40. chemical characterization of aromatic hydrocarbons, kerogen

5
40. CHEMICAL CHARACTERIZATION OF AROMATIC HYDROCARBONS, KEROGEN, AND HUMIC ACIDS IN DEEP SEA DRILLING PROJECT LEG 71 CORES 1 E. C. Copelin and S. R. Larter, Union Science and Technology Division, Union Oil Company of California, Brea, California ABSTRACT Forty three core samples from Sites 511 through 514 of DSDP Leg 71 were analyzed geochemically. The black shales at the bottom of Hole 511, in the basin province of the Falkland Plateau, contain an average of 1590 ppm ex tractable organic matter (EOM) and 120 ppm hydrocarbons. Whereas molecular type carbon number distributions of mono and polynuclear aromatic hydrocarbons and their sulphur and oxygen analogues in the black shale "aromatic hydrocarbon" fractions are very similar to those of many crude oils, other data—gas chromatography (GC) finger print, pyrolysis GC, visual kerogen analysis, H/C ratio—suggest the black shale section is thermally immature. To gether, these observations imply that many of the hydrocarbons were deposited with the original sediments or are dia genetic products of other biological compounds. Pyrograms of the humic acid and kerogen fractions from the black shale interval are typical of geopolymers derived from marine algal material. It appears that these humic acids and kero gens are derived from the same lipid stock. INTRODUCTION Forty three core samples from Sites 511 through 514 of DSDP Leg 71 were analyzed for extractable organic matter (EOM), aromatic hydrocarbons, humic acids, and kerogen. The amounts of EOM and total hydrocar bons and the molecular composition of the aromatic hy drocarbons were used to evaluate the potential of the sediments as source beds for crude oil. Pyrolysis gas chromatograms of the humic acid and kerogen fractions were recorded to define the source of the insoluble or ganic matter in the sediments. SAMPLE SEPARATIONS AND ANALYTICAL METHODS Diagrams of the separation procedures appear in Figures 1 and 2. All solutions and solvents, both organic and inorganic, were of the highest purity available. Centrifugation was used to separate sedi ments from water or organic solvent phases. Organic solvents were stripped from extracts or eluates by heating on a steam bath under a gentle stream of nitrogen. Each section of core was dispersed in and washed twice with water. Then the sediment was suspended in acetone and gently pulverized further in a Waring blender. Next it was extracted again with acetone and then twice more with a mixture of 70% toluene, 15% methanol, and I5°7o acetone (TMA). The organic extracts were combined and concentrated to yield the EOM fractions. Table 1 lists the EOM results as well as quantitative data about EOM subfractions. The EOM fractions were dissolved in a minimum amount of meth ylene chloride and deasphaltened in n pentane. Then the "oils" from the n pentane solution were elution chromatographed over silica gel (4% water). Hydrocarbons were eluted first with 2.5% toluene in n pentane and nonhydrocarbons eluted with 50% methanol toluene. Each hydrocarbon fraction was charged to a column of calcined alu mina. Saturated hydrocarbons (plus any olefins) were eluted first with 7% toluene in n pentane and aromatic hydrocarbons eluted with 50% methanol toluene. Each extracted sediment was treated with 50% HC1 to dissolve carbonates, then washed with water to remove soluble salts. Next the RESIDUE PYROLYSIS GC Figure 1. Procedures for separating kerogen and humic acid frac tions from DSDP Leg 71 samples. EXTRACTABLE ORGANIC MATTER 2.5% Tol/π C 5 SATURATES (OLEFINS) 7%Tol/n-Cs DEASPHALTENE ASPHALTENES AI 2 0 3 CHROMATOGRAPHY SiO x CHROMATOGRAPHY 50%Me0H/Tol CARBONS 50% MeOH/Tol I A P n M Δ T i r s I HIGH RESOLUTION MASS SPECTROMETRY. GC Ludwig, W. J., Krasheninnikov, V. A., et al., Init. Repts. DSDP, 71: Washington (U.S. Govt. Printing Office). Figure 2. Procedures for separating of aromatic and saturated hydro carbon fractions from EOM. sediment was suspended in 0.57V NaOH for a day. The residue was separated from the aqueous phase first by centrifugation and then by filtration through a 0.45 µm Millipore filter. After the pH was ad justed to 1.0, the solution was centrifuged to isolate the precipitated humic acids (Ishiwatari et al., 1977). Kerogens were isolated from some of the residues remaining from the Site 511 humic acid separations. After overnight treatment with hydrofluoric acid, the residues were washed with dilute HC1 and wa ter. The kerogens were separated from any remaining inorganic mate rial by flotation on 1.85 Sp. G. ZnBr 2 solution, after which they were solvent extracted and dried. Pyrolysis gas chromatograms (pyrograms) were produced by pyro lyzing up to 100 µg of very finely ground humic acid or kerogen at 1045

