38. CONTENT, TYPE, AND THERMAL EVOLUTION OF ORGANIC MATTER IN SEDIMENTSFROM THE EASTERN FALKLAND PLATEAU, DEEP SEA DRILLING PROJECT, LEG 711

Hans von der Dick,2 Lehrstuhl für Geologie, Geochemie und Lagerstàtten des Erdöls und der Kohle,Rheinisch-Westfàlische Technische Hochschule, Aachen, D-5100 Aachen, Federal Republic of Germany

andJurgen Rullkötter and Dietrich H. Welte, Institut für Erdöl und Organische Geochemie (ICH-S)

Kernforschungsanlage Jülich GmbH, D-5170 Jülich, Federal Republic of Germany

ABSTRACT

The quantity, type, and maturity of the organic matter in Recent through Upper Jurassic sediments from theFalkland Plateau, DSDP Site 511, have been determined. Sediments were investigated for their hydrocarbon potentialby organic carbon and Rock-Eval pyrolysis. Kerogen concentrates were prepared and analyzed in reflected andtransmitted light to determine vitrinite reflectance and maceral content. Total extractable organic compounds wereanalyzed for their elemental composition, and the fraction of the nonaromatic hydrocarbons was determined bycapillary column gas chromatography and combined gas chromatography/mass spectrometry.

Three main classes of organic matter can be determined at DSDP Site 511 by a qualitative and quantitative evalua-tion of microscopic and geochemical results. The Upper Jurassic to lower Albian black shales contain high amounts oforganic matter of dominantly marine origin. The content of terrigenous organic matter increases at the base of theblack shales, whereas the shallowest black shales near the Aptian/Albian boundary are transitional in composition,with increasing amounts of inert, partly oxidized organic matter which is the dominant component in all Albianthrough Tertiary sediments investigated.

The organic matter in the black shales has a low level of maturity and has not yet reached the onset of thermalhydrocarbon generation. This is demonstrated by the low amounts of total extractable organic compounds, low per-centages of hydrocarbons, and the pattern and composition of nonaromatic hydrocarbons. The observed reflectance ofhuminite and vitrinite particles (between 0.4% and 0.5% RQ at bottom-hole depth of 632 m) is consistent with this inter-pretation. Several geochemical parameters indicate, however, a rapid increase in the maturation of organic matter withdepth of burial. This appears to result from the relatively high heat flow observed at Site 511. If we relate the level ofmaturation of the black shales at the bottom of Hole 511 to their present shallow depth of burial, they appear rathermature. On the basis of comparisons with other sedimentary basins of a known geothermal history, a somewhat higherpaleotemperature gradient and/or additional overburden are required to give the observed maturity at shallow depth.

A comparison with contemporaneous sediments of DSDP Site 361, Cape Basin, which was the basin adjacent and tothe north of the Falkland Plateau during the early stages of the South Atlantic Ocean, demonstrates differences insedimentological features and in the nature of sedimentary organic matter. We interpret these differences to be theresult of the different geological settings for Sites 361 and 511.

INTRODUCTION

DSDP Site 511 (51°00.28'S; 46°58.30'W) was drilledduring Leg 71 in the basin province of the FalklandPlateau about 10 km south of DSDP Site 330 (Leg 36) ata water depth of 2589 meters.

About 630 meters of sediments of Recent to LateJurassic age were penetrated and divided into six majorlithostratigraphic units (site chapter, this volume). Amajor unconformity of about 25 m.y. separates the Ter-tiary and the Cretaceous sediments. Small layers of mid-Cretaceous black shales were first encountered in Core56 at a sediment depth of 495 meters. Drilling into theblack shales was terminated at a sub-bottom depth of632 meters, after increasing amounts of gaseous hydro-carbons with low Q/C2, C2/C3, and C2/C5 hydrocar-bon ratios were detected and a strong seismic reflectorat a sub-bottom depth of 700 meters was observed.

Our study is concerned with the content, type, andthermal maturation of organic matter in the sediments

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

2 Present address: Petro Canada Research Laboratory, 40 Research Place NW, Calgary,Alberta, Canada.

at DSDP Site 511. Investigation of the type and quantityof organic matter will provide information about theenvironmental conditions in the early South AtlanticOcean and thus contribute to one of the major objec-tives of this leg—understanding the paleoceanographicconditions of the South Atlantic.

In addition, this report investigates potential deep seahydrocarbon source rocks and chemical alterations oforganic compounds during diagenesis. As far as thermalevolution is concerned, the black shales of the FalklandPlateau have attracted attention since Comer and Little-john (1977) attested that the organic matter is still in alow stage of maturity for oil generation but overmaturefor its present depth of burial. New methods are used inthis study to reveal the stage of diagenesis and the ther-mal history of the black shales.

METHODS

Sample Material

At 18 different core locations in Hole 511, samples were takenfrom dark-colored intervals (presumably enriched in organic carbon)and were subjected to geochemical investigation. The samples werewrapped in aluminum foil on board and kept frozen until analysisstarted. Although the samples from Sections 511-3-4, 511-16-1, 511-62-5, 511-64-4, and 511-70-3 were thawed for an unknown length of

1015

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

time during transport, they seem not to have been affected judging bycomparisons with those samples which were permanently frozen.Some of the 18 units analyzed were composite samples made up ofmaterials from different sections within a core; thus, the notation511-67-1/2/4 describes a unit composed of samples from Sections 1,2, and 4 of Core 511-67.

Table 1 contains information on sample identification, depth, stra-tigraphy, and lithology. For site location and lithologic units, see thesite chapter, this volume.

Experimental Procedures

The frozen samples were freeze-dried and ground. The total car-bon content was determined using a LECO carbon analyzer, and theorganic carbon and total nitrogen were measured with a Perkin Elmer240B CHN analyzer. For the organic carbon and total nitrogen analy-sis, the weighed samples were progressively treated with hot 2N, 4N,and 6N HC1 in pure silver vessels to decompose any carbonate anddried at 80°C for 24 hr. prior to analysis.

A soxhlet apparatus (20 hr. at 55 °C) was used for the extraction ofsoluble compounds with dichloromethane. The extracts were sepa-rated into nonaromatic hydrocarbons, aromatic hydrocarbons, andheterocompounds on an Al2O3/SiO2 (1:2) column by elution withn-hexane, dichloromethane, and methanol, respectively. Where sam-ple material was sufficient, some total extracts were also analyzed forC, H, O, and N contents with the Perkin Elmer 240B analyzer.

The experimental conditions for Rock-Eval pyrolysis (Espitalié etal., 1977), gas chromatography, and gas chromatography/mass spec-trometry and the procedure of kerogen concentration for microscopicinvestigations have been described in detail in previous reports(Rullkötter et al., 1981, and in press).

RESULTS AND DISCUSSION

Amount and Type of Organic Matter

The sediments of Late Jurassic to Recent age en-countered in the basin province of the Falkland Plateauwere divided into six lithologic units which vary con-siderably in their organic matter content. Very highamounts of organic carbon in the Late Jurassic to earlyAlbian sediments of Unit 6 (Fig. 1 and Table 2) demon-strate the high potential of these sediments to accumu-late organic matter under reducing environmental con-ditions. The relative uniformity of the sediments and theuniformity of the type of organic matter—close to akerogen Type II (Fig. 2)—together with the constant,high organic carbon concentrations confirm that euxinicconditions were well established throughout Late Juras-sic to early Albian times. The drop in organic carboncontent near the Aptian/Albian boundary (Fig. 3, be-tween 520 and 490 m) indicates the development of abetter-ventilated environmental system, which resultedin much lower organic carbon preservation in Albianand younger sediments (Fig. 1, Units 1-5). This devel-opment in the early Albian period is in agreement withthe end of euxinic conditions in the southern of the twobasins in the early South Atlantic Ocean (Reyment,1980). The results from these two different environmen-tal systems are discussed hereafter.

Aptian-Upper Jurassic (Cores 58-70)

According to Rock-Eval pyrolysis, the homogeneousblack shales of Cores 59-70 are grouped between thekerogen Type II and Type III trend lines, but are close toType II (Fig. 2). The relatively high hydrogen indexvalues of these samples suggest a significant contribu-tion of liptinitic organic matter. This is supported by

microscopy and by the analysis of the extractable hydro-carbons. The maceral analysis of the typical black shalesfrom Cores 70-59 reveals that liptinitic particles are animportant or even dominant fraction of the kerogenparticles (Table 3, Fig. 4). Structured and clearly identi-fiable liptinites represent only a small part of the totalliptinites, most of which are present as detrital debrisand amorphous organic matter that may originate fromboth autochthonous and allochthonous sources. In theclearly structured liptinites (Table 3), except for Section511-57-2/3/5, pollen, spores, and cuticles of terrestrialplants predominate. This does not necessarily indicatethat terrestrial organic matter is a major contributor tothe black shales, since a large portion of the primarilystructured, autochthonous, marine organic matter mayhave followed a diagenetic pathway to form bitumi-nous, unstructured organic matter. Thus, the bitumi-nites and the abundant bituminous background matteroccurring in all these sediments as a matrix for particu-lar organic matter may originate from diagenetically de-graded marine organisms, from microorganisms, and,to some extent, from amorphous organic matter in fecalaggregates.

