The method of offline coupling of micro scale sealed vessel pyrolysis (MSSV-Py) and gas chromatography-isotopic ratio mass spectrometry (GC-IRMS) was developed using a purpose built gas sampling device. The sampling device allows multiple GC and GC-IRMS injections to quantify the molecular composition and isotopic evolution of hydrocarbon gases (n-C1 to n-C5) generated by artificial maturation of sedimentary organic matter. Individual MSSV tubes were introduced into the gas sampling device, which was then evacuated to remove air and filled with helium at atmospheric pressure. The tube was crushed using a plunger after which the device was heated at 120 °C for 1 min to thermally mobilize and equilibrate the generated gas products. Aliquots of the gas phase were sampled using a gas tight syringe and analysed via GC-FID and GC-IRMS. Hydrocarbon gas yields using this technique have been calculated and compared with those obtained previously by online MSSV pyrolysis of the same samples under the same conditions. The major objective of this study was to investigate the potential isotopic fractionation of generated gaseous hydrocarbons within the gas sampling device as a function of time and temperature. For this purpose several tests using a standard gas mixture have been performed on the GC-IRMS. The analyses showed no isotopic fractionation of C1–5 hydrocarbons within 1 hour, minor δ13C enrichment after 5 hours, and significant enrichment after 22 hours for all the compounds at a temperature of 120 °C

Berwick, Lyndon

Boreham, C.

Grice, Kliti

Horsfield, B.

Ladjavadi, Mojgan

English

Mojgan Ladjavardi

Lyndon J. Berwick

Kliti Grice

Chris J. Boreham

Brian Horsfield

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Organic Geochemistry

Rapid offline isotopic characterisation of hydrocarbon gases generated by micro-scale sealed vessel pyrolysis

