has a vapor pressure of ~11 bar at 30 °C, a customized high-pressure NMR tube was made (Fig. 1). It consists of a sapphire NMR tube (Wilmad-Labglass and Crytur, Chez Republic) and a valve assembly, including a micro-metering valve, made of organic thermoplastic polymer (PEEK, IDEX Health and Science). The sapphire tube (4.92 ± 0.05 mm OD, 3.4 ± 0.1 mm ID, and 17.8 mm long) can hold pressure as high as 400 bar. The valve assembly is rated at 500 psi (34 bar), which provides enough safety margin for liquefied propane. A sample volume of the sapphire tubes (90 μL/cm) is smaller than standard NMR tubes or LPV tubes (139 μL/cm), which contributes to longer acquisition time. Although a set of the high-pressure sapphire NMR cell and the valve assembly (13.2 g) is heavier than a LPV tube (7.2 g), they can be readily spun within an NMR probe of spectrometers. For quantitative 2H NMR measurements, neat samples were used because deuterated solvents for NMR would affect the measurement of natural abundance (2H/1H ~ 1.5 × 10−4) samples. For liquid samples (e.g., n-pentane and n-heptane), ~1.1 mL of samples were pipetted into a regular NMR tube. For propane and n-butane samples, at least 6.8 mmol (150 mL at Standard Temperature and Pressure, STP) of gas were loaded into the sapphire and LPV tubes using a vacuum

3.2. Quantitative NMR methods 3.2.1. 2H Quantitative NMR All the 2H NMR experiments were conducted with JEOL ECS 400 MHz NMR spectrometers equipped with a 5 mm broadband, auto tunable probe (40TH5AT/FG). The T1 time of 2H of the sample was measured so that repetition time can be selected as at least 7 times the longest T1. Experiment temperature was maintained at 30 °C and the NMR cells were spun at 15 Hz. Both 2H and 1H channels were tuned and matched. For the 2H NMR experiments, gradient shimming was conducted with 1H. Shimming parameters were manually adjusted to obtain symmetric peak shapes. A 90° flip was chosen for the 2H channel. Proton decoupling sequence of WALTZ-16 with decoupling offset 5 ppm was used. Scan numbers (typically 640 for propane) can be adjusted to obtain signal to noise (S/N) ratio ≥ 120 for all peaks of a sample (Martin and Martin, 1995; Rezzi et al., 2004). A single measurement for quantitative 2H NMR of natural-abundance samples may take 2 to 10 h. Propane samples in particular needed a long acquisition time, because of the smaller sample volume (~65%) of the sapphire tube relative to regular or LPV NMR tubes. 3.2.2. 13C Quantitative NMR The 13C T1 time of a sample was also measured to allow the repetition time to be at least 10 times the longest T1. A 90° flip was chosen for the 13C channel. Scan numbers can be adjusted to have S/N ratios > 1000 for all peaks. The 13C-labeled samples need only several scans to achieve that, but acquisition times of natural abundance samples ranged from 2 to 5 h. Most samples were analyzed on a Bruker 400 MHz AVANCE III HD NMR spectrometer with a dual-channel BBFO probe, using an inversegated, adiabatic COS/OIA decoupling sequence (Tenailleau and Akoka, 2007). In the 13C NMR measurements with this NMR spectrometer, 3

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sample tubes did not need to be spun to achieve a homogeneous magnetic field. The temperature of the probe was set at 30 °C. Both 13C and 1H channels were carefully tuned and matched. Details of other experimental parameters have already been described in Chaintreau et al. (2013). The neat light hydrocarbon samples (C3-C5) were also run on JEOL 400 MHz NMR spectrometers using an inverse-gated, adiabatic WURST20 decoupling sequence. Experiment temperature was set at 30 °C, spinning rate at 15 Hz. Both 13C and 1H channels were tuned and matched. For 13C NMR experiments, gradient shimming with 2H was conducted, which is more effective than 1H shimming used in 2H NMR experiments.

compounds (Cross et al., 1998; McKelvie et al., 2010). In contrast to 2H NMR experiments, Nuclear Overhauser Effect (NOE) can disturb the proportional relationship between the signal intensities and the number of 13C nuclei in resonance in 13C NMR experiments (Martin et al., 2006; Rezzi et al., 2004). Several techniques have been used to minimize NOE, such as short acquisition time and optimized inverse-gated adiabatic proton decoupling sequences (WURST-20 and COS/OIA). The latter sequence had been developed in the late 2000s specifically for determining position-specific carbon isotope analysis (Tenailleau and Akoka, 2007). Below, we have tested and compared the accuracy and precision of these two sequences for position-specific 13C/12C measurements of the light hydrocarbons.