Apart from these common features, microscopy alsodetects some differences within the otherwise homoge-neous Aptian-Barremian shales. The inertinite andhuminite particles (vitrinites of reflectance values below0.5% are termed huminites) make up 7-8% in thedeeper sections (511-70-3 and 511-69-2/3/4) and de-crease to about 5°7o in Sections 511-67-1/2/4 through511-62-5 (Table 3). As an exception, Section 511-65-1/2/3 again shows a somewhat higher amount of inert-inites and vitrinites. The very low concentrations in Sec-tions 511-61-1/2/5 through 511-59-1 suggest that thesupply of terrestrial organic matter generally decreasedduring Aptian-Barremian times. The high amounts ofmarine liptinitic matter in Section 511-57-2/3/5 (Table3) are in agreement with the occurrence of nannofossilchalks at the top of the black shales. The relatively highamounts of inertinites and huminites in Sections 511-58-1/2/4/CC and 511-57-2/3/5 (Table 3) are no excep-tion to the generally decreasing terrestrial contributionof organic debris during the Early and mid-Cretaceous.These two sections have low organic carbon contents,that are not comparable to the deeper sections (Table 1).

If we consider the Rock-Eval data (Fig. 2 and Table1) and maceral compositions (Fig. 4) and compare themwith the organic carbon contents (Table 1, Fig. 3), wemay conclude that the high organic carbon contents inLithologic Unit 6 are the result of excellent preservationof largely marine organic matter. A plot of organic car-bon content versus the amount of huminite plus inertin-ite particles normalized to the quantity of liptinites dem-onstrates the positive correlation between total organiccarbon enrichment and liptinite concentration (Fig. 5).The typical black shales form a cluster around pointsrepresenting both high amounts of organic carbon andrelatively high liptinite content. The upper shale inter-vals, Sections 511-61-1/2/5, 511-60-2/5/6, and 511-59-1, differ slightly in that they have very high liptiniteconcentrations.

1016

Table 1. Geological and geochemical data for DSDP Hole 511 samples.

Chromatographic Fractions (wt.%)Elemental Composition

of Bitumen (%/wt.) Rock-Eval Data

Core/Section(interval in cm)

Depth(m) Geological Age

Lithology andLithologic Unit

Rock (% dry wt.) Hydrocarbons Hetero- HCorg Total N Ccarb (ppm) (ppm/Corg × 10) Nonaromatic Aromatic Compounds C H

S i + S 2

Index Index (°C) (mg/g rock) (mg/g rock)

3-4, 135-150

16-1, 135-150

31-5, 120-135

55-1, 100-10655-2, 77-8155-4, 0-555-5, 0-555-6, 49-54

56-5, 123-128

57-2, 37-41,135-14057-3, 125-129, 135-13957-5, 39-44, 132-136

58-1, 9-14, 120-12558-2, 9-1458-4, 120-125CC

139

261

485

497

early OligoceneMuddy diatomaceous ooze,

Subunit 2A

early Campanian Zeolitic claystone, Unit 4

Muddy nannofossil chalk,Unit 5

early Albian

504

513 Aptian

Black shale in intervals withwhite laminae, Unit 6

Black shale in intervals withgray and greenish zones,Unit 6

59-1, 45-48, 143-147

60-2, 78-8260-5, 84-8960-*, 16-21

61-1, 139-14361-2, 0-7, 110-11561-5, 110-114

62-5, 135-150

64-4, 135-150

65-1, 41-4665-2, 0-565-3, 52-57

66-1, 23-2866-3, 145-150

67-1, 114-11867-2, 1-467-4, 146-150

68-2, 27-31, 119-121*

69-2, 145-15069-3, 144-15069-4, 0-5

70-3, 135-150

519

533

542

Barremiεui—Aptian

554

571

577

586

597

606

617

627

Late Jurassic

Black shale with whitelaminae of shell debris,Unit 6

Muddy chalk, chalk, andshale (petroliferous),Unit 6

Black shale (petroliferous;occasionally with whitelaminae of shell debrisUnit6

1.02 0.082

nd

4.11

nd

0.181

0.25

0.60 0.048 0.01

0.57 0.050 0.53

0.07 0.027 3.35

5.07 0.186 0.17

4.66 0.182 0.39

4.46 0.173 0.78

nd

0.00

161

33

5

0.68 0.033 0.00 105

0.69 0.042 0.25 69

1.88 0.084 1.95 164

8.18 0.360 0.58 1968

3.84 0.154 1.17 549

5.12 0.183 0.41 733

895

1052

606

5.02 0.195 1.12 800

5.43 0.201 0.68 565

nd

453

10.0

26.7

5.8

7.4

15.5

10.0

8.7

6.8

nd

nd

nd

15.4

17.7

22.6

13.6

15.9

10.4

nd

11.0

8.1

8.1

10.4

11.5

22.5

nd

22.8

3.4

nd

nd

nd

6.2

9.0

9.9

13.3

9.0

12.5

nd

10.3

64.0 nd nd nd nd 34

89.8 nd nd nd nd

nd 80.1 10.1 8.3 0.14

nd 74.3 12.0 9.9 trace

nd 78.8 10.1 8.4 0.17

78.4 nd nd nd nd

80.7 80.1 10.0 7.0 0.87

90.7 nd nd nd nd

71.8 79.5 11.1 5.9 0.88

83.2 80.9 10.9 4.5 0.35

nd nd nd nd nd

66.9 81.4 11.2 4.2 0.27

178 —

82.9

82.0

76.3

79.5

65.0

nd

nd

81.6

nd

nd

nd

nd

11.0

nd

nd

nd

nd

4.3

nd

nd

nd

nd

0.31

nd

nd

461

481

401

532

527

69

81

59

75

55

407

413

410

410

414

381 73 407

345 58 424

81.4 11.8 3.9 0.22 446 45 428

0.06 0.41

394

27

nd

92

375

nd

—

—

nd

0.16

0.04

nd

2.28

0.18

nd

66 169 — 0.02 0.40

45 196 417 0.01 0.28

195 127 426 <O.Ol 3.63

492 62 412 0.37 41.01

414 83 408 0.13 15.98

568 75 407 0.20 29.66

0.15

0.11

0.11

0.22

0.25

2.80

0.12

24.55

21.89

18.30

26.20

28.30

17.24

14.47

ssδo*i

oo

Note: nd = not determined.a Contaminated.

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

Uniti

Subunit 2A h

Subunit 2B I-

Unit3

Unit 4 h

Unitδ

Unit 6 (black shales) \-

I l I I l I I I I

0.01 0.02 0.04 0.06 0.08 0.10 0.2 0.4 0.6 0.8 1.0 8 10

Figure 1. Organic carbon (Corg) mean values (•Φ) and total ranges (i—i) for lithologic units or subunits ofSite 511. The variation in the mean organic carbon values correlates well with the Mn/Fe ratios as in-dicators of anoxic and oxic facies (Robert and Maillot, this volume). Ranges are calculated on thebasis of 95 samples taken along the core profile (see Table 2).

Sections 511-58-1/2/4/CC (uppermost Aptian) and511-57-2/3/5 (lower Albian) are grouped into a clustercharacterized by higher contributions of huminites plusinertinites relative to liptinites, together with significant-ly lower organic carbon concentrations. This can beinterpreted to mean a low supply of marine organic mat-ter to the sediment and/or oxidation of organic matterbefore and during sedimentation. The result is a con-siderable decrease in total organic carbon and a relativeenrichment of the more resistant terrestrial and inertorganic matter. Within this sequence, however, Section511-57-2/3/5 contains an unusually high amount of pre-dominantly marine liptinites relative to terrigenous sporesand cuticules (Table 3). This apparent contradiction isresolved by the detection of a dominant population ofparticles with a very high reflectance in addition to theliptinites (Fig. 4); huminites and the related semi-fusinites are almost absent. Obviously, the organic mat-ter which reached the sedimentary basin at that timeconsisted mainly of inert carbon and marine organismsrather than of herbaceous and woody material.