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Page 1: 40. Chemical Characterization of Aromatic Hydrocarbons, Kerogen

40. CHEMICAL CHARACTERIZATION OF AROMATIC HYDROCARBONS, KEROGEN,AND HUMIC ACIDS IN DEEP SEA DRILLING PROJECT LEG 71 CORES1

E. C. Copelin and S. R. Larter, Union Science and Technology Division,Union Oil Company of California, Brea, California

ABSTRACT

Forty-three core samples from Sites 511 through 514 of DSDP Leg 71 were analyzed geochemically. The blackshales at the bottom of Hole 511, in the basin province of the Falkland Plateau, contain an average of 1590 ppm ex-tractable organic matter (EOM) and 120 ppm hydrocarbons. Whereas molecular type-carbon number distributions ofmono- and polynuclear aromatic hydrocarbons and their sulphur and oxygen analogues in the black shale "aromatichydrocarbon" fractions are very similar to those of many crude oils, other data—gas chromatography (GC) finger-print, pyrolysis GC, visual kerogen analysis, H/C ratio—suggest the black shale section is thermally immature. To-gether, these observations imply that many of the hydrocarbons were deposited with the original sediments or are dia-genetic products of other biological compounds. Pyrograms of the humic acid and kerogen fractions from the blackshale interval are typical of geopolymers derived from marine algal material. It appears that these humic acids and kero-gens are derived from the same lipid stock.

INTRODUCTION

Forty-three core samples from Sites 511 through 514of DSDP Leg 71 were analyzed for extractable organicmatter (EOM), aromatic hydrocarbons, humic acids,and kerogen. The amounts of EOM and total hydrocar-bons and the molecular composition of the aromatic hy-drocarbons were used to evaluate the potential of thesediments as source beds for crude oil. Pyrolysis gaschromatograms of the humic acid and kerogen fractionswere recorded to define the source of the insoluble or-ganic matter in the sediments.

SAMPLE SEPARATIONS AND ANALYTICAL METHODS

Diagrams of the separation procedures appear in Figures 1 and 2.All solutions and solvents, both organic and inorganic, were of thehighest purity available. Centrifugation was used to separate sedi-ments from water or organic solvent phases. Organic solvents werestripped from extracts or eluates by heating on a steam bath under agentle stream of nitrogen.

Each section of core was dispersed in and washed twice with water.Then the sediment was suspended in acetone and gently pulverizedfurther in a Waring blender. Next it was extracted again with acetoneand then twice more with a mixture of 70% toluene, 15% methanol,and I5°7o acetone (TMA). The organic extracts were combined andconcentrated to yield the EOM fractions. Table 1 lists the EOM resultsas well as quantitative data about EOM subfractions.

The EOM fractions were dissolved in a minimum amount of meth-ylene chloride and deasphaltened in n-pentane. Then the "oils" fromthe n-pentane solution were elution-chromatographed over silica gel(4% water). Hydrocarbons were eluted first with 2.5% toluene inn-pentane and nonhydrocarbons eluted with 50% methanol-toluene.Each hydrocarbon fraction was charged to a column of calcined alu-mina. Saturated hydrocarbons (plus any olefins) were eluted first with7% toluene in n-pentane and aromatic hydrocarbons eluted with 50%methanol-toluene.

Each extracted sediment was treated with 50% HC1 to dissolvecarbonates, then washed with water to remove soluble salts. Next the

RESIDUE

PYROLYSIS GC

Figure 1. Procedures for separating kerogen and humic acid frac-tions from DSDP Leg 71 samples.

