Release of organic nitrogen compounds from Kerogen via catalytic hydropyrolysis
© The Royal Society of Chemistry and the Division of Geochemistry of the American Chemical Society 2000
Received: 26 October 2000
Accepted: 1 December 2000
Published: 7 December 2000
High hydrogen pressure pyrolysis (hydropyrolysis) was performed on samples of solvent extracted Kimmeridge Clay Formation source rock with a maturity equivalent to ca. 0.35% vitrinite reflectance. We describe the types and distributions of organic nitrogen compounds in the pyrolysis products (hydropyrolysates) using GC-MS. Compounds identified included alkyl-substituted indoles, carbazoles, benzocarbazoles, quinolines and benzoquinolines. The distributions of the isomers of methylcarbazoles, C2-alkylcarbazoles and benzocarbazoles in the hydropyrolysates were compared to a typical North Sea oil. The hydropyrolysates compared to the North Sea oil, showed increased contributions from alkylcarbazole isomers where the nitrogen group is "exposed" (no alkyl substituents adjacent to the nitrogen functionality) and appreciable levels of benzo[b]carbazole relative to benzo[a]- and benzo[c]carbazoles. Hydropyrolysis is found to be an ideal technique for liberating appreciable quantities of heterocyclic organic nitrogen compounds from geomacromolecules. The products released from the immature Kimmeridge Clay are thought to represent a potential source of nitrogen compounds in the bound phase (kerogen) able to contribute to the free bitumen phase during catagenesis.
Hydropyrolysis refers to pyrolysis assisted by high hydrogen gas pressures (> 10 MPa). In an open-system, fixed-bed reactor configuration with an active catalyst dispersed in the sample, overall conversions are typically greater than 90% for petroleum source rocks with high selectivities to soluble tar products.[1, 2] Hydropyrolysis was first developed and applied as an analytical pyrolysis method for liberating covalently-bound hydrocarbon structures from kerogen by Love et al. Subsequent work has demonstrated the unique ability of the hydropyrolysis procedure to release higher yields of aliphatic biomarker hydrocarbons (including n-hydrocarbons, hopanes, steranes and methyl steranes) from immature kerogens in comparison with solvent extraction and conventional pyrolysis methods [4–8] with excellent retention of product structural and stereochemical features. A more recent study has shown that immature biomarker hydrocarbons can also be released from oil and source rock asphaltenes and that the biomarker profiles produced from hydropyrolysis are useful for source correlation purposes.
Until now, hydropyrolysis applications in organic geochemistry have exclusively utilised molecular constituents contained within hydrocarbon fractions (aliphatics and aromatics) to provide biogeochemical information. This study represents the first attempt at determining product distributions and potential uses of organic nitrogen-containing constituents of hydropyrolysates. While organic sulfur and oxygen functionalities are susceptible to reductive removal under hydropyrolysis conditions, organic nitrogen compounds (with the exception of amides) are much more inert. Appreciable nitrogen removal from heterocyclic aromatic compounds requires much more vigorous hydro-treatment than the experimental conditions encountered in our hydropyrolysis regime can provide. Hydropyrolysis is, therefore, ideal for liberating heterocyclic organic nitrogen compounds from geomacromolecules and the products formed should be highly representative of the molecular structures which are actually covalently-bound within the host macromolecular matrix.
The distributions of organic nitrogen compounds in source rocks and related crude oils have shown large and systematic variations due to fractionation effects associated with the migration process.[12, 19, 20, 23] Both Yamamoto and Li et al. described changes amongst nitrogen compound isomers based on steric effects favouring the preferential accumulation of nitrogen shielded (alkyl group adjacent to the nitrogen group) isomers relative to nitrogen exposed (no alkyl substituents at position C-1 and C-8 of carbazole III) isomers during petroleum migration. However, it is difficult to distinguish the effects of primary migration, expulsion and secondary migration in the carrier bed upon the nitrogen compound distributions by comparing source rocks and crude oils.