Comparison of the maceral analysis of Section 511-57/2/3/5 with that of a stratigraphically higher section(Fig. 4) makes the "transitional zone" described byDeroo et al. (this volume) obvious. The relative amountof liptinite particles is comparable to that in the blackshales downhole, whereas the distribution of the par-ticles with higher reflectance is very similar to the se-quence of the Albian-Maestrichtian claystones, which islow in organic carbon.

Compared with microscopy results (Fig. 4 and Table3), the Rock-Eval pyrolysis results exhibit surprisinglylow hydrogen and high oxygen indexes for Sections 511-57-2/3/5 and 511-58-1/2/4/CC (Fig. 2). This is espe-cially pronounced for Section 511-57-2/3/5, which inthe hydrogen index versus oxygen index diagram (Fig. 2)plots close to samples with minor liptinite content buthigh amounts of inert carbon (e.g., Sections 511-56-5,and 511-31-5 in Figs. 2 and 4). The high input of inertcarbon lowers the hydrogen index and increases the ox-ygen index to some extent. The very high amount of lip-tinites present is, however, not consistent with the dataof the Rock-Eval pyrolysis. An explanation for thesecontrasting results may be that oxidation of the lip-

tinites has lowered their hydrogen potential values, assuggested by Deroo et al. (this volume) for some sam-ples from Site 511. Based on this interpretation thesetwo samples from Cores 57 and 58 in Figure 5 form acluster characterized by the high amounts of inert car-bon and oxidation of liptinites typical of a residual typeof organic matter.

The analysis of the extractable nonaromatic hydro-carbons by the gas chromatography and combined gaschromatography/mass spectrometry reflects gradation-al differences within the black shale sequence similar tothose just outlined. The capillary column chromatogramsfor the nonaromatic hydrocarbon fractions of some rep-resentative samples (Fig. 6) clearly show a more pro-nounced terrigenous influence near the base of the blackshale unit (Section 511-70-3). This is indicated by thedominance of the long-chain wax alkanes (C25 to C35) inthe high-molecular-weight range. Section 511-62-5 con-tains considerably lower amounts of these w-alkanesrelative to the complex mixture of steroid and triter-penoid hydrocarbons (Fig. 6). Finally, the peculiar fea-ture detected by microscopy and Rock-Eval pyrolysis re-sults for the composite Section 511-57-2/3/5 coincideswith an unusual /z-alkane distribution in this sample(Fig. 6). The maximum is found at n-C20, and there is aslow decrease in individual w-alkane concentrations to-ward Λ-C 3 1, with only a slight predominance of «-al-kanes at odd carbon numbers. This w-alkane distribu-tion may be the result of a mixture of rederived and bac-terially degraded organic matter.

Polycyclic nonaromatic hydrocarbons are abundantin all of the black shale samples and are reduced to someextent only in the transitional samples near the Aptian/Albian boundary. The complexity of the mixture, com-prising saturated, mono- and diunsaturated isoprenoid,steroid, and triterpenoid hydrocarbons, is shown forSection 511-62-5 in Figure 7, which is part of the chro-matogram in Figure 6, expanded to show greater detail.Compounds identified by gas chromatography/massspectrometry on the basis of relative retention times andspectral interpretation are listed in Table 4 for sevenblack shale samples. Steroid hydrocarbons are domi-nant in all of the samples; this is interpreted to be con-sistent with the microscopic detection of major amounts

1018

CONTENT, TYPE, AND THERMAL EVOLUTION OF ORGANIC MATTER

Table 2. Organic carbon and total nitrogencontents of DSDP Site 511 samples.

Core/Section(interval in cm)

1-4, 31-332,CC (7-9)2-3, 120-1222-4, 10-123-4, 41-433-5, 7-84-2, 61-624-2, 118-1205-4, 90-925-5, 7-96-3, 90-926-4, 58-6011-3, 110-11112-1, 80-8215-1, 72-7416-1, 90-9217-2, 60-6218-1, 70-7220-2, 64-6620-2, 110-11221-1, 15-1721-1, 46-4824-1, 43-4624-3, 98-9925-1, 103-10427-1, 42-4328-1, 78-8028-4, 112-11328-7, 0-129.CC30-1, 14-1531-1, 136-13931-4, 133-13532-2, 140-14433-2, 92-9433-5, 106-10934-1, 1-334-2, 113-11534-7, 51-5436-2, 63-6636-2, 141-14337-1, 50-5138-2, 41^»238-3, 64-6539-3, 84-8540-3, 31-3241-1, 123-12541-2, 115-11641-3, 36-3742-4, 97-9943-1, 23-2544-3, 51-5344-3, 52-5345-2, 91-9345-2, 93-9547-2, 84-8647-6, 72-7448-2, 97-9948A 48-5049-5, 38-4050-1, 107-10950-3, 62-6451-1, 141-14352-2, 22-2353-5, 65-6654-5, 62-6355-1, 49-5155-5, 40-4255-5, 42-4356-5, 56-5856-5, 58-6057-2, 14-1657-3, 100-10257-5, 82-8458-1, 71-7258-1, 72-7458-4, 72-7459-1, 86-8959-1, 88-8959-3, 105-10760-1, 32-3461-2, 46-4861-2, 81-8262-1, 22-2462-3, 54-5663-1, 107-109•63-3, 16-1764-3, 128-13065-3, 8-1066-3, 92-9467-3, 43-4568-2, 71-7269-3, 75-7769-5, 2-470-4, 96-98

OrganicCarbon

(% dry wt.)

0.260.170.260.070.750.960.910.790.600.700.560.710.760.760.460.690.340.400.390.440.090.280.180.150.040.230.080.270.560.300.710.780.460.300.630.630.950.700.430.410.590.930.660.740.360.490.670.100.130.510.230.300.460.490.310.350.170.070.130.110.090.020.030.080.060.160.180.070.210.560.660.292.020.640.830.630.302.922.802.064.645.394.194.752.833.253.942.640.125.284.863.893.721.403.90

TotalNitrogen

(% dry wt.)

0.0360.0360.0310.0320.0660.0680.0670.0730.0660.0570.0600.0600.0710.0720.0510.0610.0370.0410.0440.0350.0260.0250.0260.0260.0280.0310.0280.0320.0520.0310.0470.0470.3100.0230.0500.0510.0570.0550.0380.0330.0470.0550.0510.0510.0300.0400.0510.0220.0340.0390.0350.0340.0340.0360.0360.0380.0300.0220.0380.0310.0310.0260.0560.0290.0240.0320.0280.0320.0310.0350.0310.0150.0900.0330.0430.0410.0230.1260.1230.0860.1840.1790.1460.1610.1220.1340.1530.1220.0130.1970.1740.1710.1630.0850.159

1000

900 -

100 -

— 375200

Figure 2. Results of Rock-Eval pyrolysis expressed in a hydrogen in-dex versus oxygen index diagram. Core numbers in circles; for fullsample intervals, see Table 1. Core 55 sample not analyzed becauseof its low organic carbon content. I, II, III = kerogen types I—III.)

of marine liptinitic organic matter. Many of the steroidhydrocarbons have undergone skeletal rearrangement,probably induced by clay mineral catalysis (Rubinsteinet al., 1975).

The distribution of rearranged and regular sterenesand steradienes is shown in Figure 8 by their majormass-spectrometric fragment ions m/z 257 and m/z255, respectively. Also shown are the correspondinghigher homologs, probably 4-methyl sterenes and stera-dienes (m/z 271 and m/z 269). Together with varioushopanoid hydrocarbons (Ourisson et al., 1979), the 4-methyl steroids may indicate a major contribution ofmicrobial mass to the organic matter in the black shales.The saturated sterane content in Section 511-62-5 isshown in Figure 8 by the m/z 217 mass chromatogram.The concentration of these compounds, compared totheir unsaturated analogs, is relatively low in the shal-lower part of the black shales unit but increases slightlydownhole (Table 4).

Tertiary-Albian (Cores 3-57)

Both the quantity of organic carbon (Table 1) and thetype of organic matter described in detail for Section511-57-2/3/5 reflect a transition toward a type of organ-ic matter in the Albian to Tertiary sediments that con-trasts with those of the black shales. Low organic car-bon values (Fig. 1), associated with a low hydrogen in-dex but a high oxygen index (Fig. 2) and generally verylow amounts of liptinites (Fig. 4), testify to the residualcharacter of this type of organic matter.

Lithologic Subunit 2B and Units 3 and 5, consistingof pelagic clays, calcareous and zeolitic oozes, and red-

1019

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

500 -

550 -

600 -

6500.1 0.2 0.40.60.81.0 2

corg(%)4 6 810 0 0.5 1 1.5 2 2.5

AHC (ppm)0 1 2 3 4 5 6

HC (ppm)

'orgx 1 0

Figure 3. Variations and trends of organic carbon (Corg), and of aromatic hydrocarbons (AHC) and totalhydrocarbons (HC) normalized to organic carbon, as a function of depth at DSDP Site 511 (nd = notdetermined).