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Additional Information NOTICE: This is the author’s version of a work that was accepted for publication in Organic Chemistry. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Organic Chemistry, Vol. 58, May 2013 DOI: http://dx.doi.org/10.1016/j.orggeochem.2013.03.003  Accepted ManuscriptNoteRapid offline isotopic characterisation of hydrocarbon gases generated by microscale sealed vessel pyrolysisMojgan Ladjavardi, Lyndon J. Berwick, Kliti Grice, Chris J. Boreham, BrianHorsfieldPII: S0146-6380(13)00054-5DOI: http://dx.doi.org/10.1016/j.orggeochem.2013.03.003Reference: OG 2932To appear in: Organic GeochemistryReceived Date: 21 December 2012Revised Date: 7 March 2013Accepted Date: 7 March 2013Please cite this article as: Ladjavardi, M., Berwick, L.J., Grice, K., Boreham, C.J., Horsfield, B., Rapid offlineisotopic characterisation of hydrocarbon gases generated by micro scale sealed vessel pyrolysis, OrganicGeochemistry (2013), doi: http://dx.doi.org/10.1016/j.orggeochem.2013.03.003This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customerswe are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, andreview of the resulting proof before it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.  1  Rapid offline isotopic characterisation of hydrocarbon gases generated by  1 micro scale sealed vessel pyrolysis 2 Mojgan Ladjavardi1, Lyndon J. Berwick 1, Kliti Grice1*, Chris J. Boreham2, Brian 3 Horsfield3 4 1 Western Australia Organic and Isotope Geochemistry Centre, The Institute for 5 Geoscience Research, Department of Chemistry, Curtin University, GPO Box U1987 6 Perth, WA 6845, Australia 7 2 Geoscience Australia, GPO Box 378 Canberra ACT 261 Australia 8 3 Helmholtz-Centre Potsdam GFZ German Research Centre for Geosciences, Organic 9 Geochemistry, Telegrafenberg, D-14473 Potsdam 10 *Corresponding author: K. Grice (email: K.Grice@curtin.edu.au) 11  12 Abstract 13 The method of offline coupling of micro scale sealed vessel pyrolysis (MSSV-Py) and 14 gas chromatography-isotopic ratio mass spectrometry (GC-IRMS) was developed using 15 a purpose built gas sampling device. The sampling device allows multiple GC and GC-16 IRMS injections to quantify the molecular composition and isotopic evolution of 17 hydrocarbon gases (n-C1 to n-C5) generated by artificial maturation of sedimentary 18 organic matter. Individual MSSV tubes were introduced into the gas sampling device, 19 which was then evacuated to remove air and filled with helium at atmospheric pressure. 20 The tube was crushed using a plunger after which the device was heated at 120 °C for 1 21 min to thermally mobilize and equilibrate the generated gas products. Aliquots of the 22 gas phase were sampled using a gas tight syringe and analysed via GC-FID and GC-23 IRMS. Hydrocarbon gas yields using this technique have been calculated and compared 24   2  with those obtained previously by online MSSV pyrolysis of the same samples under 25 the same conditions. The major objective of this study was to investigate the potential 26 isotopic fractionation of generated gaseous hydrocarbons within the gas sampling 27 device as a function of time and temperature. For this purpose several tests using a 28 standard gas mixture have been performed on the GC-IRMS. The analyses showed no 29 isotopic fractionation of C1–5 hydrocarbons within 1 hour, minor δ13C enrichment after 5 30 hours, and significant enrichment after 22 hours for all the compounds at a temperature 31 of 120 °C.  32  33 Keywords 34 micro scale sealed vessel pyrolysis, artificial maturation, stable isotopes, δ13C, methane, 35 hydrocarbon gas, isotopic fractionation 36  37 1. Introduction 38  The formation of petroleum (oil and gas) cannot be extrapolated from 39 geochemical data derived from naturally occurring basins (Michels et al., 2002). In 40 order to overcome the limitations of natural samples, artificial maturation experiments 41 using analytical pyrolysis techniques have been employed to simulate the formation of 42 gases (Michels et al., 2002; Dieckmann et al., 2006).  The study of 13C of the 43 hydrocarbon (HC) gases, particularly when combined with compositional data, is of 44 great importance to evaluate natural gas resources (Boreham et al., 1998). A range of 45 analytical pyrolysis techniques, coupled with online or offline gas chromatography-46 mass spectrometry (GC-MS), have been used to characterise organic macromolecules 47 and simulate geochemical reactions over laboratory timescales. Micro scale sealed 48   3  vessel pyrolysis (MSSV-Py) involves heating small quantities of sample (typically 0.1–49 10 mg) enclosed within glass tubes under controlled temperature (e.g. 250–350 °C) and 50 time (e.g. hours to days) conditions (Horsfield et al., 1989). However, this approach 51 does not allow replicate analyses from a single run of a sample required to obtain 52 reliable quantitative and stable isotopic compositions. Here we report the development 53 of offline MSSV-Py that enables multiple injections on GC and GC-IRMS in order to 54 measure the molecular composition and stable isotopic values of HC gases (C1–C5) 55 generated by artificial maturation of sedimentary organic matter.  