3.2.3. NMR data processing Delta (JEOL NMR software) was used to process 2H spectra from the JEOL NMR spectrometers. The processed spectra were saved as JCAMP files to be imported to Perch software (see below). Spinworks software (developed at University of Manitoba, Winnipeg, Canada) was used to process and transform the 13C spectra with WURST-20 decoupling sequence to JCAMP files, so that they can be imported and processed by Perch software. Topspin (Bruker NMR software) was used to process 13C spectra with COS/OIA decoupling sequence from the Bruker NMR spectrometer. Spectra that were processed by Topspin can be directly imported to Perch software. Perch Software (Perch Solutions Ltd., Kuopio, Finland) was used for the deconvolution, fitting, and integration of NMR peaks. It employs a Lorentzian mathematical model with options to include a Gaussian component. It also has options to optimize the dispersion and asymmetry in the peak fitting. In some cases, neighboring peaks from a given molecule overlapped and had to be deconvoluted. To be consistent, peak fittings were done for all the peaks by Perch, even in the cases where peaks appear well separated. Fitting quality can be evaluated by visually examining residual (difference between the real spectra and fitting line) and the RMS (root mean square) values calculated by Perch. Peak areas obtained after deconvolution, fitting, and integration by Perch software discussed above were used to calculate the positionspecific isotope deviations within the light hydrocarbons (Δi). A molar fraction of heavy isotope (2H or 13C) at the position i (fi) can be calculated as.

3.3.1. 13C-labeled compounds Bi-labeled 13C ethanol and acetic acid were used to test the precision and accuracy of the 13C quantitative NMR technique. Signals from each carbon site in 13C-labeled ethanol and acetic acid are doublets with a small peak in the center (see Fig. 1 in Caytan et al., 2007b). The doublets arise from 13Ce13C coupling in the bi-labeled molecules. Because the 13C-labeled ethanol and acetic acid contain traces of 12C (< 1%), smaller peaks in between are from molecules with only one 13 C-labeled carbon site. Because the two sets of doublets arise from the same molecule, peak areas of two doublets should be identical (Caytan et al., 2007b). Any variations from this relationship can be used as a measure to evaluate the decoupling efficiency and the effect of NOE. A decoupling offset is an important parameter for quantitative NMR measurements. The influences of decoupling offsets on the accuracy of carbon position-specific isotopic composition measurements of 13C bilabeled ethanol and/or acetic acid were tested using WURST-20 and COS/OIA adiabatic decoupling sequences (Tenailleau and Akoka, 2007). Similar experiments were also carried out for two inverse-gated, adiabatic decoupling sequences (COS/OIA and WURST-20) in this study. Fig. 2 shows the results from tests of 13C bi-labeled acetic acid and ethanol by plotting decoupling offsets and position-specific isotopic deviations of methyl carbon sites from the random distribution (ΔCH3). The two carbon sites of bi-labeled molecules should have the same number of 13C, yielding a ΔCH3 value of zero. For 13C-labeled acetic acid measurements using COS/OIA, the ΔCH3 value was relatively constant, ranging from +4.0 to +6.8‰ at different decoupling offset values. The values of ΔCH3 have in general good standard deviation (< ± 0.6‰, n = 5). On the other hand, for 13C-labeled acetic acid measurements using WURST-20, the value of ΔCH3 ranged from −2.9 to −4.7‰. Due to cycling sidebands in measurement with WURST-20 decoupling sequence, the baseline identification and curve fitting were not conducted with Perch software. For this reason, uncertainty associated with WURST-20 decoupling sequence could not be rigorously evaluated. For 13 C bi-labeled ethanol measurements using COS/OIA, the value of ΔCH3 was nearly constant, ranging from 1.5 to 2.1‰. Results with WURST-20 sequence showed large deviations with increasing decoupling offsets.

fi = Si /Stot ,

(16) 2

13

where Si is a peak area of H or C spectra at the position i and Stot is a total area of all 2H or 13C peaks of a given molecule. Due to 13Ce13C coupling, small 13C satellite peaks can be observed on both sides of major peaks in 13C NMR spectra. These 13C satellite peaks must be included in the calculation of the 13C peak. However, satellite peaks are small on both shoulders of the main peak, so that deconvolution and fitting result in large errors in the area calculation of these peaks. For this reason, the contributions of satellite peaks were calculated, rather than by direct measurements. The correction factor (1 + 0.011 ∗ n) needs to be applied to a major peak, where n is the number of carbons that are attached to the carbon of interest within a given molecule (Tenailleau et al., 2004; Zhang et al., 1999). Errors in the contribution of satellite peaks caused by the correction factor are very small. For 2H NMR spectra, a correction is not necessary because each hydrogen site always has one carbon attached within hydrocarbon molecules. Thus, the correction factors are all cancelled out.

3.3.2. Natural-abundance 2H and 13C NMR of light hydrocarbons We have applied and tested 2H quantitative NMR method with WALTZ-16 decoupling sequence and 13C quantitative NMR methods with WURST-20 and COS/OIA decoupling sequence to several naturalabundance light hydrocarbons (propane, n-butane, n-pentane, and nheptane). 3.3.2.1. Propane. The customized high-pressure sapphire NMR cells for propane samples have a lower sample volume than a regular 5 mm OD NMR tube. This leads to longer acquisition times to obtain quality NMR spectra (Fig. 3). The longest T1 time for propane 2H is ~6 s, so that delay time was set at least 35 s. The number of scans were adjusted to have S/N > 120 and 350 for peaks of the central and terminal sites, respectively. For 13C quantitative NMR, the longest T1 time ranges from 5 to 9 s, depending on the concentration of the Cr(acac)3 solution. The number of scans was adjusted to have S/N > 1000 and 2000 for the