Table 3. Maceral analysis of particles 3 µm and general description ofkerogen background matter in samples from DSDP Hole 511.

Core-Sectiona

3-416-131-555-1/2/4/5/656-557-2/3/558-1/2/4/CC59-1,60-2/5/661-1/2/562-564-465-1/2/366-1/367-1/2/468-269-2/3/470-3

Inertinites+ Huminites

(vol.%)

5223.210.1

nd7114.541.82.22.55.45.47.73.65.3

Contamini8.17.2

Note: nd = not determined.a For cm ranges, see Table 1.

TotalLiptinites(vol.%)

2.825.84.9nd8.2

52169.3

16.721.113.718.66.9

14.312.5

ited-nd12.516.2

Pollen,Spores,

Cuticules(vol.%)

nd1.81.5nd1.34.5nd0.60.91.90.72.10.50.4?0.9

1.41.8

AlgalBodies(vol.%)

nd2.50.7?nd0.96.0nd0.60.51.60.20.70.10.1?0.3

0.30.6

Visual Descriptionof Kerogen

Background Matter

Inertinitic debrisHumic-partly bituminousInertinitic debrisndInertinitic debrisHigh amounts of liptinitic detritusPartly humic-bituminousBituminousBituminousBituminousBituminous-partly humicBituminous-partly humicMixed bituminous-humicBituminous-partly humicBituminous-partly humic

Mixed bituminous-humicBituminous-partly humic

dish brown claystones, exhibit low mean organic carbonvalues, about 0.2%; the dark gray oozes of Subunit 2Aand the greenish gray zeolitic clays and claystones ofUnit 4 show somewhat higher mean organic carbon con-centrations, about 0.5% (Fig. 1). These latter units rep-resent the late Eocene-early Oligocene and Coniacian-Santonian intervals, respectively; both are characterizedby a rather high sedimentation rate that probably led toa slightly better preservation of organic matter by rapidburial under oxic bottom-water conditions. Judging bythe residual character of the organic matter, the Albianto Tertiary sediments from Site 511 form a third cluster

if the Core 16 sample is excluded (Fig. 5). Particles witha high reflectance range (0.9% to about 1.5%) form thedominant fraction in the Albian to middle Maestrich-tian sediments (Sections 511-57-2/3/5, 511-56-5, and511-31-5 in Fig. 4).

The occurrence of bimacerites (single, sedimentary,organic particles composed of two maceral types) withreflectance values of 1.0-1.4% and >2.0%, respective-ly, in Sections 511-16-1, 511-31-5, and 511-67-1/2/4,suggests that the particles within the reflectance range0.9-1.6% may, at least partly, originate from recycledand oxidized organic matter. The presence and abun-

1020

CONTENT, TYPE, AND THERMAL EVOLUTION OF ORGANIC MATTER

dance of particles in that reflectance range in all sections(Fig. 4) leads to the assumption that oxidized, probablyeroded organic matter has been transported to the Site511 area throughout the whole time span covered by thesediments analyzed. Thus, particles with a higher re-flectance range (1.69b) can be defined as inertinites.Huminite particles are in a range of about 0.3-0.5% inHole 511. Semifusinites exhibit a reflectance 0.2-0.4%higher than the corresponding huminites. Thus, the par-ticles in Figure 4 that are in a reflectance range of about0.5-0.8% are determined to be semifusinites, a deter-mination sustained by their abundance, which correlateswell with the abundance of the huminites.

Section 511-16-1 of the Tertiary Subunit 2A is the on-ly section outside the black shales that exhibits a hydro-gen index similar to the shales (Fig. 2). The low organiccarbon content and the large amounts of inertinite andhuminite particles (probably oxygenated, Fig. 4) shiftthis sample close to the cluster in Figure 5 that is typicalof the "transitional zone" (e.g., Cores 57 and 58). Dis-seminated liptinitic material (not discernible under themicroscope) is probably dominant and is responsible forthe high hydrogen index, despite the relative abundanceof huminite and inertinite particles detected by micro-scopy (Fig. 4). Section 511-16-1 is also the only Tertiarysample containing significant amounts of polycyclicnonaromatic hydrocarbons. The cyclic hydrocarbonsare the products of primary deposition and an earlystage of diagenesis—for instance, 17ß(H)-hopanes (Fig.6, Table 4). The dominant compounds in the capillarychromatogram of the nonaromatic hydrocarbon frac-tion are, however, long-chain wax alkanes of terrige-nous origin (Fig. 6).

Although only a few random samples of the Albian-Tertiary sequence were analyzed, it can be concludedfrom the sediment facies and the low organic carboncontents (Fig. 1) that the organic facies of the Albianand Upper Cretaceous-Tertiary sequence is representedmainly by a residual type of organic matter, with a fewexceptions such as Subunit 2A, Section 511-16-1.

The pronounced difference between the two contrast-ing types of organic matter is also discernible from theelementary analysis of the bitumens (soluble organicmatter). Inert organic carbon yields very few or no com-pounds extractable by organic solvents. Thus, the bitu-men originates mainly from organic matter that is lipti-nitic and, to some extent, humic. Because of the markeddifferences in elementary composition between aquaticand terrestrial organic matter, black shales with a pre-dominantly marine organic input are clearly separatedfrom the overlying sequence when plotted in a C/N-O/C diagram (Fig. 9). The liptinites of the sedimentarysequence overlying the black shales contain importantor even major portions of land plant lipids and areprobably affected by oxidation. As is to be expected forterrestrial and/or oxygenated organic matter with highoxygen and low nitrogen contents (Waples, 1977), bitu-mens in Sections 511-56-5, 511-55-1/2/4/5/6, and 511-31-5 exhibit much high C/N and O/C ratios than theblack shales. The variation within the black shale sam-ples is, later in this chapter, tentatively explained as an

effect of diagenetic evolution. Characterization of thetype of organic matter by the atomic C/N-O/C ratiosof the bitumen generally confirms the results based onRock-Eval pyrolysis (Fig. 2) and microscopy (Table 3,Figs. 4 and 5); it differs principally in that it does not in-clude any contribution of inert carbon, as Rock-Evaland microscopy do. This fact has to be taken into con-sideration when, for example, Rock-Eval data fromSection 511-58-1/2/4/CC are compared with C/N-O/Cratios of bitumen. The low C/N ratio suggests a pre-dominantly marine lipid source; the somewhat higherO/C ratio may reflect oxidation as described earlier andsuggested by Deroo et al. (this volume).

Thermal Maturation of the Black Shales

The stage of thermal evolution of the organic mattercan be determined by both geochemical and microscop-ical data.

The reflectance of disseminated vitrinites in a rock iswidely accepted as an indicator of the thermal history oforganic matter and commonly applied for rank deter-minations (e.g., Bartenstein and Teichmüller, 1974; Bos-tick and Alpern, 1977). At DSDP j>ite 511, vitrinite re-flectance ranges between 0.32%JR.0 at the top of theblack shales and about 0.45% Ro at the bottom-holedepth of 632 meters (Fig. 10). Oil_generation is believedto start at a level of 0.5-0.6% Ro (Hunt, 1979); thusthese shales are still in a low stage of maturity.

The reflectance values given for Sections 511-57-2/3/5, 511-59-1, and 511-60-2/5/6 in Figure 10 are notcompletely certain because of the low amounts of well-characterized humic particles (Fig. 4). In a few cases—for instance, Sections 511-62-5, 511-67-1/2/4, and 511-70-3—bimodal distributions of huminites/vitrinites maylead to an equivocal determination of maturity, so thateither the first or the second peak is accepted as valid fora rank determination. Humic material originates frommany different plant constituents that generate a largevariety of such particles in coals and rocks during diage-netic alteration (Jacob, 1980). Thus bimodal distribu-tions may be caused by differences in the precursorcompounds of the humic material. The bimodal distri-butions have been ignored in this case, since the meanvalues over the entire range best fit the over- and under-lying sections. Since maturation is progressive with in-creases in temperature and depth, it would be unreason-able to assume, for instance, a lower or significantlyhigher huminite/vitrinite reflectance in Section 511-67-1/2/4, compared with the unimodal distribution in theoverlying Section 511-66-1/3 (Fig. 4).

Rank determination by measurement of vitrinite re-flectance is supported by other geochemical parameterswhich all point to a low level of maturity. The kerogensof the black shales still have high hydrogen and oxygenindex values, indicative of a diagenetic stage (Fig. 2).The amounts of total extractable organic compounds(normalized to organic carbon) and the relative propor-tions of hydrocarbons are still low (Table 1). The mod-erate amounts of generated light hydrocarbons agreewith this interpretation (Schaefer et al., this volume).