During this study we aim to use high hydrogen pressure pyrolysis to release organic nitrogen compounds from immature kerogen and identify the nitrogen compound types and distributions that may be present in source rock organic matter prior to catagenetic release from the bound phase as primary migration, petroleum expulsion and secondary migration. We report the preliminary results as part of our continuing studies on the origin and fate of organic nitrogen compounds in the geosphere.
General sample details for 2 Kimmeridge Clay Formation samples
Total organic carbon
Recovery of nitrogen compounds by solid phase extraction
The organic nitrogen compounds were isolated from the hydropyrolysates (KC3 and KC4) and KCF-derived North Sea oil (Miller Field), using a modified version of the solid phase extraction (SPE) methods described in Bennett et al. Following complete removal of CH2C12 and methanol, the hydropyrolysate residue was dissolved in n-hexane (agitated via sonication) and then transferred to a C18 non-endcapped SPE cartridge (Jones Chromatography, UK). The oil sample may be applied directly onto the SPE cartridge. Firstly, the aliphatic and aromatic hydrocarbon fraction is eluted with n-hexane (5 ml). The polar non-hydrocarbon fraction containing organic nitrogen compounds was recovered in CH2C12 (5 ml) and then the solvent reduced to minimum volume under nitrogen gas prior to GC-MS analysis.
Gas chromatography-mass spectrometry
The organic nitrogen compounds were analysed on a fused silica capillary column (HP-5; 95%/5%, methyl/phenyl silicone; dimensions, 30 m × 0.32 mm id, 0.25 μm film thickness (Hewlett-Packard)). The GC oven temperature program was 40°C held for 2 min, then 4°C min-1 to 300°C and held at the final temperature for 20 min.
Mass spectral characterisation of the organic nitrogen compounds was carried out using combined gas chromatography-mass spectrometry (GC-MS) on a Hewlett-Packard 5890 GC (using splitless injection) interfaced to a HP 5970B quadrupole mass selective detector (electron input energy 70 eV, source temperature 250°C).
Compound identification was based on relative retention times, comparison of mass spectra with published mass spectra and, where standard compounds were available, by co-chromatography.
Quantification of organic nitrogen compounds
A standard stock solution of N-phenylcarbazole was prepared in CH2Cl2. The standard was added to the organic nitrogen compound-enriched fraction prior to GC-MS analysis. Peak area integration during GC-MS analysis was by MASS LAB. The relative response factor (RRF) between N-phenylcarbazole and related compounds was assumed to be 1.
Results and discussion
General sample details of the two Kimmeridge Clay Formation shales employed for this study are shown in Table 1. The total organic carbon content and carbonate content identify differences in gross composition. With reference to vitrinite reflectance (ca. 0.35% Ro) the samples were assigned as "immature". The polar non-hydrocarbon fraction isolated from the hydropyrolysates was found to be enriched in aromatic nitrogen compounds of the pyrrolic (indoles, carbazoles and benzocarbazoles) and pyridinic (quinolines and benzoquinolines) structural types. The characterisation and determination of pyrrolic and pyridinic nitrogen compounds are described below.
Concentrations (μg g-1yrolysate or oil) of alkylcarbazoles, benzocarbazoles and alkylindoles in hydropyrolysates generated from solvent extracted Kimmeridge Clay Formation (samples KC3 and KC4) and Miller Field (North Sea) production oil
[b] [a] + [c]
[a] [a] + [c]
The partial mass chromatograms (m/z 181) showing the distributions of methylcarbazoles isolated from the hydropyrolysates recovered from samples KC3 and KC4 are displayed in Fig. 1a and 1b, respectively. The dominant methylcarbazole in sample KC3 is 4-methylcarbazole, although a shoulder from a neighbouring unidentified compound, also with a m/z 181 ion may contribute to the peak (Fig. 1a). The 4 isomers of methylcarbazole (excluding 9-methylcarbazole) are present in similar relative abundance in sample KC4 (Fig. 1b). For comparative purposes, the methylcarbazole distribution obtained from a typical North Sea oil (from the Miller Field) is shown in Fig. 1c. A characteristic feature of the methylcarbazole distribution in the Miller oil is shown by the elevated relative abundance of 1-methylcarbazole compared to other methylcarbazole isomers (compare Fig. 1b and 1c).