3.3. Results of quantitative NMR methods Deuterium (2H) has a spin number of 1, so that its relaxation is dominated by a quadrupolar mechanism. Therefore, in 2H quantitative NMR experiments, the signal intensities or peak areas are proportional to the numbers of deuterium nuclei in resonance. A standard protondecoupling sequence of WALTZ-16 has been successfully used for position-specific hydrogen isotope analysis of many organic and biological 4

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ΔCH3(per mil)

8

3.3.2.3. n-pentane. Quantitative 2H NMR (WALTZ-16) and 13C NMR (WURST-20 and COS/OIA) were conducted for the n-pentane sample (Fig. 5). The longest T1 time was 3.0 s, and the delay time were set at 24 s for 2H NMR. The longest T1 times were 3.8 and 3.2 s, and the delay times were set at 40 and 50 s for 13C quantitative NMR experiments with WURST-20 and COS/OIA, respectively. For 2H measurements, the scan number was 1300, and the S/N were larger than 490, 340 and 170 for the hydrogen sites 1, 2 and 3, respectively (see Fig. 5 for site assignment). For 13C measurements, the scan number was 224 with the S/N over 3000, 3000 and 1500 for the carbon sites 1, 2 and 3, respectively in the WURST-20 experiments, while the S/N is larger than 900 for all carbon sites with 256 scans in the COS/OIA experiments. The two middle sites (2 and 3) are heavier than bulk average in 2H, while the terminal site is depleted in 2H. For the 13C position-specific isotope ratios, the two middle sites (2 and 3) are also enriched compared to the terminal site. In this n-pentane sample, 2H and 13C have similar patterns of position-specific isotopic compositions. The precision was 1.1, 3.7 and 8.0‰ for the H sites 1, 2, and 3, respectively, and 0.7, 0.6 and 0.8‰ for the C sites 1, 2, and 3, respectively in 13C COS/OIA experiments (Table 3).

COS/OIA WURST-20

4

decoupling offset (ppm) 0 -4

0

4

8

labeled acetic acid

-4

b

0 0

4

decouplong offset(ppm)

3.3.2.4. n-heptane. Quantitative 13C NMR with COS/OIA decoupling sequence was conducted for the n-heptane sample that was previously measured at University of Nantes (Julien et al., 2016). The longest T1 time was 6 s, and the delay time was set at 60 s. The scan number was selected at 180 to obtain the smallest S/N > 1200. The results show that the terminal carbon site is depleted in 13C compared to other three middle carbon sites (Table 4 & Fig. 6). We have processed our data with Perch software with two different curve-fitting optimization methods (dispersion only and dispersionasymmetric-Gaussian), while the Nantes group used the dispersion only optimization. With the same processing procedures (with dispersion optimized only), the results from the two labs matched very well with very comparable standard deviation (Table 4 & Fig. 6). Comparing the two different processing procedures (dispersion optimized only vs. dispersion, Gaussian and asymmetric all optimized), the results were very close, while the dispersion only processing procedure had better standard deviation. With all three parameters (dispersion-Gaussianasymmetric) optimized, the fitting quality was naturally better with smaller RMS values than that with dispersion only processing.

-2

COS/OIA WURST-20

labeled ethanol

-4 ΔCH3(per mil)

Fig. 2. Relationship between decoupling offsets and isotopic compositions of methyl carbon sites relative to bulk molecule isotopic compositions (ΔCH3) in 13 C labeled acetic acid (a) and ethanol (b), using COS/OIA and WURST-20 decoupling sequences. ΔCH3 = (SCH3 / Stotal) / 0.5–1) ∗ 1000 (see Eqs. (10) and (16)).

central and terminal sites, respectively. The two 2H peaks are well separated, while the two 13C peaks are slightly overlapped (Fig. 3). 13Csatellite peaks of the two 13C peaks are not symmetric due to a secondorder coupling effect and they were not included in curve-fitting with the Perch software as discussed above. This propane has a depleted 2H isotope ratio at the center site compared to the terminal site (Table 1). The precision for the center H site ( ± 8.3‰) is worse than that of the terminal H's ( ± 2.8‰), because of the different numbers of hydrogens at the two sites (2 vs. 6). The results of 13C quantitative NMR using the two different decoupling sequences (WURST-20 and COS/OIA) were consistent with similar precision (0.3–0.6‰).

4. Purification of propane from natural gases 4.1. Sample collection and purification Pre-evacuated commercial propane tanks (Bernzomatic) were used to collect samples of natural gases in oil-gas fields and store them in the laboratory. The propane tanks can contain ~17.8 L gas with ~240 psi (16.5 bar) maximum gas pressure, which is controlled by a pressure release valve. A stainless-steel flexible tube with a pressure gauge was connected to the propane tanks through a Quick-Connects (Swagelok). A 2″ or 1″ male NPT connection is attached in the other end of the tube and a 15-μm filter (Swagelok) is inserted between. At a wellhead in oil fields, a flexible tube with the NPT connection is attached to one of outlet valves. The flexible tube was flushed to remove air and possible liquid petroleum in the outlet valve system and the sampling tube. Then, propane tanks were slowly filled with the natural gas sample until pressure equilibrates. Gas pressure at wellhead ranges up to 150 psi (10.3 bar). The sampling at a wellhead usually takes only 10–15 min. A propane component of the natural gas samples collected from oilgas fields was separated and purified in the laboratory, using a vacuum line with a variable-temperature cold trap. One aliquot of natural gas (about 2 L at 8 bar) was expanded from the propane tank by passing slowly through a series of chemical traps to remove H2S, CO2, and H2O components, using SULFURTRAP H2S Scavenger (Chemical Products),

