474
Energy & Fuels 1994,8, 474-480
Free Aliphatic Acids in Sulfur-Rich Lacustrine Sediments: Their Origin and Relation to Hydrocarbons Assem 0. Barakat' Department of Chemistry, Faculty of Science, Alexandria University, P.O.Box 426, 21321 Alexandria, Egypt
Jiirgen Rullkotter' Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky Universitlit Oldenburg, P.O.Box 2503,D-26111 Oldenburg, Germany Received August 23, 1993. Revised Manuscript Received November 15,199P
Free fatty and hopanoic acids, and hydrocarbons of five organic- and sulfur-rich lacustrine sediments from Nardlinger Ries black shales (southern Germany), have been studied by gas chromatography and computerized GC-MS. The results indicate that Nordlinger Ries sediments contain abnormally high concentrations of those free acids (up to 57% of total extractable bitumen). Besides n-alkanoic, monomethyl, and isoprenoid acids common to any other immature sediments, high concentrations of C16 and c18 10-oxoalkanoic acids are unusual constituents of these sediments. The high concentrations of fatty acids in the bitumens are attributed to early diagenesis. The carbon distributions of the acids are consistent with an origin mainly from planktonic and microbial organisms. Only the acids in the sediments from well NR-30 located closer to the rim of the basin indicate a slightly elevated contribution from terrestrial sources. Distributions of n-alkanoic acids (C11-C22) in the samples from the center of the basin (well NR-10) show striking similarities to the n-alkane distributions, indicating that both series may, at least in part, be diagenetically related by decarboxylation of the acids. Isoprenoid and hopanoic acid distributions indicate common precursors but clearly show that decarboxylation is not a significant pathway to convert acids into hydrocarbons.
Introduction Fatty acids are major lipid components of most living organisms. Due to their geochemical stability, they are widespread chemical fossils and have frequently been considered as a source of petroleum hydrocarbons after decarboxylation.' The occurrence of fatty acids in sediments and petroleum has been intensely investigated to determine sources of organic to assess the depositional environment of sediments,a10 and to study their thermal maturity.ll-l3 Lacustrine sediments in the Nordlinger Ries, a former impact crater (20 km in diameter) in southern Germany
* Author to whom correspondence should be addressed. * Abstract published in Advance ACS Abstracts, January 1, 1994.
(1) Cooper, J. E.; Bray, E. E. Geochim. Cosmochim.Acta 1963, 27, 1113-1127. (2) Eglinton, G . In Advances in Organic Geochemiatry, 1968;Schenck, P.A., Havenaar, I., Eds.; Pergamon Press: Oxford, England, 1969, pp 1-24. (3) Cranwell, P. A. Chem. Geol. 1974,14, 1-14. (4) Seifert, W. K. In Progress in the Chemistry of Organic Natural
Products; Herz, W., Grisebach, H., Kirby, G. W., Eds.; Springer Verlag: New York, 1975; Vol. 32, pp 1-49. (5) Van Dorsselaer, A. Ph.D. Thesis, Universit4 Louis Pasteur, Strasbourg, France, 1974. (6) Schmitter,J. M.;Arpino,P. J.;Guiochon, G . Geochim. Cosmochim. Acta 1981,45, 1951-1955. (7) Mataumoto, G. I.; Watanaki, K. Org. Geochem. 1990,15,199-208. (8)Matauda, H.; Koyama, T. Geochim.Cosmochim. Acta 1977, 41, 777-783. (9) Mackenzie,A. S.;Wolff,G. A.;Maxwell,J. R. In Advances in Organic
Geochemistry, 1981; Schenck, P. A., de Leeuw, J. W., Lijmbach, G . W. M., Eds.; Pergamon Press: Oxford, 1983, pp 637-649. (10) Behar, F. H.; Albrecht, P. Org. Geochem. 1984,6, 597-604. (11) Suzuki, N.; Taguchi, K. Org. Geochem. 1984, 6, 125-133. (12) Kawamura, K.; Ishiwatari, R. Org. Geochem. 1984, 7, 121-126. (13) Kawamura, K.; Ishiwatari, R. Org. Geochem. 1985,8, 197-201.