1021

s[72] Q Legend

Liptinites

[101] u

Huminites(vitrinites)

I I I II I I I Il l l l l l l l l

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0 SernifüsüiReflectance (%) -

40

35 H511 161

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0Reflectance (%) *-

40 η

35-511-58-1/2/4/CC [92]

Inertinites andrederived vitrinite

Inertinites

a- 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0" Reflectance (%) »-

1 40 π

35-

3 0 -

25-

20-

15-

10-

5-

0

511-31-5

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0> 2.0

Reflectance (%) * -

24 η

2 1 -

,rmrH0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) •40-i

35-

30-

25-

20-

15-

10-

5 -

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0> 2.0

Reflectance (%) ^ -

40-Λ

511-56-5

-with rederivedites?

i-wiin reαi/ vitrinit

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) »-

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) »-

O

1JO

αn

r

o••

s

511-61-1/2/5

1.0 1.2 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) — » -

80 η511-62-5

70 H

[112]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance I

[110] [J

Woodfragment

*+u -

30-

20-

1 0 -

0 - ll0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) * -0.2 0.3 0.4 0.5

32-,

28-

24-

20-

16-

12-

8 -

4 -

511-65-1/2/3[48] Q

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) »-

[120] Q

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0Reflectance (%) ^ –

40-i,511-67-1/2/4 [136] Q

t 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0° Reflectance (%) — f c -

π511-69-2/3/4 [192] |J

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) * -

40 η _51170-3

[97] Q

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0>2.0

Reflectance (%) »-

Figure 4. Reflectance histograms of liptinites, huminites (vitrinites), and inertinites larger than 10 µm in kerogen concentrates of sediments from DSDP Site 511. Liptinitesare not representative in all cases because of their translucency and internal reflections. The occurrence of bimacerites in some sections and the distribution pattern of par-ticles in the reflectance range 0.85-1.6% RQ allows a further classification of the inertinites into semifusinites and mixed rederived vitrinites and inertinites. Core 55 samplenot analyzed because of its low organic carbon content; Core 68 sample contaminated.

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

= 2

1

0.8

0.6

0.4

0.2

0.1

Mainly inertorganic matter

Oxygenated liptinitic andinert organic matter

Field of humic coals

1 I 1 l_L

Predominantly

bituminous

organic matter

0.1 0.2 0.4 0.6 0.8 1.0 6 8 10 20 30

'org λ

Figure 5. Kerogen classification based on ratios of (huminites + inertinites>/liptinites as revealed bymicroscopy versus organic carbon content. Cores 57 and 58 of the "transitional zone," characterizedby both high amounts of inert carbon and/or oxygenated liptinitic material, have an intermediate posi-tion between the black shales and samples dominated by an inert type of organic matter. Section511-16-1, exhibiting a rather high hydrogen index but low organic carbon content (Table 1), forms anexception. Although the kerogen background matter is not considered here, the pattern correlates wellwith the data obtained by Rock-Eval pyrolysis (Fig. 2). (Core number in circles; for full sample iden-tification, see Table 1.)

The investigation of the extractable C 1 5 + nonaro-matic hydrocarbons provides further support both forthe low stage of maturity of the organic matter in theblack shales and also for a slight increase in maturitydownhole within the black shale sequence. The most ob-vious indication of the low maturity is the great abun-dance of unsaturated steroid and triterpenoid hydrocar-bons in all black shale samples. Regular sterenes (III—VII; see Appendix for compound structures) decrease inconcentration relative to regular steranes (VIII) as depthof burial increases (Table 4). A similar trend for the re-arranged sterenes (I,II) could not be detected. Neohop-13(18)-enes (IX) and hop-17(21)-enes (X) are abundantin all samples and there is no definite change in relativeconcentrations with depth, except for the decreasingamounts of trisnorneohop-13(18)-ene relative to trisnor-hop-17(21)-ene. 17ß(H)-hopanes (XII) predominate overthe thermally more stable 17α(H)-hopanes (XI) (VanDorsselaer et al., 1977) in all samples, and 17/3(H)-homohopane is the most abundant single compound

within the homologous series. Finally, the predomi-nance of the odd-numbered, long-chain /z-alkanes overtheir even-numbered homologs in the deepest sampleanalyzed (Section 511-70-3; Fig. 6) also shows that theblack shales have not yet reached the zone of thermalhydrocarbon generation.

Considering the short depth interval of about 140meters, the increase of the vitrinite reflectance from0.32% RQ at the top of the shales to about 0.45% RQ at630 meters sediment depth appears to be rather high andwould suggest an extremely rapid increase of matura-tion with increasing depth of burial (Fig. 10). The dif-ficulties in establishing a valid trend certainly arise inpart from the short interval over which the black shalesamples were investigated. Furthermore, it is well knownthat, at the same level of maturity, vitrinite reflectanceappears to be lower in the presence of abundant bitumi-nous organic matter (Hutton and Cook, 1980). The val-ues obtained from the upper, highly bituminous part ofthe shales are perhaps too low. Shifting these data to

1024

CONTENT, TYPE, AND THERMAL EVOLUTION OF ORGANIC MATTER

Section 511-16-1139.4 m depth

Section 511-57-2/3/5500.9-506.4 m depth

Section 511-62-5553.9 m depth

Section 511-70-3626.9 m depth

10 min.

Figure 6. Capillary column gas chromatograms of the nonaromatic hydrocarbon fractions of four sediment samplesfrom DSDP Site 511. Numbers indicate n-alkanes; lettered compounds were identified by GC/MS and are listedin Table 4. Conditions: Varian 3700 gas chromatograph with FID detector; 20 m x 0.3 mm ID, SE 54; carriergas, helium; temperature program, 80-265°C at 3°C/min.)

higher values would result in a lower maturation gra-dient than shown in Figure 10. The reflectance value ob-tained from the wood fragment seems, however, to bemost reliable, since the huminites are in a stage of tran-sition from a texto-ulminite to an eu-ulminite. This tran-

sition takes_place in a reflectance interval between 0.4%and 0.6% Ro (Hagemann, pers. comm.).

An increasing maturation with depth of burial is alsodiscernible from the atomic C/N and O/C ratios of thebitumen (soluble organic matter), which decrease slowly

1025

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

Section 511-62-5553.9 m depth

Temperature (°C)- isothermal -*- 265 240 220

Figure 7. Expanded partial capillary column gas chromatogram of the nonaromatic hydrocarbon fraction of Section511-62-5 (steroid and triterpenoid hydrocarbon range). (Numbers indicate n-alkanes; lettered compounds arelisted in Table 4. Conditions: Varian 3700 gas chromatograph with FID detector; 20 m × 0.3 mm ID, SE 54; car-rier gas, helium; temperature program, 80-265 °C at 3°C/min.)

for O/C and increase rapidly for C/N with increasingdepth (Fig. 9). The rapid increase of the C/N ratios withdepth is affected not only by diagenesis but also by theincreasing contribution of land-derived organic matterdownhole, which shifts the values to higher C/N andO/C ratios. Despite the increasing terrestrial input, theO/C ratios are, however, slowly decreasing with depth.Thus, the increasing C/N and decreasing O/C ratios ofthe bitumen as depth of burial increases are interpretedas a loss of functional groups from heterocompoundsduring the early stages of thermal evolution. The slightincrease, with depth, in the amounts of hydrocarbons(normalized to organic carbon, Fig. 3) supports this in-creasing maturation and is consistent with the inter-pretation that some organic compounds have lost func-tional groups.

In most cases it was not possible to determine thematuration in the Upper Cretaceous and Tertiary clay-stones because of the residual nature of the organic mat-ter. Apparently, the majority of the huminite particlesare affected by oxidation. Thus, the reflectance value of0.26% RQ from a few particles in the Tertiary Section511-16-1 seems plausible but is not substantiated.

Apart from the increase in the maturation of organicmatter, the absolute level of maturity appears to berather high, considering the actual shallow depth ofburial. This fact has already been observed by Comerand Littlejohn (1977) for the black shales at DSDP Sites327 and 330. The deepest sample at Site 511, Section511-70-3, has already reached a fairly high level ofmaturation, corresponding to 0.45% ‰ Interpreted asa mixture of thermogenic and biogenic origin (Burke etal., 1981), the low Q/C2 ratios calculated from thecomposition of core liner gases (site report, this volume)

together with the "paraffinic" character of light hydro-carbons (Schaefer et al., this volume) support the ideaof a relatively high level of maturity at shallow depth.