3.3.2.2. n-butane. Quantitative 2H NMR (WALTZ-16) and 13C NMR (WURST-20 and COS/OIA) were conducted for the n-butane sample (Fig. 4). The longest 2H T1 time was 3.2 s and the delay times were set as 30 s. The longest 13C T1 times were 4.0 and 4.2 s, and the delay times were set 41 and 45 s for WURST-20 and COS/OIA 13C experiments, respectively. For 2H measurements, the scan number was 1000, and the measured S/N values were larger than 400 and 600 for the central and terminal hydrogen sites, respectively. For 13C measurements, the scan number was 192 with the S/N > 3000 for both carbon sites in the WURST-20 experiments, while the S/N > 1200 for both carbon sites with 96 scans in the COS/OIA experiments. The spectra of n-butane show that both 2H and 13C peaks are well separated (Fig. 4). The center site is slightly enriched in both 2H and 13C compared to the terminal site in the sample (Table 2). The precision was 1.1‰ and 0.8‰ for the center and terminal H sites, respectively, and 0.9‰ for 13C COS/OIA experiments. The 13C experiments using the two different decoupling sequences yield comparable results. 5

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Hterminal

a

Hcenter

Hterminal

Hcenter

b

Cterminal

Ccenter

Ccenter

Cterminal

Fig. 3. 2H (a) and 13C (b) NMR spectra (blue line) and Perch fitting (red line) of propane with residual (green line). The lower panels are close-up NMR spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Non-condensable gases, including air and methane, were pumped away. Then, the C2+ components were transferred to a variable-temperature cold trap to separate C2+ gas components cryogenically according to their vapor pressures at given temperatures. The variabletemperature cold trap, designed after Des Marais (1978), is made of Pyrex glass, consisting of a liquid N2 cold trap outside and an inner trap wrapped with thin Cu foil and heating wires. Two type-K(J) thermocouples are attached at bottom and middle of the inner the cold trap. Helium slowly and consistently flows through the annular space. This variable-temperature trap can control temperature between −190 °C to room temperature within ± 1 °C. First, ethane was separated from other C2+ components at around −158 °C. Then, propane was released from the trap at around −137 °C. A small aliquot was taken at each separation step and its gas compositions were analyzed by GC-FID or

Table 1 13 C and 2H position-specific deviations of a commercial propane. Site

Center Terminal

2

H with WALTZ-16 (n = 10)

13

C with WURST-20 (n = 10)

13 C with COS/OIA (n = 10)

Δi/‰

std/‰

Δi/‰

std/‰

Δi/‰

std/‰

−19.8 6.6

8.3 2.8

−1.6 0.8

0.6 0.3

−0.3 0.2

0.6 0.3

Soda Lime (Fisher Scientific), and Drierite (W.A. HAMMOND DRIERITE), respectively. Then, liquid N2 cold traps were used to collect C2+ (C2-C5) hydrocarbon components from the remaining gas. At liquid N2 temperature, methane vapor pressure is relatively high (~13 mbar).

6

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a

H1 H2

H2

b

H1

C2

C1

C2

Fig. 4. 2H (a) and 13C (b) NMR spectra (blue) and Perch fitting (red) of n-butane with residual (green). The lower panels are close-up NMR spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

components that are previously separated from a sample of natural gases and the neat reagent propane (71.7 mbar in 3.2 L flask), whose bulk and position-specific isotope compositions are already determined (see Section 3.3.2). This synthetic sample of natural gas was processed by the same separation and purification processes as for natural samples. After separation and purification, 25.4, 73.8, and 14.6 mbar in 3.2 L flask of C2, C3, and C4+, were collected, corresponding to the yields of 84, 103, and 100%, respectively. The purity of separated propane component was about 93.6% and the remaining fraction is composed of 3% C2, 1.4% isoC4, 1.8% nC4 and trace of C1 and C5+. The yield of the introduced propane was about 95%. The NMR spectra of propane purified from the synthetic sample shows that it contains impurities of ethane, n-butane and trace amount of other hydrocarbons (Fig. 7). For 2H quantitative NMR spectra, a peak from a small fraction (3%) of ethane is clearly seen on the right shoulder of the terminal site 2H peak of propane. Although the two peaks are partially overlapped, they were successfully deconvoluted with Perch software. Peaks from butane (1.4% isoC4 and 1.8% nC4) were too small to be observed because of its low concentration and relatively low sensitivity of 2H NMR spectroscopy. Therefore, even if they were very closely overlapping with propane peaks, their

Table 2 13 C and 2H position-specific deviations of a commercial n-butane. Site

1 2

2

H with WALTZ-16 (n = 12)

13

C with WURST-20 (n = 9)

13 C with COS/OIA (n = 5)

Δi/‰

std/‰

Δi/‰

std/‰

Δi/‰

std/‰

−2.8 4.1

0.8 1.1

−3.1 3.1

0.7 0.7

−0.4 0.4

0.9 0.9

GC–MS to monitor purity. Usually this purification has to be repeated to obtain 95% or higher purity of a propane component. A typical sample size of propane required for the quantitative NMR technique is about 150 mL STP (6.8 mmol).