formed during the Miocene, were deposited under saline conditions and contain highly bituminous laminite series with up to 25 % total organic carbon and 5 5% sulfur. The organic facies varies among the black shale layers, and maceral analysis from organic petrographic studies do not always match the distributions of extractable hydrocarbons but these comprise only a minor fraction of the total extracts.l4 The amount of bitumen of the Niirdlinger Ries black shales is unusually high considering the mild thermal history,14J5but probably related to the incorporation of high amounts of sulfur into the organic matter as known from carbonates. The present study was undertaken to clarify in detail the occurrence and distribution of free fatty acids in five organic- and sulfur-rich sediments from Niirdlinger Ries black shales. The purpose of this study was to investigate major constituents in the polar fractions of the extractable bitumens in order to obtain information about the biological origin of the organic matter and its depositional environment and diagenetic history, and to put further constraints on reasons for organic facies variations. The distribution of fatty acids will also be discussed in relation to the distribution of hydrocarbons in order to assess possible diagenetic relationships between both classes of compounds. (14) Rullkbtter,J.;Littke,R.;Schaefer,R. G. InGeochemistryofSulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Seriea 429; American Chemical Society: Washington DC, 1990; pp 1 4 H 6 9 . (15) Hollerbach, A.; Hufnagel, H.; Wehner, H. Geol. Bavarica 1977, 75, 139-153.
oaa1-0~241941250a-04~4~04.5010 0 1994 American Chemical Society
Sulfur- Rich Lacustrine Sediments
Energy & Fuels, Vol. 8, No. 2, 1994 475
Table 1. Sample Descriptions, Carbon Analyses, Lipid Yields, and Bulk Organic Matter Properties of the NBrdlinger Ries Sediments Examined" well
depth (m)
TOC (%)
extracta (mg/g of TOC)
S (%)
HI (mg HC/g of TOC)
terrigenous
NR-10 NR-10 NR-10 NR-30 NR-30
151.5 170.5 250.0 215.1 222.9
12.3 25.5 14.4 10.3 8.4
206 433 200 185 63
1.7 1.0 3.0 3.5 4.3
470 920 863 626 480
2.5 0.6 0.0 1.3 8.1
macer& % alginite ground mass ~
a
0.0 96.6 93.9 70.8 61.7
Data obtained from present work.
Table 2. Yields of Fatty Acids in the NBrdlinger Ries Sediments Investigated total concentrations % of GC of GC amenable esters % of acids amenable mg/g of mg/g sample inTEB4 esters rock ofTOC NR-10 (151.5 m) 28 52 3.700 30 NR-10 (170.5 m) 57 69 44.Ooo 170 NR-10 (250.0 m) 37 58 6.100 43 NR-30 (215.1 m) 10 29 0.500 5 NR-30 (222.9 m) 12 31 0.200 2 a TEB = total extractable bitumen.
Experimental Section Ndrdlinger Ries sediment samples investigated in this study were selected at various depths from the bituminous sediments in two boreholes of BEB Erdgas und Erd61GmbH. The geological background of the sediments has been discussed earlier." The sample descriptions and bulk organic matter properties are given in Table 1. All solvents (pro analysis grade from Merck, Darmstadt, Germany) were distilled in glass stills before use. Water was penta-distilled in an all-glass apparatus. All chemicals were analytical grade. Total lipids were obtained by extracting the dried and pulverized sedimenta with CHzClz in a Soxhlet apparatus for a period of 1 week. The resulting extract was reduced in volume by rotary evaporation and dried by a gentle stream of Nz. The solvent-extractable lipids were separated by medium-pressure liquid chromatography (MPLC) into nonaromatic hydrocarbons (saturated and olefiis), aromatic hydrocarbons, and heterocomponenta.16 Aliphatic acids were separated from the heterocomponenta by MPLC over a column of silica gel impregnated with KOH. Free aliphatic acids were washed down from the column with CH&lZ-HCOOH ( 9 9 1 ) , and distilled water was added followed by extraction and phase separation. The aqueous layer was further extracted with CHzC12, and the combined CHzClz extracta were washed with water, dried over anhydrous NazSOd, and concentrated with a rotary evaporator. The aliphatic acid extracta were derivatized with diazomethane in ether and the esters were analyzed by capillary gas chromatography (GC) and computerizedgas chromatography-mass spectrometry (GC-MS). GC was carried out on a Carlo Erba 6180 gas chromatograph equipped with aflame ionization detector, an on-column injector and a capillary column coated with SE-54(25 m X 0.32 mm, df = 0.25 pm). The temperature conditions for the GC were initial column temperature 60 "C for 1 min, 30 OC/min to80 OC followed by heating at 4 "C/min to 300 "C and held isothermal for 30 min. Helium was used as carrier gas at a flow rate of 3 mL/min. Components were quantitated using androstane as internal standard. All GC data were stored and processed by a multichrom on-line data system (VG Instruments). GC-MS analyses were performed on a Finnigan gas chromatograph coupled to a Finnigan MAT 5100 quadrupole mass spectrometer operating at 70 eV. Samples were injected onto a fused silica capillary column (50 m X 0.32 mm, d j = 0.4 rm) (16) Radke, M.; Willsch, H.; Welte, D. H. Anal. Chem. 1980,52,406411.