56  57 2. Experimental 58 2.1. Kerogen sample 59  Kerogen was isolated from Hovea-3 well core from the basal Kockatea Shale, 60 Perth Basin Western Australia at 978.3–978.4 m, which is a representative of the 61 Sapropelic Interval, Hovea Member. The uppermost Permian interval of the Hovea 62 Member consists of inertinitic kerogen (Intertinitic Interval) whereas the Lower Triassic 63 Sapropelic Interval contains Type II algal-rich kerogen (Thomas and Barber, 2004; 64 Thomas et al., 2004; Grice et al., 2005, 2007). The sampled kerogen contained 61% 65 total organic carbon (TOC) and yielded Rock-Eval Hydrogen Index (624 mg/g TOC), 66 Oxygen Index (22 mg/g TOC) and a Tmax at 428 °C. 67             68 2.2. MSSV-Py 69  Artificial maturation experiments were performed using the principles of MSSV-70 Py (Horsfield et al., 1989). Aliquots of kerogen (0.87 and 2.75 mg) were loaded into 71 MSSV glass capillary tubes (5 cm x 5 mm i.d.), the void volume was filled with 72   4  thermally pre-cleaned (400 ºC overnight) glass beads (60–80 mesh) and the tubes were 73 flame sealed. The MSSV tubes were heated in an Al block at 300 ºC. The temperature 74 was increased at 0.7 ºC/min and the tubes were removed from the heating block at 389 75 ºC and 415 ºC, representing kerogen transformation ratios (TR) of 0.3 and 0.7, 76 respectively (Horsfield and di Primio, 2010). The method developed (see below) was 77 then applied to determine the gas yields of the artificially matured Kockatea shale 78 samples for comparison with previously published results from the same samples and 79 under the same conditions using online MSSV-Py (Horsfield and di Primio, 2010). 80  81 2.3. Offline gas sampling 82  The apparatus and procedure for offline sampling of the generated HC gases is 83 shown in Fig. 1. The individual MSSV tubes were placed between the side port and 84 main chamber of the gas sampler. The device was evacuated for 5 minutes by 85 connecting the side port of the sampler to a vacuum line using a custom Swagelok 86 fitting. The device was then filled with helium at atmospheric pressure after which the 87 MSSV tube was crushed by winding down the threaded glass stopcock. The main 88 chamber of the sampling device containing the crushed sample was heated to 120 ºC by 89 inserting through a vacant injector port of a Hewlett Packard 5890 GC oven. After 90 cooling to ambient temperature the gaseous products released were sampled using a gas-91 tight syringe and analysed by GC and GC-IRMS. 92  93 2.4. GC and GC-IRMS analyses 94  GC analysis of n-C1 to n-C5 HC gases was performed using a Hewlett Packard 95 5890 gas chromatograph equipped with a HP-PlotQ column (30 m x 0.32 mm i.d.; film 96   5  thickness 20 µm). Helium carrier gas was used at a constant pressure of 16 psi and a 97 split ratio of 20:1. The GC oven temperature was held at 200 ºC to achieve optimum 98 separation of gases and the flame ionisation detector at 250 ºC. A gas standard 99 consisting of C1–5 hydrocarbons (Table 1) was used to confirm baseline resolution of the 100 individual components and to check linearity, reproducibility and detection limits for 101 GC analysis. Calibration curves for quantitation of individual compounds were 102 developed by performing multiple injections of different volumes of the gas standard 103 from gas sampling bags filled at known pressure (1 atm). Peak areas were plotted as a 104 function of the number of moles of each component, calculated from the ideal gas law 105 (PV = nRT); where P is the partial pressure of the gas, V is the volume of gas injected, 106 and T is the laboratory temperature (22 °C). The calibration was repeated every 3 days 107 to ensure stability and reproducibility. The yields (µg/g sample) of generated HC gases 108 from the Kockatea shale kerogen were calculated by injecting a 200 µl aliquot from the 109 gas sampling device (total volume 1.6 ml) at known internal pressure (1 atm). 