EXTRACTABLEORGANICMATTER

2.5% Tol/π C5

SATURATES(OLEFINS)

7%Tol/n-Cs

DEASPHALTENEASPHALTENES

A I 2 0 3 CHROMATOGRAPHY

SiOx CHROMATOGRAPHY

50%Me0H/Tol

C A R B O N S

50% MeOH/Tol I A P n M Δ T i r s I

HIGH RESOLUTIONMASS SPECTROMETRY.GC

Ludwig, W. J., Krasheninnikov, V. A., et al., Init. Repts. DSDP, 71: Washington(U.S. Govt. Printing Office).

Figure 2. Procedures for separating of aromatic and saturated hydro-carbon fractions from EOM.

sediment was suspended in 0.57V NaOH for a day. The residue wasseparated from the aqueous phase first by centrifugation and then byfiltration through a 0.45-µm Millipore filter. After the pH was ad-justed to 1.0, the solution was centrifuged to isolate the precipitatedhumic acids (Ishiwatari et al., 1977).

Kerogens were isolated from some of the residues remaining fromthe Site 511 humic acid separations. After overnight treatment withhydrofluoric acid, the residues were washed with dilute HC1 and wa-ter. The kerogens were separated from any remaining inorganic mate-rial by flotation on 1.85 Sp. G. ZnBr2 solution, after which they weresolvent extracted and dried.

Pyrolysis gas chromatograms (pyrograms) were produced by pyro-lyzing up to 100 µg of very finely ground humic acid or kerogen at

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Page 2: 40. Chemical Characterization of Aromatic Hydrocarbons, Kerogen

E. C. COPELIN, S. R. LARTER

Table 1. Organic geochemical data for Sites 511 through 514, DSDPLeg 71.

Sub-bottomCore/ Depth

Section21 (m)

(ppm)

HC HAC

Extractable Organic Matter(wt. %)

HC SAT AROM HET ASP

Hole 511

3-46-39-516-1

18-1

24-4

28-5

31-5

34-5

37-1

40-4

43-4

46-249-4

52-555-4

58-3

60-5

62-5

64-4

66-4

68-2

70-3

Hole 512

3-29-215-2

Hole 513

4-49-5

Hole 513 A

2-65-48-312-3

15-617-4

20-2

28-3

31-6

Hole 514

2-28-214-2

20-2

26-2

32-2

20.5

47.5

79.0

139.5

158.5

210.5

231.0

259.5

288.0

310.5

343.5

372.0

397.5

429.0

459.0

486.0

513.0

535.0

554.0

571.5

590.5

606.5

627.0

9.033.9

58.9

34.0

83.0

75.0

110.0

137.0

175.0

208.0

224.0

249.3

327.0

360.0

4.130.6

57.0

83.4

109.8

136.2

177130153245142384324161151124793119141548

2017246814111939978727

1314327

9342

1409196324216955025

34810520016917687

241415352522271011975142221137914

206123921686370

29146

2522

4722241523654169

2243714192011

6431180

c17501580234_682307214126122

0_155

12789228271598140

_

26512918089__17242060

4210—

3800__

141110141858624231501129687047623010579710

223221

2752

332425465540573233

64358111113

996101410504112433253620333826534636

172917

2548

311720382333442830

56237798

524444812119784325014244523344

534

24

27583271343

8121425

464957374834314542351732915332333314640564250

303954

4630

454652292733313639

314354525760

40403349348713271572392315201537594953355140

482925

2718

223023251827123228

52238373227

a All samples from interval 120-130/135 cm." Humic acids were not purified by dialysis.c Not isolated.