4.2. Verification of the purification protocol The separation and purification procedure of propane from natural gases discussed above was tested and verified, using a synthetic sample of natural gas. A synthetic sample was made by combining C2 (30.1 mbar in 3.2 L flask) and C4+ (14.5 mbar in 3.2 L flask) 7

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a

H1 H2

H3

H3

H2

H1

b

C2

C1

C3 Acetone-d6 as solvent

C3

C2

C1

Fig. 5. 2H (a) and 13C (b) NMR spectra (blue) and Perch fitting (red) of n-pentane with residual (green). The lower panels are close-up NMR spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 3 13 C and 2H position-specific deviations of a commercial n-pentane. Site

1 2 3

2

Table 4 Carbon position-specific isotope compositions of n-heptane.

H with WALTZ-16 (n = 4)

13 C with WURST-20 (n = 7)

13

Δi/‰

std/‰

Δi/‰

std/‰

Δi/‰

std/‰

−21.4 25.1 14.1

1.1 3.7 8.0

−9.1 7.0 4.1

0.3 0.3 0.5

−5.5 3.7 3.5

0.7 0.6 0.8

C with COS/OIA (n = 5)

Site

1 2 3 4 Bulk

contributions can be safely neglected. For 13C quantitative NMR spectra, the peak from ethane is obvious along with some other small peaks in the spectra. However, all these peaks from the impurities are well separated from the propane peaks. Likely impurities (C2 and C4) do not overlap propane (Silverstein et al., 2005).

a b c

8

University of Nantes (n = 5)

Texas Tech University with D&G&Hb (n = 6)

Texas Tech University with Dc (n = 6)

δi/‰

std/‰

δi/‰

std/‰

δi/‰

std/‰

−38.17 −26.12 −25.23 −23.13 −28.88a

0.24 0.26 0.25 0.59

−36.84 −26.68 −25.66 −23.80 −28.88a

0.89 0.54 0.72 0.83

−36.83 −26.85 −25.62 −23.57 −28.88a

0.21 0.35 0.30 0.62

Bulk carbon isotope value was measured by University of Nantes. D&G&H-dispersion, Gaussian, and asymmetry. D-dispersion.

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the decoupling range, the more effective the decoupling becomes. This is because it becomes harder to decouple effectively large ranges of chemical shifts (Bayle et al., 2014). Our results with COS/OIA decoupling sequence show that a better accuracy (smaller ΔCH3 value) was achieved for 13C-labeled ethanol (chemical shifts, 39 ppm) relative to 13 C-labeled acetic acid (chemical shift, 157 ppm). A very similar relation has been observed before in a more systematic study (Bayle et al., 2014). Also, different NMR spectrometers and/or NMR probes yield varying levels of accuracy (Bayle et al., 2014). Given small ranges of chemical shifts for light hydrocarbons in this study (0.33, 11.8, 21.8, and 18.2 ppm for propane, n-butane, n-pentane and n-heptane, respectively), relative to those of ethanol and acetic acid (39 and 157 ppm, respectively), quantitative 13C NMR measurements with our Bruker 400 MHz NMR spectrometer and COS/OIA decoupling sequence very likely achieve sufficiently high precision (≤1‰). Measurements using WURST-20 showed somewhat mixed results, indicating that WURST-20 could also be effective if decoupling offsets are selected properly. Our tests of 13C quantitative NMR measurements of natural-abundance light hydrocarbons (propane, n-butane, n-pentane, and n-heptane) indicate that WURST-20 decoupling must be as effective as COS/ OIA for propane since the difference in the chemical shifts between the two carbon sites of propane is very small (0.33 ppm) and a proper decoupling offset was chosen (Table 1). The 13C experiments of n-pentane using the two different decoupling sequences yield the same patterns of intramolecular isotope fractionation, but the results from WURST-20 show slightly larger fractionations. With increasing chemical shifts (21.8 ppm), WURST-20 may become less effective in properly decoupling with minimum NOE (Fig. 2). In the subsequent measurements of 13 C quantitative NMR measurements of propane, COS/OIA inversegated, adiabatic proton decoupling sequence has been used. A comparison of two different curve-fitting optimization methods (dispersion only and dispersion-asymmetric-Gaussian) with Perch software showed that they produce very similar results of 13C quantitative NMR of n-heptane. The quality of fitting is naturally better with the dispersion-asymmetric-Gaussian optimization, rather than dispersion only. It is likely that any possible systematic errors in fitting with the dispersion only mode were largely cancelled out when calculating the ratio of two peak areas. In the following study of propane, we have elected to use the dispersion-asymmetric-Gaussian optimization of Perch to better deconvolute overlapping propane spectra with asymmetric satellite peaks (Fig. 3). We have shown that 2H and 13C NMR spectra of liquefied light hydrocarbons (C3-C5) with high S/N ratios can be obtained, using regular 5 mm OD tube, a LPV NMR tube, and the custom-made highpressure sapphire cells. High precision (≤ ± 10 and ≤ ± 1‰ for position-specific 2H and 13C isotope compositions, respectively) can be routinely achieved. Accuracy of similar levels is also likely achieved for 13 C quantitative NMR, based on (a) measurements of 13C-labeled compounds (ethanol and acetic acid), (b) good agreements between the two different, inverse-gated, adiabatic proton-decoupling sequences (WURST-20 and COS/OIA), and (c) inter-lab comparison of n-heptane with different NMR spectrometers and probes, which had previously been measured at University of Nantes (Julien et al., 2016). Our test with the synthetic natural gas demonstrates that the separation-purification procedure developed in this study can be used to determine accurately position-specific isotope compositions of propane from natural gases by the quantitative NMR methods presented above. Previously studies based on IRMS (Gao et al., 2016; Gilbert et al., 2016; Piasecki et al., 2016b) have reported 13C position-specific isotope ratios of propane without demonstrating accuracy of the data. To our knowledge, our study is the first that reports both carbon and hydrogen position-specific isotope compositions of natural propane with demonstrated accuracy and precision.