97.5 2.7 6.1 27.9 30.2
coated with CP-Sil-5. Helium was the carrier gas and the temperature programmed from 110 to 320 "C at 3 "C/min with an initial hold time of 2 min and a fiial hold time of 20 min. Spectra acquisition and processing were performed using an INCOS 2300 data system. Compounds were identified from retention time data, coinjection with standards, and comparison of mass spectral data with those of standards or published data.5J7J8 Mass fragmentography was also used to reveal homologousseries of compounds and structural isomers.
Results Free fatty acids isolated represent a remarkably high proportion of 12-57% of the total extractable bitumens. The summed amounts of the GC amenable acids range from 0.2 to 43.5 mg/g of rock and are greatest for the three samples from well NR-10. The highest value was obtained for the sample from 170.5m depth (-17 7% of total organic carbon). The yields of acidic products recovered and the proportions of the GC amenable acids are given in Table 2. Gas chromatograms of the methyl esters of the aliphatic acids are shown in Figure 1. The GC-MS results revealed that they contain mainly five compound types: n-alkanoic acids, branched alkanoic acids, 10-oxoacids, homologous and stereoisomeric hopanoic acids, and n-alkenoic acids. Each compound type identified from the GC-MS analysis is described in more detail below. The carbon number range, concentration, and estimated total proportion of each compound type are summarized in Table 3. Normal alkanoic acids are the most abundant compound type. Figure 1 shows remarkable differences in the distribution patterns of n-alkanoic acids, particularly in samples obtained from different locations. The distribution of n-alkanoic acids in samples from well NR-10 are characterized by a predominance of n- CISacid followed by n-Cla acid and a very strong even/odd carbon number predominance (CPI = 17-590). These features are consistent with those in recent surficial lacustrine sediments.lS2l The distributions of n-alkanoic acids are significantly different in samples obtained at 215.1 and 222.9 m depth from well NR-30, showing maxima at n-& and n-Cz2 acids with a significantly lower even/odd carbon number predominance (CPI = 6.4 and 4.0, respectively). Two series of branched alkanoic acids were found; these are isoprenoid acids and is0 and anteiso acids. The relative abundances of these two types of branched acids differ strongly; they are more abundant in those N6rdlinger Ries sediments containing substantial amounts of well preserved algal bodies (cf. maceral composition in Table 1). (17) McLafferty, F. W. A d . Chem. 1959, 31, 82-87. (18) McEvoy,J. Ph.D. Thesis,Univemity of Bristol, Brietol,U.K., 1983. (19) Brooks,P.W.;Eglinton,G.;Gaakell,S.J.;McHugh,D.J.;Maxwell, J. R.; Philp, R. P. Chem. Geol. 1976,18, 21-38. (20) Meyers, P. A.; Takeuchi, N. Org. Geochem. 1979,1, 127-138. (21) Kawamura, K.; Ishiwatari, R. Chem. Geol. 1985, 51, 123-133.
Barakat and Rullkotter
476 Energy & Fuels, Vol. 8, No. 2, 1994 NR-10 (151.5 m) 3700 mglg rock 3 0 mglg TOC LIH = 1.92
140
'E
-
? 100
c
80
P
> E
100
i e
.-m
.-c>: v)
22
$ 60
-
OI
C
e
5
NR-10 (170.5 m) 44000 mg/g rock 170 mg/g TOC LIH = 11.4
120
60
-
218
l4 12
20
-.