600 °C for 1 s using a Chemical Data Systems Pyroprobe 120-filamentpyrolyzer modified for use with a high resolution WCOT fused silicaGC column. The pyrolysis effluent was swept, via a 30:1 sample split-ter, to a 25 m × 0.2 mm ID fused silica capillary column coated with aSP 2100 liquid phase. Nitrogen was used as a carrier gas, the flow ratethrough the pyrolyzer adjusted to minimize secondary pyrolysis ef-fects. Following pyrolysis, the GC oven was programmed from 40-270 °C at 4°C/min. Similar conditions were used to record gas chro-matograms of the aromatic and saturated hydrocarbon fractions.Aromatic hydrocarbon fractions were analyzed in detail with a CEC-110B high-resolution mass spectrometer. Low-ionizing-voltage condi-tions were employed to measure the intensities of molecular ions from70-350 mass units with an accuracy of 5-10 millimass units. Thegeneral approach to computer processing and interpreting the massdata is like that described by Aczel and Lumpkin (1972).

RESULTS AND DISCUSSION

In the Initial Core Descriptions of DSDP Leg 71, theblack shales at the bottom of the Site 511 core were re-ported as being highly petroliferous, mature, and al-

600

100

Extractable Organic Matter(ppm)

1000

Figure 3. Concentrations of extractable organic matter in samplesfrom DSDP Leg 71.

ready containing some oil or gas. Our work shows (Fig.3) that this interval contains 10 times more EOM thanmost other Leg 71 sediments. The average EOM concen-tration is 1590 ppm, which is often considered adequatefor a crude oil source bed. However, the average hydro-carbon content of the black shale interval is 120 ppm,which is marginal compared to the amount of EOMpresent. Mainly because of these results, our geochemi-cal investigations of Leg 71 core samples were confinedpredominantly to organic matter from the black shalesection of the Site 511 core.

Molecular Analysis of AromaticHydrocarbon Fractions

The only sediment samples which yielded sufficient(>3 mg) aromatics to analyze by high-resolution, low-ionizing-voltage mass spectrometry were Cores 60through 70 from Hole 511. The molecular type-carbonnumber distributions of the aromatic hydrocarbons andthe S-and O-heterocyclic compounds in the aromaticsfraction from Section 511-66-4 are shown in Figure 4.The aromatic fractions from the other core samplesfrom the same interval had similar molecular type distri-butions. Aromatic molecular types from benzenes (Z-6)

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Page 3: 40. Chemical Characterization of Aromatic Hydrocarbons, Kerogen

Figure 4.sity.

Carbon Number

Molecular type-carbon number distributions of aromatic hydrocarbons and S- and O-heterocyclic compounds from Section 511-66-4. Ends of distribution curves are at zero inten-

-4

>

o

1nx

Page 4: 40. Chemical Characterization of Aromatic Hydrocarbons, Kerogen

E. C. COPELIN, S. R. LARTER

through naphchrysenes (Z-26) (Fig. 4) have distribu-tions which are like those found in crude oils (Rullkötterand Welte, 1981). In addition, there are sharp maximafrom C22 to C25 in the distributions for pyrenes (Z-22),tetraaromatics (Z-24), and naphchrysenes (Z-26). Thesemaxima are not caused by sterenes, because the accu-rately determined mass weights are those of polynucleararomatics, not sterenes, and the sterenes were elutedwith the saturated hydrocarbons in the elution chro-matographic separation steps. The most abundant sin-gle molecular type in Figure 4 is perylene (Z-28, C2oH!2),a common constituent of sediments (Aizenshtat, 1973).Alkyl derivatives of perylene are minor. Benzperylene(Z-32, C22H12) is present without homologs. The singlemolecular type for the Z-30 distribution has been tenta-tively assigned the structure of diacepyrene (C20H10).

Aromatic thiophenes and furans were identified inthe aromatic fractions, and their molecular distributionsare shown on the right side of Figure 4. The distribu-tions of the components are similar to those of manycrude oils, and the relative abundances of the varioussubtypes are very similar. Tribenzfurans (Z-22) are pres-ent, but tribenzthiophenes (Z-22) were not detected.