-30

bulk

13

Ci (per mil)

-24

University of Nantes TTU with D&G&H TTU with D only

-36

C1

C2

C3

C4

position in n-heptane Fig. 6. Comparison of 13C position-specific compositions of n-heptane sample from two labs: D-dispersion; D&G&H-dispersion, Gaussian, and asymmetry.

Bulk isotopic compositions of C1 to C3 hydrocarbons in the natural gas samples were also measured. Natural gas samples (10 μL for C1 and C2, 50 μL for C3) were manually withdrawn from serum sample vials and injected into a GC-C-IRMS system using a gas-tight syringe. The system is comprised of TRACE GC Ultra (Thermo Scientific) with a PoraPlot Q capillary column (Varian, #CP7551), GC Isolink (Thermo Scientific) coupled with ConFlo IV (Thermo Scientific) and Delta V IRMS (Thermo Scientific). The isotopic compositions of natural gas samples were calibrated with a lab standard of natural gas mixture, which has been calibrated with the natural gas 13C and 2H reference standards from USGS (Dias et al., 2016). The precision is 0.8 and 5‰ for δ13C and δ2H values, respectively. 4.3. Results of the purification of propane from natural gases Table 5 compares the bulk (δbulk) and position-specific 2H and 13C isotopic deviations (Δi) of the neat propane sample and the propane purified from the synthetic sample. The position-specific isotopic deviations agree well within the error ranges: < 0.5 and < 5‰ for Δi(13C) and Δi(2H), respectively. The bulk isotopic compositions also agree within 0.8 and 5‰ for δ13C and δ2H values, respectively. Therefore, the purity of ≥95% propane is sufficient and possible isotope fractionation during the separation and purification process of propane from natural gases is negligible. The test with the synthetic natural gas demonstrates that the separation-purification procedure developed in this study can be used to determine accurately position-specific isotope compositions of propane from natural gases by the quantitative NMR methods presented above (Section 3). 5. Discussion 5.1. Quantitative NMR methods NOE that is introduced by proton decoupling during the acquisition and relaxation delay non-uniformly interferes with the intensity of the 13 C NMR spectra, preventing quantitative measurements. Therefore, 1 H-decoupling must be performed efficiently and uniformly over the entire range of chemical shifts. We have tested two, inverse-gated adiabatic proton decoupling sequences (WURST-20 and COS/OIA) for 1 H-decoupling. The effectiveness of 1H-decoupling in quantitative 13C NMR is inversely correlated with the decoupling range: the narrower 9

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Hterminal

a

Hcenter Ethane

Hcenter

Hterminal

Ethane

b Cterminal

Ccenter acetone ethane

Ccenter

Cterminal ethane

Fig. 7. 2H (a) and

13

C (b) NMR spectra of propane purified from the synthetic sample. The lower panels are close-up NMR spectra.

5.2. Position-specific isotope compositions of propane from natural gases

Table 5 Bulk isotope compositions and 13C and 2H position-specific deviations of propane from the synthetic sample.

Bulk δ13C Bulk δ 2H 13 C center (Δcenter) 13 C terminal (Δterminal) 2 H center (Δcenter) 2 H terminal (Δterminal) a b

Neat propane/‰

Propane from synthetic sample/ ‰

−31.43 ± 0.53a (n = 7) −61.7b −0.3 ± 0.6 (n = 9)

−32.24 ± 0.77a (n = 3) −66.8b 0.2 ± 0.4 (n = 5)

0.2 ± 0.3

−0.1 ± 0.2

−19.8 ± 8.3 (n = 10) 6.6 ± 2.8

−23.7 ± 4.9 (n = 8) 7.9 ± 1.6

Natural gas samples were collected from several oil-gas fields, including the Permian Basin in West Texas, Woodford shale in the Arkoma Basin in Oklahoma, and Eagle Ford shale in southern Texas. Here, some select data are presented to illustrate and briefly discuss position-specific isotope compositions of propane from different sources. The samples include the neat propane of a commercial source (Matheson Tri-gas) and 5 propane gases that are recovered from natural gases: a conventional oil reservoir of the Spraberry Formation in the Permian Basin (PB-1), an unconventional gas field from Woodford shale in the Arkoma Basin (Woodford-1 and Woodford-2), and an unconventional oil field from Eagle Ford Formation (Eagle Ford-1 and Eagle Ford-2). Full discussion on each set of natural gases from the different locations (Permian Basin, Woodford, and Eagle Ford) will be presented in forthcoming manuscripts.