110  The carbon isotope analysis of the generated gases was performed using a 111 Micromass IsoPrime isotope ratio monitoring-mass spectrometer (irm-MS) interfaced to 112 a Hewlett Packard HP 6890 gas chromatograph (GC) with a GS-Carbon plot column 113 (30 m x 0.32 mm i.d x 3 µm film thickness). The GC oven was initially held at 70 ºC for 114 1 min, heated at rate of 20 ºC/min to 250 ºC, and held for 5 min. Helium was used as 115 carrier gas. Analyses were performed using two different sized gas loops (250 µl and 116 500 µl) with a split ratio of 20:1.  Stable carbon isotopic compositions are expressed in 117 ‰ (parts per thousand) relative to the international carbon isotope reference material 118 Vienna Pee Dee Belemnite (VPDB). 119  120   6  3. Results and discussion 121 3.1. Gas composition and yield 122  The procedure was applied to Kockatea shale kerogen samples, artificially 123 matured to transformation ratios (TR) of 0.3 and 0.7, to confirm reproducibility and 124 compare the HC gas yields using the offline technique with those obtained previously 125 by online MSSV pyrolysis (Horsfield and di Primio, 2010). Excellent reproducibility 126 was observed for duplicate offline pyrolysis experiments (Table 2). HC gas yield 127 increased with temperature due to increased conversion of kerogen to volatile products. 128 However, it is evident that the gas yields from the offline injection method are between 129 1.6–4.8 times lower than those obtained by online MSSV-Py. This is most likely related 130 to the different injection conditions employed for each method. Online MSSV-Py 131 involves crushing the sample tube directly inside a heated injector system (typically 300 132 ºC) under a constant flow of helium. In contrast, the offline approach involves heating 133 the crushed sample to 120 ºC to mobilise and equilibrate the gaseous components within 134 the sampling device prior to GC analysis. Adsorption of gas products to the exposed 135 pyrolysed kerogen residue may explain the lower yields. However, no significant 136 differences in gas yields were observed by adding a known amount of the standard gas 137 mixture to the device with and without kerogen residue. In addition, heating the main 138 chamber of the sampling device to higher temperatures (350 ºC) after crushing duplicate 139 MSSV pyrolysed kerogen samples also showed no significant difference in gas 140 distribution and yields at the higher temperature, indicating minimal, if any, adsorption 141 to the kerogen residue. We therefore attribute the lower gas yields compared with the 142 previous online study not to thermal transfer effects but to the absence of He carrier gas 143 flushing within the offline system. 144   7  3.2. 13C of gaseous hydrocarbons 145  The gas sampling device was filled with the standard gas mixture at atmospheric 146 pressure to investigate the potential for isotopic fractionation due to temperature and 147 time. Table 1 shows that no significant isotopic fractionation was observed for any C1–5 148 hydrocarbons as a result of the mild heating (120 ºC) used to mobilise/equilibrate the 149 gaseous products within the device. Fig. 2 shows changes in δ13C values for C1–4 150 hydrocarbons over a 23 h period using 250 µl and 500 l gas loops. δ13C values showed 151 no isotopic fractionation within 1 hour and only minor fractionation to more enriched 152 values within 5 hours. However, significant fractionation was observed after 22 h. The 153 δ13C values for all the compounds become heavier over time, most likely due to 154 preferential loss of the isotopically lighter components from the system through the 155 punctured septa of the gas sampling device. It is recommended that the generated gas 156 hydrocarbons should be sampled within 1 hour to avoid isotopic alteration. 157  158 4. Conclusions 159  An offline sampling technique coupled to GC and GC-IRMS was developed in 160 order to determine the gas yields and δ13C values of hydrocarbons (C1–5) generated from 161 MSSV-Py artificial maturation of kerogen from the Kockatea Shale (Perth Basin, WA). 162 The HC gas yields using this offline approach were slightly lower than previous results 163 for the same sample using online MSSV-Py, most likely due to the absence of He 164 carrier gas flushing within the offline system. GC-IRMS results showed no significant 165 isotopic fractionation of HC gases within 1 hour of crushing the MSSV sample, nor due 166 to the mild heating used to mobilise the gaseous components within the sampling 167 device. This offline sampling technique is rapid and inexpensive, enabling multiple 168   8  isotopic analyses of gases generated by laboratory maturation. The approach has great 169 scope for characterising gas formation in sedimentary systems. 