The relatively high maturation level may reflect twomain circumstances: (1) the shales may actually havebeen more deeply buried in the Mesozoic, but there oc-curred an uplift and a part of the Upper Cretaceous/Paleocene sedimentary sequence was eroded; and/or (2)a higher temperature gradient may have acted on thesediments in the past. Although the vitrinite reflectanceof 0.26% RQ in the Tertiary claystones at a depth of 139meters has been determined only tentatively and thevitrinite data of the upper part of the Cretaceous blackshales appear to be too low, there is no indication of amajor break in the vitrinite reflectance trend at the Cre-taceous/Tertiary unconformity.

A significantly higher overburden in the past there-fore seems unlikely. Paleontological considerations (dis-cussions of the shipboard party) and the data of the po-rosity trend (Bayer, pers. comm.) also suggest that therewas no additional overburden above the unconformityexceeding 400 meters. Comer and Littlejohn (1977) con-sidered the time interval of the unconformity to be tooshort for significantly deeper burial and subsequent ero-sion.

Thus the question arises whether the present geo-thermal heat flow at Site 511 may account for thematurity, or whether there have been changes in heatflow during the geological past. The problem may besolved by comparing the temperature regime and thematuration level of organic matter on the FalklandPlateau with those of other sedimentary basins with aknown geological and thermal history. Compared withsamples from the Landau 2 borehole (Rhine Valley,

1026

CONTENT, TYPE, AND THERMAL EVOLUTION OF ORGANIC MATTER

Table 4. Branched and cyclic compounds identified in the nonaromatic hydrocarbon fractions of DSDP Hole511 samples.

CompoundGeneral

Structure8116-1 57-2/3/5 64-4 66-1/3 69-2/3/4 70-3

prpha

b

cd

efg

h

i

k1m

n0

P

q

rs

t

uV

wX

y

za'b 'c'

d'e'f

g'

h'

i 'j 'k'

rm'n'

o'

P'q'r's't 'u '

pπstanephytanetetracyclic alkane

(M + 328,BP313,243,231,217)C27-sterene

(M +370,355,327,314,257,239)C27-rearranged sterenesterene?

(M + 37O,BP2O6,355,219,121)C27-rearranged sterene?C27-rearranged stereneC27-sterene

(M+ =BP370,355,279,257,215)C28-sterene

(M + 384,BP121,369,257,206)C2β-steradiene (rearranged?)C29-sterene

+ other sterenes/steradienes(M + 384,M + 382,M + 370)

C29-sterene (M + 398,257,220)C28-rearranged stereneC28-sterene

(M +384,369,314,257,215)C29-steradiene (rearranged?)50-cholestane+ 4-methylsteradieneC29-rearranged sterene

22)29,30-trisnorneohop-13(18)-ene4-methyl-C27-rearranged sterene

(M +384,271)cholest-4-ene4-methyl-C27-sterene

+ cholest-2-enecholest-5-ene

+ 5α-cholestane22,29,30-trisnorhop-17(21)-ene24-methylcholesta-4,22-diene

+ 4-methyl-C27-sterene24-methylcholesta-5,22-diene22,29,30-trisnor-17α(H)-hopane24-methyl-50-cholestane

+ 4-methyl-C27-sterene4-methyl-C27-sterene22,29,30-trisnor-17|8(H)-hopane24-methylcholest-4-ene24-methylcholest-5-ene

+ 24-methyl-5α-cholestane24-ethyl-5/3-cholestane30-nor-17α(H)-hopane24-ethylcholest-4-ene

+ 30-norneohop-13(18)-ene+ 30-norhop-17(21)-ene

24-ethylcholest-5-ene+ 24-ethyl-5α-cholestane

hop-17(21)-ene

30-normoretane4-methyl-C29-sterene4-methyl-C29-sterene17α(H)-hopanefern-8-eneneohop-13(18)-ene

30-nor-17/3(H)-hopane+ fern-9(l l)-ene

homohop-17(21 )-ene

17α(H)-homohopane17/3(H)-hopane17/3(H)-homohopane17|8(H)-bishomohopanelycopane

I, R = H

I, R = H

I, R = C H 3

VIII, R = H

I, R = C2H5IX, R = HII

IV, R = H

IIIV, R = HVIII.R = HX, R = HVI

VIIXI, R = HVIII, R = C H 3

XII, R = Hiv j> CHiV,R = C H 3

VIII, R = C H 3

VIII, R = C2H5XI, R = C 2 H 5

IV, R = C2H 5

IX, R = C2H5X, R = C2H5V, R = C2H5VIII, R = C2H 5

X, R = CH(CH 3 ) 2

XIII

XI, R = CH(CH 3 ) 2

XIVIX, R = CH(CH 3 ) 2

XII, R = C 2 H 5

XVX, R = CH(CH 3 )C 2 H 5

XI, R = CH(CH 3 )C 2 H 5

XII, R = CH(CH3)2XII, R = CH(CH3)C2H5

XII, R = CH(CH3)C3H7XVI

+ +

+ +

+ +tr

Note: Compounds are marked in the gas chromatograms shown in Figures 6 and 7 (letter symbols), and are listed in order of elution from the capillary col-umn. Estimated abundances: + + + = major, + + = intermediate, + = minor, tr = trace amounts. M + = molecular ion; BP = basic peak of mass spec-trum (100%).

a See Appendix to this chapter for compound structures.

West Germany) and the Northwest African continentalmargin, the organic matter at DSDP Site 511 reaches anequivalent level of maturity at a very shallow depth(Table 5). On the Northwest African continental mar-gin, the comparable level at much greater depths is un-doubtedly associated with the much lower temperaturegradient, 42°C/km. If one does not assume an addi-tional overburden on the Falkland Plateau in the past,the comparison clearly shows that at DSDP Site 511there must have been a higher paleotemperature gra-dient, which can be estimated using the time-tempera-ture relationship of the organic matter maturation.

The rapid increase of maturation with depth shownin Figure 10 (this seems to be too high, for reasonsdiscussed earlier) and the generation trend of light hy-drocarbons (Schaefer et al., this volume) would lead usto expect the onset of oil generation at an extrapolateddepth of about 800-900 meters, in sediments whichwould have an estimated age of at least 130 m.y. At thisdepth, a reaction temperature of about 80°C is neces-sary for such a level of maturity (Connan, 1974; Wright,1980). Thus, assuming no additional overburden, apaleothermal gradient of about 100°C/km can be calcu-lated. This, of course, would be a very unlikely situa-

1027

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

511-62-5

553.9 m depth

Λ^Λ>—^.ΛΛ ^ T Λ ^ Λ - X / ^

900 1100

Scan Number

1300

Figure 8. Mass chromatograms of 5 key fragment ions indicating the sterene/steradiene (m/z 257/255), 4-methylsterene/steradiene (m/z 271/269), and sterane (m/z 217) distributions in the nonaromatic hydrocarbon fractionof Section 511-62-5. (Conditions: Varian MAT 112 S mass spectrometer, 70 eV, source temperature, 220°C, 2.3s/scan, coupled directly to a Varian 3700 gas chromatograph; GC conditions as in Fig. 6.)

tion, not normally seen in nature except in regions ofvolcanic activity.

Allowing for an additional overburden of approxi-mately 400 meters above the unconformity, the bottom-hole sediments of DSDP Hole 511 reach a level of matu-rity comparable to that of Landau 2 at a depth of be-tween 800 and 900 meters and 1100 meters (Table 5). Inthis case, the present temperature gradient of 70°C/km,if it has lasted for the past 120 m.y., would be sufficientto account for the observed maturity. This is in agree-ment with the geological considerations advanced by the

DSDP Leg 71 shipboard party, which excluded an addi-tional overburden of more than 400 meters and a lowerpaleotemperature gradient.

PALEOENVIRONMENTAL COMPARISONWITH THE BLACK SHALES OF THE

CAPE BASIN; CONCLUSIONS

Sediments of the black shale facies of Cretaceous agehave been encountered in many South and North Atlan-tic DSDP holes (Tissot et al., 1979, 1980). Withoutdoubt, they reflect periods of reducing conditions dur-

1028

CONTENT, TYPE, AND THERMAL EVOLUTION OF ORGANIC MATTER

700 -

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10Atomic O/C of Bitumen

Figure 9. Atomic O/C and C/N ratios of bitumen (organic extracts)of some sediment samples from DSDP Site 511. The samples ofthe black shales facies (circles) show much lower O/C and C/N ra-tios (also depending on diagenesis) compared with the samplescontaining a residual type of organic matter (squares). (Samplesare identified by core numbers; for full identification see Table 1.)

ing deposition in a wide range of water depths, fromshallow to about 3000 meters (Thiede and van Andel,1977).