Gas chromatograms of the aromatic hydrocarbonfractions from the black shale interval of Site 511 werevery similar. Also, the complementary saturate fractionsproduced gas chromatograms which were generallyalike. An example (from Section 511-66-4) of a chroma-togram of each type is shown in Figure 5. The aromatichydrocarbon fractions were very complex mixtures, andthose few peaks that could readily be identified by Re-tention Index are so marked in Figure 5. The general ap-pearance of the saturates gas chromatogram shown inFigure 5 indicates the black shales may potentially be-come crude oil source rocks. The pristane/phytane ratiois about 0.5, and a complex assemblage of steroidal hy-drocarbons is present between C25 and C32. The rela-tively high abundance of /7-alkanes in the C17-C24 regionindicates a predominantly marine source. The high CPI

PeryleneBenz-perylene

25 30n-Alkane Carbon Number

Figure 5. Gas chromatograms of aromatic and saturated hydrocar-bon fractions from Hole 511 black shale samples.

of the fl-alkanes in the C25-C31 region suggests a rela-tively low rank level for the organic matter in this sec-tion. This may explain why the gas chromatographicfingerprint of the aromatic hydrocarbon fraction in Fig-ure 5 is not like that from a thermally mature sedimentor petroleum. These observations of low rank level oforganic matter, together with similar results in the kero-gen and humic acid work to be described, suggest thatthe hydrocarbons in the black shale section of Hole 511are not entirely of a thermal origin. They appear to bepredominantly hydrocarbons preserved in the sedimentssince their initial deposition.

Pyrolysis Gas Chromatography of Humic Acidand Kerogen Fractions

Figure 6 shows pyrolysis gas chromatograms (pyro-grams) of the humic acid and kerogen fractions fromSections 511-68-2 and 511-64-4, respectively. The pyro-grams are typical for the humic acids and kerogens fromthe interval, rich in organic matter, at the bottom ofHole 511 core, both materials having pyrolysates char-acterized by abundant products, particularly w-alkenesand H-alkanes, with less than 20 carbon atoms. Althoughthe pyrograms do contain higher molecular weight ma-terial, it represents only a minor fraction of the wholepyrolysate. A dominance of relatively low molecularweight aliphatic materials in the pyrograms is typical ofgeopolymers derived from marine algal material (van deMeent et al., 1980).

The peak eluting near 1730 (Retention Index) posi-tion in both pyrograms in Figure 6 is tentatively identi-fied as prist-1-ene (Larter et al., 1979). A CH isoprenoidalkene is similarly identified eluting between the «-C13alkene and alkane. A like abundance of both C14 andC19 isoprenoid moieties was noted in pyrograms of ma-rine, algally derived kerogen from the Cariaco Trench(van de Meent et al., 1980), and indeed the pyrogramfingerprints from both studies are very similar.

HUMIC ACIDS511-68-2

I I I I I I I I I I I I I I I I I I I15 20

π-Alkane Carbon Number

25 30

Figure 6. Pyrograms of kerogen and humic acids from Hole 511 blackshale core samples, i = C1 4 isoprenoid alkene; pr = prist-1-ene;t = «-C13 alkane; t ' = «-C13 alkene.

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Page 5: 40. Chemical Characterization of Aromatic Hydrocarbons, Kerogen

AROMATIC HYDROCARBONS, KEROGEN, AND HUMIC ACIDS

Visual kerogen analysis indicated that the kerogensfrom Site 511 black shales are composed almost entirelyof relatively low rank, amorphous material similar inappearance to many marine kerogens. This kerogen typewas found throughout Hole 511 with varying amountsof admixed terrigenous material down to the black shalesection. Elemental analysis indicated that the kerogen inthe black shales has atomic H/C ratios as high as 1.3.Although there are differences between the humic acidand kerogen pyrograms in Figure 6, the similar molecu-lar weight range of the pyrolysate components, the abun-dant presence of specific pyrolysate components in bothpyrograms, and a general similarity of the pyrogramfingerprints suggest that both humic acid and kerogenwere derived from a common marine, lipid stock. It hasbeen suggested that humic acids represent a kerogenprogenitor phase (Hue and Durand, 1977). The datapresented here certainly suggest a common, if not se-quential, origin for both materials.

