Energy & Fuels 1997, 11, 1237-1249
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Nitrogen Fate during Laboratory Maturation of a Type I Kerogen (Oligocene, Turkey) and Related Algaenan: Nitrogen Mass Balances and Timing of N2 Production versus Other Gases B. Gillaizeau,*,†,‡ F. Behar,‡ S. Derenne,† and C. Largeau† Laboratoire de Chimie Bioorganique et Organique Physique-UA CNRS 1381, Ecole Nationale Supe´ rieure de Chimie de Paris, 75231 Paris cedex 05, France, and Institut Franc¸ ais du Pe´ trole, BP 311, 92506 Rueil-Malmaison cedex, France Received April 15, 1997. Revised Manuscript Received July 28, 1997X
Parallel pyrolytic studies were carried out on an immature, ultralaminae-rich, type I kerogen (Go¨ynu¨k oil shale kerogen) and a related algaenan (isolated from the extant green microalga Scenedesmus communis). Isothermal, closed, pyrolyses were performed under argon, in sealed gold tubes, under various temperature/time conditions (ranging from 260 to 550 °C and 1 to 72 h). Nitrogen distribution was determined, through independent measurements, in the different fractions obtained from these pyrolyses. Such analyses allowed, for the first time to the best of our knowledge, to establish complete nitrogen mass balances for kerogen pyrolyses and to derive comparisons with an algaenan. Sulfur distribution in the pyrolysis products was examined as well. The nature and the abundance of the individual constituents of the gas fractions were also determined. The latter analyses provided information on (i) the timing of CO2, N2, CH4, and H2S production, upon thermal stress, to be expected from ultralaminae-rich kerogens and source algaenans and (ii) the composition of the gas fractions which should be generated from such kerogens during the different stages of thermal, natural, maturation.
Introduction Nitrogen, along with other heteroatoms like oxygen and sulfur, ubiquitously occurs in sedimentary organic matter.1 This element is however found in relatively low abundances, when compared to the two others, and levels ranging from 0.5 to 3 wt % of total organic matter are usually observed in low-rank coals, oil shales, and immature kerogens.1-8 Nitrogen is still less abundant in petroleum and typically accounts for less than 0.1 wt % of most crude oils.5,8,9 In spite of these low levels, the presence of nitrogen in fossil organic matter and fossil fuels is associated with a number of major problems. Thus, for example, nitrogen compounds can poison petroleum-reforming catalysts and make liquid fuels unstable upon storage due to tar formation. Health and environmental hazards are also related to the presence * Corresponding author. † Ecole Nationale Supe ´ rieure de Chimie de Paris. ‡ Institut Franc ¸ ais du Petrole. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984. (2) Mu¨ller, E. P.; Goldbecher, K.; Botnewa, T. A. Z. Angew. Geol. 1973, 19, 494-499. (3) Boudou, J. P.; Mariotti, A.; Oudin, J. L. Fuel 1984, 63, 15081510. (4) Oh, M. S.; Taylor, R. W.; Coburn, T. T.; Crawford, R. W. Energy Fuels 1988, 2, 100-105. (5) Baxby, M.; Patience, R. L.; Bartle, K. D. J. Pet. Geol. 1994, 17,2 211-230. (6) Krooss, B. M.; Littke, R.; Mu¨ller, B.; Frielingsdorf, J.; Schwochau, K.; Idiz, E. F. Chem. Geol. 1995, 126, 291-318. (7) Littke, R.; Krooss, B.; Idiz, E.; Frielingsdorf, J. AAPG Bull. 1995, 79, 410-430. (8) Barth, T.; Rist, K.; Huseby, B.; Ocampo, R. Org. Geochem. 1996, 24, 889-895. (9) Curiale, J. A.; Bromley, B. W. 17th EAOG Meeting Proc. 1995, 327-328.