Palynological and geological arguments (Harris,1977; Shipboard Scientific Party et al., 1977) suggest apredrift position of the eastern part of the FalklandPlateau (Maurice Ewing Bank) close to the Mozam-bique Ridge. Thus there should be a correlation betweenthe shales of the Plateau and equivalent shales fromDSDP Site 361 (marginal Cape Basin). The lower Al-bian(?) and Aptian sapropelic shales at Site 361 (Unit 7,Site 361; Bolli et al., 1978) comprise more than 300meters of sediments. Thus, at Site 361 much more sedi-ment was accumulated during the Early Cretaceous thanthe extrapolated total sequence of about 200 meters ofAptian-Barremian (or older?) age at Site 511. However,the typical black shale layers at Site 361 average only 20

400 -

E 500 -

600 -

700

cal

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ich.

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. 1

ian

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1 B

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ia

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o?

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)

Oooo

0

o0

1

-

-

_

0

1 1

0.2 0.3 0.4Vitrinite Reflectance (% Rn)

0.5 0.6

Figure 10. Mean vitrinite reflectance plotted as a function of depth.Although there seems to be^a trend, its validity over such a shortdepth interval is questionable. Bituminous organic matter is sup-posed to have affected the reflectance values, especially in the shal-lower black shale samples.

cm each in thickness and are interbedded by massivesilty and sandy layers and coarse (turbiditic) coaly sand-stones. Kagami (1978) calculated 24% of Unit 7 fromSite 361 to be black shales.

In addition to strata dominated by a marine type oforganic matter (Tissot et al., 1980), terrigenous plantdetritus forms an important or even the main portion ofthe organic fraction of the sediments at Site 361 (Ray-naud and Robert, 1979). A similarly wide variety of ke-rogen types has been determined in young sedimentsfrom the Gulf of California (DSDP Leg 64; Rullkötteret al., in press). Site 361 and the Gulf of California Sites474, 478, 479, and 481 were, and still are, situated innarrow, rifting basins of at least partly anoxic watersreaching more than 3000 meters in depth (Thiede andvan Andel, 1977). They then, as now, accumulated largemasses of inorganic and organic detritus from the adja-cent continents.

This is different from the situation on the Early Cre-taceous Falkland Plateau. On the Plateau, which actedas a southern barrier in the spreading early South Atlan-tic, the black shales were deposited at shallow waterdepths. They do not occur in layers interbedded withmany silty and sandy deposits but as a massive sequenceof dark-colored, mostly black clays and mudstones. Ex-cept for the transitional zone, fewer than 3% of 145samples analyzed by Deroo et al. (this volume) and usover a cored interval 120 meters long contain less than1% of organic carbon. A predominantly marine sourceof organic matter has been determined for the majorityof these samples.

1029

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

Table 5. Levels of organic maturation and temperature realms in three sedimentary basins.a

Sedimentary Province

Northwest AfricanContinental Rise:DSDP Site 397

Rhine Valley, West Germany:Landau 2 borehole

Falkland Plateau:DSDP Site 511

Geological Age

Early Cretaceous

Tertiary

Late Jurassic-Early Cretaceous

Depth at Which0.45% Ro Level

Is Reached(m)

1450

1100

630

PresentTemperature

( °Qa t0.45% Ro Level

62

No data

48

PresentTemperature

(°C/km)

42

77

70

CalculatedPaleotemperature

Gradient(°C/km)

No data

58 (45 forNeogene, 70for Palaeogene)

<IOO>70

a Data from Teichmüller and Teichmüller (1979), Espitalié (1979), and Buntebarth (1979) for Landau 2 borehole, and from Cornfordet al. (1979) for DSDP Site 397.

Several basins and basement highs separated the Falk-land Plateau from the South African continent in pre-drift times (Jones and Plafker, 1977; Thompson, 1977).When the rift basin developed during Late Jurassictimes, West Gondwana broke up and the masses of con-tinental runoff originating from southern Africa werecaptured mainly in the spreading basin. This led to a re-duced but more homogeneously distributed terrigenousinput of prograded upslope sedimentation in the easternpart of the basin province of the Plateau. If the MauriceEwing Bank in the northeast formed an emerged micro-continent at that time, as suggested by Ludwig (this vol-ume), a part of the inorganic and organic debris depos-ited at Site 511 probably also originates from that east-ern part of the Falkland Plateau. The influx of terrige-nous plant debris decreased as the spreading proceededduring the Early Cretaceous and the Plateau subsided.

The preservation of high amounts of autochthonousorganic matter in a thick section of black shales can berelated to very strong reducing conditions during depo-sition at shallow water depths. A strong mid-water oxy-gen minimum layer, originating from respiration andoxidation of organic material produced in surface wa-ters, was likely to be well established. The change fromreducing to oxic conditions on the Maurice Ewing Bankoccurred at a time corresponding to the transitional zonenear the Albian/Aptian boundary. At Site 361, this im-portant change in environmental conditions is indicatedby the lithologic change from Unit 7 to Unit 6 (Natland,1978), which occurred either simultaneously with that ofSite 511 or somewhat later in the lower Albian (the Ap-tian/Albian boundary is not definitively identified atSite 361; Bolli et al., 1978).

Thus, it seems plausible that euxinic conditions devel-oped within the early basins of the South Atlantic, start-ing from a strong mid-water oxygen minimum layer andproceeding downward to deep bottom water. This end-ed at the moment the Plateau subsided and/or when adeep seaway between the Indian and Atlantic oceanswas established, improving the water circulation. Firstindications of this change are already discernible withina transitional zone in the late Aptian, where the organiccarbon content drops to moderate values and liptinitesare presumably affected by oxidation. The bituminousorganic matter originating from marine precursor or-ganisms is an abundant constituent of the Aptian andBarremian black shales. As a consequence of the devel-

opment of a better-ventilated, oxygenated system, thebituminous matter disappears within the "transitionalzone," since marine organic matter is very sensitive tooxidation (Lyons and Gaudette, 1979). Thus, only lip-tinitic debris (apparently affected by oxidation) but nobituminites are left in Core 57 (early Albian). DuringAlbian and post-Albian times, only small amounts of arefractory, residual type of organic matter were able tosurvive under the highly oxidizing conditions. This re-sidual type of organic matter, originating mainly fromcontinental runoff, can be traced along all cored sec-tions of Site 511. Within the black shales, the autoch-thonous marine organic material is superimposed uponthis refractory matter.

With respect to the potential for generating hydrocar-bon at elevated temperatures, the Upper Jurassic andLower Cretaceous shales are excellent hydrocarbonsource rocks. In contrast, the Upper Cretaceous andTertiary sediments have no potential for hydrocarbongeneration—neither oil nor gas.

Given the vitrinite reflectance of 0.4-0.5% Ro at abottom-hole depth of 632 meters and the downhole trendof several geochemical parameters, the onset of oil gen-eration can be expected in depths slightly deeper thanterminal depth at Hole 511, provided that potential Ju-rassic source rocks occur. Considering the present shal-low depth of burial, the shales appear to be rather ma-ture. Despite the present high thermal gradient (7°C/100 m), either a higher paleothermal gradient (up to10°C/100 m), an additional overburden in the past abovethe unconformity (up to 400 m), or a combination ofboth is required to explain the observed maturity. Theheat flow measured at Site 511 is higher than in averageoceanic basins and surprisingly high for old basementcrust. It is not yet clear whether the "overmaturity" ob-served for both Sites 330 and 511 is due to a local heat-ing event or is characteristic of larger areas on the Falk-land Plateau. On the basis of theoretical considerations,Royden et al. (1980) predicted a vitrinite reflectance of0.5% RQ at a sub-bottom depth of 3100 meters for thebasin province of the Falkland Plateau. This maturitylevel at such a great depth of burial would imply an ex-tremely low heat flow over geological times and is in ex-treme contrast with the measured results of Site 330 and511. Although a local heating cannot be excluded forthese sites, the vitrinite reflectance predicted by Roydenet al. (1980) at that depth appears erroneously low.

1030

CONTENT, TYPE, AND THERMAL EVOLUTION OF ORGANIC MATTER

ACKNOWLEDGMENTS

We would like to thank I. Schwartz, U. Disko, and S. Huhn fortechnical assistance and Dr. H. Gormly for Rock-Eval pyrolysis data.Our thanks go to Dr. G. Deroo (Institut Français du Pétrole, Paris,France) and Dr. C. Cornford (British National Oil Corporation,Glasgow, United Kingdom) for carefully reading and reviewing thefinal manuscript and to Dr. H. W. Hagemann (Technische-Hoch-schule Aachen) for constructive criticism.

Samples were made available through the participation of theDeutsche Forschungsgemeinschaft (DFG) in the DSDP/IPOD pro-gram. This is gratefully acknowledged, as also is financial support bythe DFG, grants No. We 346/23 and We 346/24. Hans von der Dickextends thanks to the Organic Geochemistry Panel of JOIDES, whichmade it possible for him to participate on Leg 71.