40
\
24
20
J . ,I.
0
E
0
.-5 250 2
16
0
w
- 150
c!
C
26
0
I
22
B
:11 \;: [ e
II 10 10
50
24
70
50
30
28
90
50
30
10
21
NR-30 (222.9 m) 200 mglg rock 2 mglg TOC LIH = 0.12
-
100
70
9(
Rettention time, min
Retention time, min
30
I
32
I d
> E
2r
20
24 2$30
10
30
50
70
90
Retention time, min
50
55
60
65
70
Retention time, min
Figure 1. Capillary column gas chromatograms of the methyl esters of the carboxylic acid fractions. Normal alkanoic acids are indicated by number of carbon atoms, I = isoprenoid acids, A = unidentified, IS = internal standard (androstane). Hopanoic acids are identified in Scheme 1. L/H = ratios of the amounts of lower molecular weight (C9-Cls) n-alkanoic acids to those of higher molecular weight (C20433) n-alkanoic acids.
The ratios of isoprenoids to total n-alkanoic acids vary from 0.0 to 0.1. Phytanic acid dominates the isoprenoid acid distributions followed by pristanic acid. Iso- plus anteiso- to n-alkanoic acid ratios vary between 0.00 and 0.05. Iso- and anteiso-Clb were always the major components in this series. Besides these two series, inspection
of the gas chromatogram of the sample obtained at 222.9 m depth from well NR-30 in Figure 1 shows a relatively intense peak (A) eluting between n-Cza and n-Cas acids. The mass spectrum of this unknown compound is characterized by a molecular ion at mlz 466, an intense ion at mlz 87, and fragment ions at mlz 74, 143, 256, and 284.
Sulfur-Rich Lacustrine Sediments
Energy & Fuels, Vol. 8, No. 2, 1994 477
Table 3. Summary of Fatty and Hopanoic Acids Identified by GC-MSAnalyses of Their Methyl Esters NR-10 (151.5 m) NR-10(170.5 m) NR-10 (250.0 m) NR-30(215.1 m) NR-30 (222.9 m) carbon m d g carbon number o f - w t number compoundtype range' TOC % range n-alkanoic acids Cg-Csz 25.2 86.1 Cir-Cas isoprenoid acids Cl&7, 2.5 8.6 C1e-Cz1 iso-alkanoicacids CII-CIS 0.9 3.1 anteiso-alkanoicacids Cll, c13, 0.4 1.4 ClS, c11 10-oxoacids n-C1a 0.2 0.8 t~-Cla - - n-cu - hopanoic acids
mg/g carbon mg/g of w t number ofTOC % range TOC 73.0 42.9 ciz-& - CZO
-
-
-
94.2 2.6
-
c1s Clb
-
55.3 n-Cla 1.5 n-C16
-
-
wt
%
carbon number range
mg/g o f - wt TOC 5% 3.8 88.8 0.32 7.5
n-Clal
0.1 n-Clal
NR-30 (215.1 ml 4.7% of total extract LIH = 2.05
1000
1
BOO
400
E i U
'3 300 E
U
200
15
I
100 r)
d10
Retention time, min
Retention time. min
31
250
> E
JR-30 (222.9 ml !.l% of total extract .IH = 0.46
a
200
.=i150
K
4d
s 100
I
33
65
70
75
Retention time, min Figure 3. Capillary column gas chromatograms of the saturated hydrocarbon fractions of the Nerdlinger Ries samples studied. Normal alkanes are indicated by number of carbon atoms, ph = phytane, pr = pristane, IS = internal standard (squalane). Labeled peaks are identified in Table 4. L/H= ratios of the amounts of lower molecular weight (C~&IS) n-alkanes to those of higher molecular n-alkanes. weight (cp0-C~)
from (a) common biological precursor(s) by different diagenetic pathways in the subsurface. Extended hopanoids (>C31)found in sediments are thought to arise from tetra- and pentahydroxybacteriohopane,3'3~3' which have been shown to be major constituents of bacteria and blue-green algae.38
Conclusions Free fatty and hopanoic acids are major constituents of extractable lipids in organic-matter-and sulfur-richblack shales from the NGrdlinger Ries, a former crater lake of Miocene age in southern Germany. Saturated normal
480 Energy &Fuels, Vol. 8, No. 2, 1994
1
loo
Barakat and Rullkatter Table 4. Identifications of Labeled Peaks in Figure 4
1
TIC
Peak a b C
d e
f B h i j
k
so -
df
,b 2000
=A
'k+, & P q : 2200
f: 2400
1 m n 0
Scan number
P
Figure 4. Partial total ion current and mass chromatogramsof mlz 191 and mlz 367 of the aliphatic hydrocarbon fraction of the
9 r
sample from NR-10 (222.9 m). Labeled peaks are identified in Table 4.