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of such compounds in coals and oils, e.g. via formation of toxic combustion products like nitrogen oxides.10,11 In addition, nitrogen-containing constituents of oils are known to affect wettability in reservoirs.12 As a result, extensive studies were performed on the identification of nitrogen functionality in coals5,13-15 and of nitrogen compounds in crude oils,10,11,16-21 in shale oils,22 in source rock pyrolysates,17 and in coal liquefaction products.23 Nevertheless, the origin of nitrogen functions in fossil materials and the diagenetic mechanisms implicated in their formation are still far from being established. Indeed, almost no work dealing with organic nitrogen functionality in recent sediments had been carried out before the study of samples from the Peru upwelling area.24 Similarly, nitrogen functionality (10) Schmitter, J. M.; Vajta, Z.; Arpino, P. J. Adv. Org. Geochem. 1979 1980, 67-76. (11) Dorbon, M.; Schmitter, J. M.; Garrigues, P.; Ignatiadis, I. Org. Geochem. 1984, 7, 111-120. (12) Crocker, M. E.; Marchin, L. M. J. Petrol. Technol. 1988, 470474. (13) Bartle, K. D.; Wallace, S.; Perry, D. L. Fuel 1989, 68, 14501455. (14) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100-104. (15) Kelemen, S. R.; Vaughn, S. N.; Gorbaty, M. L.; Kwiatek, P. J. Fuel 1993, 72, 645-653. (16) Schmitter, J. M.; Arpino, P. J. Mass. Spectrom. Rev. 1985, 4, 87-121. (17) Bakel, A. B.; Philp, R. P. Org. Geochem. 1990, 16, 353-367. (18) Yamamoto, M.; Taguchi, K.; Sasaki, K. Chem. Geol. 1991, 93, 193-206. (19) Wilhelms, A.; Patience, R. P.; Larter, S. R.; Jorgensen, S. Geochim. Cosmochim. Acta 1992, 56, 3745-3750. (20) Li, M.; Larter, S. R. 16th EAOG Meeting Proc. 1993, 576-579. (21) Chen, M.; Larter, S. R.; Petch, G. S.; Bowler, B.; Aplin, A. C. 17th EAOG Meeting Proc. 1995, 288-289. (22) O ¨ nen, A.; Sarac, S. J. Anal. Appl. Pyrol. 1990, 17, 227-235. (23) Schiller, J. E. Anal. Chem. 1977, 49, 2292-2294.
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1238 Energy & Fuels, Vol. 11, No. 6, 1997
in kerogens has only been examined in the past few years and on a limited number of samples.8,25-29 Thus, the origin of nitrogen groups in sedimentary organic matter is still a matter of debate. A number of accumulations of natural gases throughout the world show high contents of molecular nitrogen, above 50% with some values up to almost 100%.6,7,30-32 The occurrence of these N2-rich natural gases represents a serious exploration risk and is still not fully explained. The potential sources of nitrogen in such accumulations have been extensively discussed6,7,30,32 and two main types of pathways are considered, namely a “direct” pathway from N2 of lithosphere or atmosphere and an “indirect” one via thermal degradation of the nitrogencontaining constituents of sedimentary organic matter. Recent studies showed that the latter pathway is likely to be the major process in the case of the N2-rich gas accumulations occurring in North Germany.6,7,32,33 A relatively large number of laboratory pyrolyses, aimed at examining the fate of nitrogen functions upon a thermal stress, were carried out, especially in the past few years.4,6-8,27,28,32-37 These studies generally focused on coals but some kerogens, oil shales and recent samples were examined as well. Nevertheless, the quantification of N2 evolution was only performed in a limited number of the above studies.6,7,32-34,37 Furthermore, a complete nitrogen mass balance was not established for any of these pyrolysis experiments. Accordingly, the general problem of the fate of nitrogen functions during sedimentary organic matter maturation is still far from being solved. In the present work we examined the pyrolysis products generated from an immature, type I, kerogen exhibiting a significant nitrogen content and from resistant biomacromolecules (algaenans) isolated from the extant microalga Scenedesmus communis. The examined kerogen was isolated from the Go¨ynu¨k oil shale, an important Oligocene lacustrine deposit from the North West of Turkey with estimated reserves of ca. 109 tons.38 In a previous study39 the origin and the mode of formation of kerogen in this important deposit were established. When examined by transmission electron microscopy (TEM) at high magnification, the Go¨ynu¨k kerogen appears to be chiefly composed of (24) Patience, R. L.; Baxby, M.; Bartle, K. D.; Perry, D. L.; Rees, A. G. W.; Rowland, S. J. Org. Geochem. 1992, 18, 161-169. (25) Derenne, S.; Largeau, C.; Casadewall, E.; Berkaloff, C.; Rousseau, B. Geochim. Cosmochim. Acta 1991, 55, 1041-1050. (26) Derenne, S.; Largeau, C.; Taulelle, F. Geochim. Cosmochim. Acta 1993, 57, 851-857. (27) Sinninghe Damste´, J. S.; Eglinton, T.; De Leeuw, J. W. Geochim. Cosmochim. Acta 1992, 56, 1743-1751. (28) Gelin, F.; Boussafir, M.; Derenne, S.; Largeau, C.; Bertrand, Ph. Lect. Notes Earth Sci. 1995, 57, 31-47. (29) Knicker, H.; Hatcher, P. G.; Scaroni, A. W. 17th EAOG Meeting Proc. 1995, 943-945. (30) Jenden, P. D.; Kaplan, I. R.; Poreda, R. J.; Craig, H. Geochim. Cosmochim. Acta 1988, 52, 851-861. (31) Whiticar, M. J. Org. Geochem. 1990, 16, 531-547. (32) Krooss, B. M.; Leythaeuser, D.; Lillack, H. Erdo¨ l Kohle-ErdgasPetrochem. 1993, 46, 271-276. (33) Idiz, E.; Kross, B. M.; Horsfield, B.; Littke, R.; Mu¨ller, B. 17th EAOG Meeting Proc. 1995, 1089-1091. (34) Klein, J.; Ju¨ntgen. Adv. Org. Geochem. 1971, 647-656. (35) Harrison, W. E. Chem. Geol. 1978, 21, 315-334. (36) Rohrback, B. G.; Peters, K. E.; Sweeney, R. E.; Kaplan, I. R. Adv. Org. Geochem. 1981, 819-823. (37) Boudou, J. P.; Espitalie´, J. Chem. Geol. 1995, 126, 319-333. (38) Pu¨tu¨n, E.; Akar, A.; Ekinci, E.: Bartle, K. D. Fuel 1988, 67, 1106-1110. (39) Gillaizeau, B.; Derenne, S.; Largeau, C.; Berkaloff, C.; Rousseau, B. Org. Geochem. 1996, 24, 671-679.