The partial mass chromatograms (m/z 195) showing distributions of the C2-alkylcarbazoles are displayed in Fig. 1d and lefor the hydropyrolysates. Identification of individual isomers was made by comparison with the assignments published in Bowler et al. Dramatic relative abundance variations amongst the C2-alkylcarbazole isomers in the m/z 195 mass chromatogram are seen between the hydropyrolysates (Fig. 1d and 1e) and the Miller oil (Fig. 1f). In general, the hydropyrolysates show reduced contributions from 1,8-dimethylcarbazole, with significant increase in contributions from the peaks labelled 11–17 (Fig. 1d and 1e), while the converse is found in Miller oil (Fig. 1f). The peaks labelled 11–17 in the m/z 195 mass chromatograms (e.g., Fig. 1e), represent alkylcarbazole isomers (except 1,2-dimethylcarbazole, peak 13) where the nitrogen functionality is "exposed" due to absence of alkyl substituents in positions C-1 and C-8 (i.e. adjacent to the nitrogen functionality) e.g., 2,7-(12) and 3,4-(17) dimethylcarbazole. Therefore hydropyrolysis of immature KCF has generated alkylcarbazole distributions with significant contributions of the "exposed" isomers, relative to a typical KCF-generated oil.
Although pyrolytically generated, the relative content of "shielded" C2-alkylcarbazole isomers to "exposed" isomers distributions of alkylcarbazoles in hydropyrolysates reveals similar features to alkylcarbazole distributions described in source rocks. The alkylcarbazole distributions generated during hydropyrolysis may provide an indication of the isomeric distributions present in source rocks prior to the processes associated with continuing source maturation.
The partial mass chromatograms (m/z 217) representing the distribution of benzocarbazoles are displayed in Fig. 1g and 1h for the hydropyrolysates. Coinjection studies performed with sample KC3 and authentic standards of benzocarbazoles led to the assignments of benzo[a]- (IV), benzo[b]- (V) and benzo[c]-carbazoles carbazoles (VI). The hydropyrolysate from sample KC4 contained just over 5 × more benzocarbazoles than sample KC3 (see Table 2). The interesting feature of the m/z 217 mass chromatograms (Figs. 1g and 1h) of the hydropyrolysates compared to the oil (Fig. 1i) is the enhancement of benzo[b]-carbazole relative to benzo[a]carbazole and benzo[c]carbazole also confirmed by the ratio of benzo[b]carbazole to both benzo[a]- and benzo[a]carbazole isomers abbreviated to [b]/([a] + [c]) (Table 2). Based on the literature, there are 4 possible processes which may give rise to an enhancement of benzo[b]carbazole relative to benzo[a]carbazole and benzo[c]-carbazole:
(i) Indicators of terrestrial contribution: in general, benzo[a]carbazole and benzo[c]carbazole are predominant in rock extracts and crude oils from clastic and carbonate marine source systems, whereas benzo[b]carbazole is either absent or occurs in relatively minor abundances.[20, 21] Harrison et al. reported increased contributions of the benzo[b]carbazole isomer relative to [a] and [c] isomers in Carboniferous coals of northwestern Europe, therefore, benzo[b]carbazole may indicate a terrestrial (coal derived) contribution to source organic matter.
(ii) Pyrolysis products: unusually high levels of benzocarbazoles, in particular high abundance of benzo[b]carbazole were identified in produced oils from the Jedburgh well (Canadian Williston basin). The oils were thought to be generated by high temperature, short-term pyrolysis of thermally immature organic matter. In this case, the high abundance of benzo[b]carbazole is concurrent with the high abundance of aromatic hydrocarbons with a linear structure relative to the angularly condensed structures, as indicated by high anthracene to phenanthrene ratio. Appreciable quantities of all benzocarbazole isomers and enhanced contributions from the benzo[b]carbazole were identified in hydropyrolysates from KCF during this study.