Comparison with Geochemical Results for Site 330 andHole 327A, DSDP Leg 36

Site 511, DSDP Leg 71, is located about 10-15 kmsouth of Sites 327 and 330, DSDP Leg 36, in the basinprovince of the Falkland Plateau. Comer and Littlejohn(1977) analyzed samples from Site 330 and Hole 327Afor organic matter content and maturity. Sapropelic clay-stones equivalent in age to the black shale interval ofSite 511 occur in the middle of Hole 330 and the lowerthird of Hole 327A. (The ages of the black shale intervalof Site 511 were just revised to Albian-Aptian throughBarremian for Cores 58 to 60 and Tithonian throughKimmeridgian for Cores 63 to 70.) In those intervalsLeg 36 sapropelic claystones contain predominantlyamorphous kerogen and an average of 287 ppm ofEOM. In comparison, the organic matter in the Site 511black shale interval is also amorphous but containsmore than five times as much EOM.

CONCLUSIONSThe black shales at the bottom of Hole 511 contain

ten times more extractable organic matter than mostother Leg 71 sediments. Based on EOM (1590 ppm) andhydrocarbon (120 ppm) contents alone, this intervalwould classically be considered a potential source bedfor crude oil. The composition of the organic matter(rt-alkane CPI, percentage of hydrocarbons, kerogenH/C ratio, etc.), however, suggests that the interval hasnot reached full thermal maturity. These data and thelow rank of this section indicate that many of the hydro-carbons were deposited with the original sediments.

One- through four-ring aromatic hydrocarbons fromthe black shale interval have molecular type-carbonnumber distributions which are similar to those of manypetroleums. The overall aromatic hydrocarbon gas chro-matographic fingerprint, however, is not like that froma thermally mature sediment or petroleum. Mono- anddibenzthiophenes and mono-, di-, and tribenzfurans arealso present. Both heterocyclic types have carbon num-ber distributions similar to those of many crude oils.

Pyrograms of the humic acids and kerogens from theblack shale interval of Hole 511 are very similar. Thegeneral fingerprint of the pyrograms is typical of geo-polymers derived from marine algal material. Indeed,humic acids and kerogens here may be derived from thesame lipid stock. Visual analysis of the kerogen indi-cates it is composed entirely of low rank, amorphousmaterial similar in appearance to many marine kero-gens. Above the black shale interval the amorphouskerogen is admixed with varying amounts of terrigenousmaterial.

ACKNOWLEDGMENTS

The authors wish to thank the Union Oil Company of Californiafor permission to carry out and publish this work. Sample separationswere capably carried out by C. J. Moderow. Manuscript review andsuggestions for improvement were made by Drs. G. H. Smith and R.E. Sweeney.

REFERENCES

Aczel, T., and Lumpkin, H. E., 1972. Detailed characterization ofgas oils by high and low resolution mass spectrometry. 164th Natl.ACS Mtg. (New York, 8/27/72), Div. Petrol. Chem. Preprints,F66-F80.

Aizenshtat, Z., 1973. Perylene and its geochemical significance. Geo-chim. Cosmochim. Acta, 37: 559-567.

Comer, J. B., and Littlejohn, R., 1977. Content, composition andthermal history of organic matter in Mesozoic sediments, FalklandPlateau. In Barker, P. F., Dalziel, I. W. D., et al., Init. Repts.DSDP, 36: Washington (U.S. Govt. Printing Office), 941-944.

Hue, A. Y., and Durand, B. M., 1977. Occurrence and significance ofhumic acids in ancient sediments. Fuel, 56: 73-80.

Ishiwatari, R., Ishiwatari, M., Rohrback, B. G., and Kaplan, I.R., 1977. Thermal alteration experiments on organic matter fromRecent marine sediments in relation to petroleum genesis. Geo-chim. Cosmochim. Acta, 41: 815-828.

Larter, S. R., Solli, H., Douglas, A. G., de Lange, F., and de Leeuw,J. W., 1979. Occurrence and significance of prist-1-ene in kerogenpyrolysates. Nature, 279: 405-408.

Rullkötter, J., and Welte, D. H., 1981. in press. Oil-oil and oil-con-densate correlation by low eV GC-MS measurements of aromatichydrocarbons. Adv. Org. Geochem., 1979: New York (PergamonPress), pp. 93-102.

van de Meent, D., Brown, S. C , and Philp, R. P., 1980. Pyrolysis-high resolution gas chromatography and pyrolysis gas chromatog-raphy-mass spectrometry of kerogens and kerogen precursors.Geochim. Cosmochim. Acta, 44: 999-1013.

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