Gillaizeau et al.
accumulations of very thin lamellar structures (ca. 15 nm thick) associated into bundles. This type of structure was recently detected by TEM observations, often in large amounts, in a number of kerogens and termed ultralaminae.40,41 Parallel studies on extant microalgae, reviewed in ref 42, showed that a number of species, including S. communis, comprise very thin outer walls composed of algaenans, i.e., of highly resistant (nonhydrolyzable) biomacromolecules. Tight morphological and chemical correlations demonstrated that ultralaminae originate from the selective preservation of such algaenan-composed outer walls.25,39,43,44 Fossil ultralaminae and their source algaenans are characterized by the formation of specific nitrogen compounds, n-alkanenitriles, upon pyrolysis.25,39,43,44 Moreover, examination by solid-state 15N NMR spectroscopy of the algaenan isolated, via drastic base and acid hydrolyses, from S. communis grown on 15N-labeled nitrate26 showed a spectrum dominated by a peak corresponding to amide functions. The nonhydrolyzable nature of these amide groups shall reflect their steric protection within the macromolecular network of the algaenan. The presence of n-alkanenitriles in the pyrolysates of this type of algaenan and of derived kerogens is probably due to the thermal cleavage, followed by a fast dehydration, of some of the protected amide groups which are associated to n-alkyl chains,26 whereas the other protected groups likely correspond to protein-derived units. It was recently demonstrated39,45 that solid-state 15N NMR spectra can be also obtained with natural 15N abundance from soil organic matter, algaenans, and kerogens. The spectrum thus obtained, for the kerogen of the Go¨ynu¨k oil shale, is markedly different from the one of S. communis algaenan (Knicker, personal communication). Amide functions could be present in this fossil material, but nitrogen now chiefly occurs in heterocyclic aromatic structures (mainly pyrrolic groups with also, possibly, minor pyridinic units). S. communis algaenan and Go¨ynu¨k oil shale kerogen thus appear as suitable materials for examining nitrogen fate upon maturation. The present work is a part of a more general study, under progress, focused on this topic. The main purposes of this work were to (i) establish bulk mass balances, along with nitrogen distributions and complete mass balances (gases, soluble pyrolysis products, insoluble residues), via examination of the fractions obtained under various temperature/ time conditions, (ii) determine the nature and the abundance of the nitrogen-containing gases generated, and (iii) compare the timing of their production with the production of other gases. To this end, closed pyrolyses were performed in sealed gold tubes under an inert atmosphere since this method was previously shown46-49 to provide suitable conditions for simulating (40) Largeau, C.; Derenne, S.; Casadevall, E.; Berkaloff, C.; Corolleur, M.; Lugardon, B.; Raynaud, J. F.; Connan, J. Org. Geochem. 1990a, 16, 889-895. (41) Largeau, C.; Derenne, S.; Clairay, C.; Casadevall, E.; Raynaud, J. F.; Lugardon, B.; Berkaloff, C.; Corolleur, M.; Rousseau, B. Meded. Rijks Geol. Dienst. 1990b, 45, 91-101. (42) Derenne, S.; Largeau, C.; Berkaloff, C.; Rousseau, B.; Wilhelm, C.; Hatcher, P. G. Nanochlorum eucaryotum. Phytochem. 1992a, 31, 1923-1929. (43) Derenne, S.; Le Berre, F.; Largeau, C.; Hatcher, P. G.; Connan, J.; Raynaud, J. F. Org. Geochem. 1992b, 19, 345-350. (44) Derenne, S.; Largeau, C.; Hatcher, P. G. Org. Geochem. 1992c, 18, 417-422. (45) Knicker, H.; Fru¨nd, R.; Lu¨demann, H. D. Naturwissenschaften 1993, 80, 219-221.
Laboratory Maturation of a Type I Kerogen
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Table 1. Elemental Composition, Rock-Eval Analysis and H/C and O/C Atomic Ratios of S. Communis Algaenan and Go1 ynu 1 k Oil Shale Kerogena samples
C (%)
H (%)
O (%)
N (%)
S (%)
Tmax (°C)
S2 (mg HC/g)
HI (mg HC/g C)
H/C
O/C
algaenan kerogen
61.61 69.52
6.88 8.81
28.61 15.21
2.24-2.29 1.46-1.47
ndb 3.92
426 430
218 412
350 768
1.34 1.52
0.35 0.16
a
No significant amount of ash (