(iii) Thermodynamic relative stability: calculation using the semi-empirical PM 3-method (Adri van Duin, pers comm) produces the heats of formation for the following compounds: benzo[a]carbazole, 70.71 kcal mol-1; benzo[b]carbazole, 71.21 kcal mol-1; and benzo[c]carbazole, 69.93 kcal mol-1. Thus, benzo[b]carbazole is defined as the least stable isomer of the benzocarbazoles. Since hydropyrolysis is a conservative technique which favours the release of products with retention of structural and stereochemical features, the increased yields of benzo[b]carbazoles in hydropyrolysates may be a consequence of the "soft" pyrolysis conditions.
(iv) Fractionation processes: the polar nature of benzocarbazoles enables strong interactions with minerals/organic matter through hydrogen bonding. The solubility parameters estimated for benzocarbazoles are similar at 24.9 MPa1/2 indicating the strong affinity of benzocarbazoles to organic matter. The molecular shape also proved an important consideration during petroleum migration where the selective removal of the rod-shaped benzo[a]carbazole relative to the subspherical benzo[c]carbazole. The benzo[b]carbazole is a linearly fused aromatic structure, and therefore, in addition to hydrogen bonding interactions it may be amenable to a molecular sieving mechanism. As a consequence, benzo[b]carbazole may be selectively retained in the source rock and during petroleum expulsion may not be expelled as efficiently as benzo[a]- and benzo[c]carbazole. Similarly, both 1-methylcarbazole and 1,8-dimethylcarbazole ("shielded" isomers) are abundant isomers in the Miller oil, while the hydropyrolysates showed enhanced quantities of nitrogen "exposed" isomers compared to the "shielded" isomers. Therefore, using simple chromatographic theory, the preferential retention of nitrogen "exposed" isomers by the source rock may explain the different distributions of benzocarbazoles (and alkylcarbazoles) in the hydropyrolysates and the Miller oil.
Analysis of Miller oil failed to identify the presence of indoles. Since the indoles are known to be highly unstable, as exemplified by their ability to contribute to sediment formation in middle distillates and fuel oils, their absence from Miller oil may be due to alteration at some stage prior to sample preparation, in the subsurface and/or during sampling and storage. However, it appears that the indoles are more commonly associated with pyrolysis products and shale oil processing. Significant quantities of indoles have been identified in retorted Condor oil shale and Green River oils and 700–850°F distillate from a Californian crude oil. The alkylindole content of the hydropyrolysates represents a major component of the pyrrolic nitrogen species present in kerogen, and although not identified in Miller oil, the indoles represent a potential source of aromatic nitrogen species ultimately contributing to the nitrogen compounds formed during burial or on catagenetic release from the source rock.
(i) Hydropyrolysis of solvent-extracted Kimmeridge Clay Formation shales released significant quantities of nitrogen compounds including carbazoles, benzocarbazoles, quinolines, benzoquinolines and indoles.
(ii) Comparable amounts of both "shielded" and "exposed" isomers in the methyl- and C2-alkylcarbazoles as well as increased abundance of benzo[b]carbazole relative to benzo[a]-and benzo[c]carbazole typify the hydropyrolysates compared to a North Sea oil.
(iii) The indoles represent a major component of the nitrogen compounds in immature kerogen and through cyclisation and aromatisation of alkylindoles during further maturation, it may be possible to generate carbazoles.
(iv) The production of vast quantities of carbazoles and benzocarbazoles from the immature (0.35% Ro) KCF indicate that aromatic heterocyclic nitrogen compounds are formed at a relatively early stage of diagenesis.
(v) The nitrogen compounds produced during hydropyrolysis of the KCF provide a means to indicate me mass balance of nitrogen compounds through providing an initial nitrogen compound distribution prior to effects of primary migration, expulsion and secondary migration.
The authors acknowledge Prof. Colin E. Snape and the University of Strathclyde for use of the high pressure hydrogen pyrolysis rig. The POLARIS consortium (Exxon, Shell, Statoil and Texaco) is also thanked for funding. We thank Paul Donohoe and Kim Noke for performing GC-MS, Jane Barnard for providing Kimmeridge Clay samples and Prof. Steve Larter and Dr. Adri van Duin for useful discussions. Andrew Fleet (B.P.) is thanked for kindly donating the Miller platform crude oil.
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