Relative to V-PDB. Relative to TTU standard. 10

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Table 6 13 C and 2H position-specific compositions of propane from natural gases. Sample name

Basin/Formation

Δcenter 13

ΔH2-1(per mil)

PB-1 Eagle Ford-1 Eagle Ford-2 Woodford −1 Woodford −2

Permian Eagle Ford Eagle Ford Woodford Woodford

C/‰

−2.8 4.4 5.2 2.3 1.7

Δterminal 2

H/‰

−140.4 28.7 14.4 19.7 −40.6

13

C/‰

1.4 −2.2 −2.6 −1.2 −0.9

50

ΔC2-1(per mil) 0 0

-50

-180

5

10

δ

46.8 −9.6 −4.8 −6.6 13.5

−36.1 −25.0 −24.2 −33.0 −34.7

H/‰

13

C/‰

Terminal δ H/‰

δ

−296 −84 −94 −112 −196

−32.0 −31.4 −31.8 −36.4 −37.2

2

13

C/‰

δ 2H/‰ −143 −119 −111 −135 −151

and they could have been thermally equilibrated. Equilibrium temperatures calculated from the carbon and hydrogen position-specific isotopic compositions of Woodford-1 propane are 244 ( ± 41) and 246 ( ± 55) °C, respectively, based on Piasecki et al. (2016a)'s calculations. Webb and Miller (2014)'s equilibrium calculations give temperatures of 215 ( ± 69) and 228 ( ± 106) °C from the carbon and hydrogen position-specific isotopic compositions, respectively. The calculated temperatures are generally consistent with the maturity of the shale (Ro = 1.6%), which in turn may correspond to 150–210 °C (Hunt, 1979). Based on Piasecki et al. (2016a)'s model, equilibrium temperatures of Eagle Ford-1 propane are at 153 ( ± 30) and 190 ( ± 33) °C for the carbon and hydrogen position-specific isotopic compositions, respectively Webb and Miller (2014)’s equilibrium model gives temperatures of 78 ( ± 39) and 136 ( ± 53) °C for the carbon and hydrogen position-specific isotopic compositions, respectively. The hydrogen position-specific isotope composition of Eagle Ford-1 propane indicates higher equilibrium temperatures because it falls slightly below the equilibrium areas (Fig. 8). Eagle Ford-1 sample is collected from an oil well from Eagle Ford that is located within an oil/wet gas window. Therefore, the maturity is likely in the range of Ro = 0.6–1.0% (correspond to 60–130 °C, Hunt, 1979), again matches reasonably well with the calculated temperatures. These results suggest that position-specific hydrogen and carbon isotopic composition of propane (Δ2–1) could serve as a single-molecule isotope thermometer. A process of kerogen/oil cracking to produce propane can occur in different modes of isotope fractionations, representing an instantaneous, small fraction to a cumulative, large fraction of the total propane produced during cracking. In fact, propane produced at late stages of a Rayleigh-type cracking process can possesses very negative value of both Δ2–1 (13C) and Δ2–1 (2H). Cracking temperature likely progressively increases upon further burial within sedimentary basins. These two samples (Eagle Ford-1 and Woodford-1) are also generally consistent with a Rayleigh-type fractionation process, either instantaneous or cumulative products of propane. However, a majority of the data (Fig. 8) clearly deviates from the trajectories by the simple equilibrium or Rayleigh-type kinetic cracking models. Many variables and processes in the formation, migration, and degradation of natural gases can affect the position-specific isotope compositions of propane. It is very likely that source organic compounds for thermogenic gases (kerogen and oil) have non-statistical distribution of carbon and hydrogen isotopes (Gilbert et al., 2013) and that source materials from different formations may have different patterns of position-specific isotope ratios. And thermogenic propane is expected to inherit position-specific isotopic compositions from its source materials. Maturity levels of source rocks may also impact position-specific isotopic compositions of propane. It is also very likely that position-specific isotope compositions are affected by post-production processes (e.g., diffusion, mixing, adsorption/desorption, and biodegradation). Adsorption/desorption of vanillin on a chromatographic column has been shown to induce large intramolecular isotope fractionations (Botosoa et al., 2008), while diffusion itself may induce only small fractionation, if any (Piasecki et al., 2016a). The large differences in Δ2–1 (13C) and Δ2–1 (2H) values between a conventional (Permian Basin) and unconventional (Woodford and Eagle Ford)

100

-5

Center 2

15

Equilibrium model (Piasecki et al., 2016a) Equilibrium model (Webb and Miller, 2014) Kinetics model (Ni et al., 2011) Woodford shale Commercial propane Permian basin Eagleford shale

-200

Fig. 8. Position-specific isotope compositions of propane (Δ2–1 = Δcenter − Δterminal) from different sources: Permian Basin, Eagle Ford, Woodford, and commercial sample. Three theoretical calculations (two equilibrium models and one kinetic cracking model) were also shown. Different points in the theoretical calculation plots represent different temperature: 300, 350, 400, 450, and 500 K (Piasecki et al., 2016b and Ni et al., 2011), 300, 400, 500, and 600 K (Webb and Miller, 2014).