170  171 Acknowledgements 172 The authors thank Geoff Chidlow and Stephen Clayton for GC-FID and GC-IRMS 173 technical support. Mojgan Ladjavardi thanks Curtin University, Geoscience Australia 174 and GFZ Germany for an International Postgraduate Award scholarship and Australia-175 China LNG project for a PhD top-up scholarship. KG acknowledges ARC for QEII 176 Fellowship and John de Laeter Centre for ARC LIEFP support.  177  178 Associate Editor – Cliff Walters 179  180 References 181 Boreham, C.J., Golding, S.D., Glikson, M., 1998. Factors controlling the origin of gas 182 in Australia Bowen Basin coals. Organic Geochemistry 29, 347-362. 183 Dieckmann, V., Ondrak, R., Cramer, B., Horsfield, B., 2006. Deep basin gas; New 184 insights from kinetic modelling and isotopic fraction in deep-formed gas 185 precursors. Marine and Petroleum Geology 23, 183–199. 186 Grice, K., Cao, C., Love, G.D., Bottcher, M.E., Twitchett, R., Grosjean, E., Summons, 187 R.E., Turgeon, S., Dunning, W.J., Jin, Y., 2005. Photic zone euxinia during the 188 Permian-Triassic superanoxic event. Science 307,706-709. 189 Grice, K., Nabbefeld, B., Maslen, E., 2007. Source and significance of selected 190 polycyclic aromatic hydrocarbons in sediments (Hovea-3 well, Perth Basin, 191 Western Australia) spanning the Permian-Triassic boundary. Organic 192 Geochemistry 38, 1795-1803. 193 Horsfield, B., Disko, U., Leistner, F., 1989. The micro-scale simulation of maturation: 194 Outline of a new technique and its potential application. Geologische Rundschau 195 78, 361–374. 196   9  Horsfield, B., di Primio, R., 2010. Oil and gas generation characteristics of a kerogen 197 concentrate and an oil asphaltene as inferred from Phase Kinetic analysis. GeoS4 198 Report 20100703. 199 Michels, R., Raoult, N.E., Elie, M., Mansuay, L., Faure, P., Oudin, J.L., 2002. 200 Understanding of reservoir gas compositions in a natural case using stepwise 201 semi-open artificial maturation. Marine and Petroleum Geology 19, 589–599. 202 Thomas, B.M., Barber, C.J., 2004. A re-evaluation of the hydrocarbon habitat of the 203 northern Perth basin. Australian Petroleum Production and Exploration 204 Association Journal 44, 59–92. 205 Thomas, B.M., Willink, R.J., Grice, K., Twitchett, R.J., Purcell, R.R., Archbold, N.W., 206 George, A.D., Tye, S., Alexander, R., Foster, C.B., Barber, C.J.,  2004. Unique 207 marine Permian/Triassic boundary section from Western Australia. Australian 208 Journal of Earth Sciences 51, 423-430. 209  210 Tables 211 Table 1. Composition and carbon isotopic values of the standard gas mixture and  212 stable carbon isotope measurements of the gases within the sampling device at ambient 213 temperature (22 ºC) and after heating to 120 ºC. Numbers in parentheses are standard 214 deviations; superscript indicates the number of replicate analyses. 215         216 Component Mole Fraction (%) δ13C (‰) 22 °C 120 °C Methane 75.9 -46 -45.6 [0.11]2   -45.6 [0.08]2 Ethane 9.14 -31.3 -30.9 [0.07]2 -30.9 [0.09]2      Propane 5.87 -35.1 -35.2 [0.16]2  -35.2 [0.18]2 iso-Butane 2.97 -29.3 -30.0 [0.21]2  -30.0 [0.15]2  n-Butane 3.01 -25.2 -25.3 [0.19]2 -25.3 [0.22]2 iso-Pentane 1.07 -27.1 -23.6 [0.31]2  -23.7 [0.26]2 n-Pentane 1.05 -23.8   CO2 0.989 -8.2     217  218 Table 2.  Comparison of gas yields (µg/g) from Kockatea shale kerogen at two different 219 transformation ratios (0.3 and 0.7) using offline (this study) and online (Horsfield and di 220 Primio, 2010) GC analysis. Duplicate values for the new offline technique are provided. 221  222   TR 0.3   TR 0.7    10  Component Offline  Online  Offline  Online  Methane 1471, 1423 2569 3868, 3978 6334 Ethane 850, 818 1968 2422, 2462 5881 Propane 916, 908 2123 2750, 2806 5837 iso-Butane 242, 220 617 718, 786 1432 n-Butane 424, 406 1462 1862, 1896 4222 iso-Pentane 174, 160 797 616, 644 1777 n-Pentane 244, 214 915 1743, 1761 2977  223  224 Figures 225  226 Fig. 1. Schematic representation of gas sampling device; (a) Sample loading and 227 evacuation (b) Sample crushing and gas analysis. 228  229  230  231  232  233  234  235  236  237  238  239 Fig. 2. Stable carbon isotope measurements of HC gases over time using (a) 500 µl and 240 (b) 250 µl gas loops. 241  242   Stopcock O-rings Side-arm Swagelok fittings Main chamber Sample in MSSV tube    Analysis by GC-FID & GC-IRMS Crushed sample Gas tight syringe     Time (h) (a) 500 µl (b) 250 µl δ13C (‰)  

Rapid offline isotopic characterisation of hydrocarbon gases generated by micro scale sealed vessel pyrolysis

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Rapid offline isotopic characterisation of hydrocarbon gases generated by micro scale sealed vessel pyrolysis

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