monocarboxylic acids (Cg-c32) are the dominant series, while isoprenoid acids (C14-C21, except CIS), iso- and anteiso-acids (C11-&), hopanoic acids (C90-C33) and a monounsaturated n-CIS acid are minor constituents. Moreover, 10-oxo acids (n-Cu and n-Cls) are present in unusually high concentrations in the samples from well NR-10. The carbon number distributions of the acids indicate a major contribution of autochthonous sources in samples from well NR-10, and mixed autochthonous and allochthonous sources in samples from well NR-30. The high concentrations of fatty acids in the bitumens as well as the isomer distribution of hopanoic acids point to an early diagenetic stage of the organic matter where hydrolytic breakdown of lipids from biomacromolecules(e.g., cell membranes) is still highly active. The distributions of n-alkanoic acids (C11-C22) in the samples from the center of the basin (well NR-10) show (36)Van Dorseelaer,A.; Ensminger,A.; Spyckerelle,C.;Dastillung, M Sieskind,0.; Arpino,P.;Albrecht,P.;Ourisson,G.Tetrahedron Lett. 1974,14, 1349-1352.
(37)Ourisson,0.; Albrecht,P.;Rohmer,M.Pure Appl. Chem. 1979,
51, 709-729.
(38)Rohmer, M.;Ourieeon, G.;Tetrahedron Lett. 1976, 40, 36333644.
8
compound hop-l7(21)-ene 17@(H),21a(H)-30-norhopane 17a(H),21fl(H)-hopane neohop-13(18)-ene 17fl(H),21@(H)-30-norhopane 17@(H),2la(H)-hopane (22S)-homohop17(21)-ene (22R)-homohop-l7(2l)-ene (22S)-bishomohop-l7(21)-ene gammacer-2-ene 17fl(H),21@(H)-hopane (22R)-bishomohop-l7(21)-ene (22S)-trishomohop17(21)-ene (22R)-trishomohop-l7(21)-ene 17@(H),218(H)-homohopane (22S)-tetrakishomohopl7(21)-ene (22R)-tetrakiehomohop-l7(21)-ene (22S)-pentakiehomohop-l7(21)-ene (22R)-pentakishomohop-l7(2l)-ene
formula ~&60
CzDH60 cas2 CSOH60
CZOH60 C&SZ (&iHsz C3lH62
CS!aHM C&60 c&62
CSZHM
CSHM CSHM Caih ~ d a s
CuHse C& CsHm
striking similarities to the n-alkane distributions, indicating that both series may, at least in part, be diagenetically related by decarboxylation of the acids. Terrigenous contribution of long-chain straight-chain compounds (>C19) provides only broad similarities of n-alkanoic acids and n-alkanes. Isoprenoid and hopanoic acid distributions indicate common precursors but clearly show that decarboxylation is not a significant pathway to convert acids into hydrocarbons. Finally, the occurrence and isomer distributions of extended hop-l7(21)-enes further confirm the deposition of Ndrdlinger Ries sediments under saline conditions.
Acknowledgment. We are grateful to BEB Erdgas und Erdol GmbH (Hannover) for providing the Niirdlinger Ries samples and for permission to publish the results. We thank Drs. M. Radke and R. G. Schaefer (KFA Jiilich, Germany) for providing facilities for extraction/liquid chromatography and gas chromatography. Technical support by A. Fischer, U. Disko, R. Harms, J. Htiltkemeier, B. Kammer, F. J. Keller, W. Ludtke, and H. Willsch (KFA Jiilich, Germany) is appreciated. A.O.B. thanks the Alexander von Humboldt Foundation for a grant.