The data show wide variations of position-specific isotope deviations: Δ2–1 = Δ2 − Δ1 = −187.2 to +38.3‰ (2H) and −4.2 to +7.8‰ (13C) (Table 6 and Fig. 8). The commercial propane sample has a small Δ2–1 (13C) value between the two sites, while the center site is slightly depleted in 2H relative to the terminal site. Sample PB-1 from a conventional sandstone reservoir in the Permian Basin is the only natural sample with negative Δ2–1 values for both 2H and 13C: the two hydrogen sites have a very large fractionation (−187.2‰). Four propane samples from unconventional gas (Woodford) and oil (Eagle Ford) shales have all positive Δ2–1 (13C) values, while the values of Δ2–1 (2H) vary widely. The field of position-specific isotope geochemistry of hydrocarbons is still in its infancy and our understanding is very limited. Theoretical calculations for equilibrium fractionation on position-specific isotopic compositions of propane (Cheng and Ceriotti, 2014; Pasecki et al., 2016a; Webb and Miller, 2014) indicate that 2H and 13C are enriched at the center site of propane at equilibrium and that the enrichments decrease with increasing temperature: the study of Cheng and Ceriotti (2014) is an exception, which predicted that 2H is depleted at the center site. Most natural gases are formed by thermal cracking of kerogen and oil. Kinetic isotope effects associated with CeC bond breaking of noctane (nC8) are also calculated during thermogenic formation of propane (Ni et al., 2011; Tang et al., 2005; Tang et al., 2000). This model also predicts that 2H and 13C are enriched at the center site of the produced propane and that the isotope effects decrease with increasing formation temperature (Fig. 8). The two processes, equilibrium vs. kinetic cracking, produce very similar trajectories (Fig. 8), but the kinetic isotope effects for position-specific isotope effects are much larger than those of equilibrium effects. Two samples (Eagle Ford-1 and Woodford1) fall very close to the predicted trajectories of the equilibrium model 11

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reservoirs may be due to differences in their migration processes and history. Microbial degradation processes with high selectivity of C and H positions would also alter position-specific isotope compositions of propane (Gao et al., 2016; Jaekel et al., 2014). Gilbert et al. (2017) reported a likely case of microbial influence where the δ13C value of the center site of propane from Southwest Ontario increases by around 10‰ with increasing bulk propane isotopic composition, while the δ13C value of the terminal site remains the same. The very low Δ2–1 (2H) value (−187‰) of PB-1 propane with low δ13C value of methane (−58.1‰) might have been caused by microbial contribution and/or degradation. With position-specific isotope compositions of propane (δi), which can be obtained from the values of δbulk and Δi (Eq. (13)), we can better understand these processes by monitoring the isotope effects at the reactive sites of propane.

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6. Conclusions We have developed a quantitative NMR method to determine position-specific carbon and hydrogen isotopic compositions of propane from natural gases with natural isotope abundances. The several tests conducted with 13C-labeled compounds (ethanol and acetic acids) and natural-abundance light hydrocarbons (C3, nC4, nC5, and nC7), using different NMR spectrometers and 1H-decoupling sequences, demonstrate that the quantitative 2H and 13C NMR measurement methods developed in this study can achieve high accuracy and high precision (< ± 1 and ± 10‰ for 13C and 2H, respectively) for position-specific isotope compositions of light hydrocarbons. This quantitative NMR technique for position-specific isotope compositions requires large samples of propane (150 mL STP), but is currently the only technique that achieves high-precision and high-accuracy analysis. We have also developed and tested a protocol for the collection and purification of propane from natural gases for the quantitative NMR technique, even from natural gases with < 1% propane. Position-specific isotopic compositions can also be determined on other light hydrocarbons (butane, pentane, and others), if sufficient amounts can be recovered from natural gases. A preliminary dataset of position-specific isotope compositions of propane from the different sources (commercial and natural gases of conventional and unconventional reservoirs) show very large variations. Possible new applications of the position-specific isotope compositions include a single-molecule isotope thermometry. Along with conventional, compositional and bulk-isotope data, position-specific isotope compositions of propane and other light hydrocarbons are expected to provide much deeper insight into the origin and history of light hydrocarbons and sedimentary geochemistry in general. To achieve this goal, systematic, synergistic studies of experimental, theoretical, and field-based approaches are needed. Acknowledgement We thank Gérald Remaud, Serge Akoka, and other scientists from the University of Nantes for their advices on method developments. We thank oil companies (Chesapeake Energy, Pablo Energy II LLC, and Napier Engineering) and Toti Larson (Bureau of Economic Geology, The University of Texas at Austin) for their help on sample collection. Financial support was provided by ACS-PRF (51786-ND2) and U.S. Department of Energy Geosciences program (DE-SC0016271). NMR facilities used in this study were supported by National Science Foundation Grant 1048553 (Texas Tech University) and 1428605 (West Texas A&M University). We would like to thank the reviewers for their valuable comments to improve the quality of the paper. References Abelson, P.H., Hoering, T.C., 1961. Carbon isotope fractionation in formation of amino acids by photosynthetic organisms. Proc. Natl. Acad. Sci. U. S. A. 47, 623.

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