REFERENCES

Bartenstein, H., and Teichmüller, R., 1974. Inkohlungsuntersuchung-en, ein Schlussel zur Prospektierung von palaozoischen Kohlen-wasserstoff-Lagerstatten? Fortschr. Geol. Rheinld. Westfalen, 24:129-160.

Bolli, H. M., Ryan, W. B. F., and the Shipboard Scientific Party,1978. Cape Basin continental rise—Sites 360 and 361. In Bolli, H.M., Ryan, W. B. F., et al., Init. Repts. DSDP, 40: Washington(U.S. Govt. Printing Office), 29-182.

Bostick, N. H., and Alpern, B., 1977. Principles of sampling, prepara-tion and constituent selection for microphotometry in measure-ment of maturation of sedimentary organic matter. / . Microscopy,109(Pt. l):41-47.

Buntebarth, G., 1979. Eine empirische Methode zur Berechnung vonpalaogeothermischen Gradienten aus dem Inkohlungsgrad organ-ischer Einlagerungen in Sedimentgesteinen mit Anwendung aufden mittleren Oberrhein-Graben. Fortschr. Geol. Rheinld. West-falen, 27:97-108.

Burke, R. A., Jr., Brooks, J. M., and Sackett, W. M., 1981. Lighthydrocarbons in Red Sea brines and sediments. Geochim. Cosmo-chim. Acta, 45:627-634.

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.

Connan, J., 1974. Time-temperature relation in oil genesis. Bull. Am.Assoc. Pet. Geol., 58:2516-2521.

Cornford, C , Rullkötter, J., and Welte, D., 1979. Organic geochem-istry of DSDP Leg 47A, Site 397 eastern North Atlantic: Organicpetrography and extractable hydrocarbons. In von Rad, U., Ryan,W. B. F., et al., Init. Repts. DSDP, 47, Pt. 1: Washington (U.S.Govt. Printing Office), 511-522.

Espitalié, J., 1979. Charakterisierung der organischen Substanz undihres Reifegrades in vier Bohrungen des mittleren Oberrhein-Grabens sowie Abschatzung der palaogeothermischen Gradienten.Fortschr. Geol. Rheinld. Westfalen, 27:87-96.

Espitalié, J., Laporte, J. L., Madec, M., Marquis, F., Leplat, P.,Paulet, J., and Boutefeu, A., 1977. Methode rapide de carac-terisation des roches-meres, de leur potential petrolier et de leurdegre devolution. Rev. Inst. Fr. Pet., 32:23-42.

Harris, W. K., 1977. Palynology of cores from Deep Sea Drilling Pro-ject Sites 327, 328, and 330, South Atlantic Ocean. In Barker, P.F., Dalziel, I. W. D., et al., Init. Repts. DSDP, 36: Washington(U.S. Govt. Printing Office), 761-815.

Hunt, J. M., 1979. Petroleum Geochemistry and Geology: San Fran-cisco (Freeman and Company).

Hutton, A. C , and Cook, A. C , 1980. Influence of alginite on thereflectance of vitrinite from Joadja, NSW, and some other coalsand oil shales containing alginite. Fuel, 59:711-714.

Jacob, H,, 1980. Die Anwendung der Mikrophotometrie in der organ-ischen Petrologie. Leitz-Mitt. Wiss. Techn., 7:209-216.

Jones, D. L., and Plafker, G., 1977. Mesozoic megafossils fromDSDP Hole 327A and Site 330 on the eastern Falkland Plateau. In

Barker, P. F., Dalziel, I. W. D., et al., Init. Repts. DSDP, 36:Washington (U.S. Govt. Printing Office), 845-850.

Kagami, H., 1978. Sedimentary features of Cape Basin and AngolaBasin sediments, DSDP Leg 40. In Bolli, H. M, Ryan, W. B. F., etal., Init. Repts. DSDP, 40: Washington (U.S. Govt. Printing Of-fice), 525-540.

Lyons, W. B., and Gaudette, H. E., 1979. Sulfate reduction and thenature of organic matter in estuarine sediments. Org. Geochem.,1:151-159.

Natland, J. H., 1978. Composition, provenance, and diagenesis ofCretaceous clastic sediments drilled on the Atlantic continental riseoff southern Africa, DSDP Site 361—implications for the earlycirculation of the South Atlantic. In Bolli, H. M., Ryan, W. B. F.,et al., Init. Repts. DSDP, 40: Washington (U.S. Govt. Printing Of-fice), 1025-1061.

Ourisson, G., Albrecht, P., and Rohmer, M., 1979. The hopanoids.PureAppl. Chem., 51:709-729.

Raynaud, J. F., and Robert, P., 1979. Microscopical survey of organicmatter from DSDP Sites 361, 362, and 364. In Bolli, H. M., Ryan,W. B. F., et al., Init. Repts. DSDP, Suppl. to Vols. 38, 39, 40, and41: Washington (U.S. Govt. Printing Office), 663-669.

Reyment, R. A., 1980. Paleo-oceanology and paleo-biogeography ofthe Cretaceous South Atlantic Ocean. Oceanol. Acta, 3:127-133.

Royden, L., Sclater, J. G., and Von Herzen, R. P., 1980. Continentalmargin subsidence and heat flow: Important parameters in forma-tion of petroleum hydrocarbons. Bull. Am. Assoc. Pet. Geol., 64:173-187.

Rubinstein, I., Sieskind, O., and Albrecht, P., 1975. Rearrangedsterenes in a shale: Occurrence and simulated formation. J. Chem.Soc. Perkin Trans. 1, (No. 19): 1833-1836.

Rullkötter, J., von der Dick, H., and Welte, D. H., 1981. Organicpetrography and extractable hydrocarbons of sediments from theeastern North Pacific Ocean, Deep Sea Drilling Project Leg 63. InYeats, R. S., Haq, B. U., et al., Init. Repts. DSDP, 63: Washington(U.S. Govt. Printing Office), 819-836.

, in press. Organic petrography and extractable hydrocarbonsof sediments from the Gulf of California, Deep Sea Drilling ProjectLeg 64. In Curray, J. R., Moore, D. G, et al., Init. Repts. DSDP,64: Washington (U.S. Govt. Printing Office).

Shipboard Scientific Party, Harris, W., and Sliter, W. V., 1977. Evolu-tion of the southwestern Atlantic Ocean basin: Results of Leg 36,Deep Sea Drilling Project. In Barker, P. F., Dalziel, I. W. D., et al.,Init. Repts. DSDP, 36: Washington (U.S. Govt. Printing Office),993-1013.

Teichmüller, M., and Teichmüller, R., 1979. Diagenesis of coal (coalifi-cation). In Larsen, G., and Chilingar, G. V. (Eds.), Developments inSedimentology, 25A: New York (Elsevier).

Thiede, J., and van Andel, Tj. H., 1977. The paleoenvironment ofanaerobic sediments in the late Mesozoic South Atlantic Ocean.Earth Planet. Sci. Lett., 33:301-309.

Thompson, R. W., 1977. Mesozoic sedimentation on the eastern Falk-land Plateau. In Barker, P. F., Dalziel, I. W. D., et al., Init. Repts.DSDP, 36: Washington (U.S. Govt. Printing Office), 877-891.

Tissot, B., Demaison, G., Masson, P., Delteil, J. R., and Combaz, A.,1980. Paleoenvironment and petroleum potential of middle Creta-ceous black shales in Atlantic basins. Bull. Am. Assoc. Pet. Geol.,64:2051-2063.

Tissot, B., Deroo, G., and Herbin, J. P., 1979. Organic matter inCretaceous sediments of the North Atlantic: Contribution to sedi-mentology and paleogeography. In Talwani, M., Hay, W., andRyan, W. B. F. (Eds.), Deep Drilling Results in the Atlantic Ocean:Continental Margins and Paleoenvironment. Am. Geophys. Union,Maurice Ewing Series 3:362-374.

Van Dorsselaer, A., Albrecht, P., and Ourisson, G., 1977. Identificationof novel (17αH)-hopanes in shales, coals, lignites, sediment andpetroleum. Bull. Soc. Chim. Fr., pp. 165-170.

Waples, D. W., 1977. C/N ratios in source rock studies. Colo. Sch.Mines Miner. Ind. Bull., 20:1-7.

Wright, N. J. R., 1980. Time, temperature and organic maturation—the evolution of rank within a sedimentary pile. J. Pet. Geol.,2:411-425.

1031

H. VON DER DICK, J. RULLKOTTER, D. H. WELTE

APPENDIXCompound Structures

I, rearranged sterenes I I , 4-methyl rearranged sterene I I I , cholest-2-ene

IV, ster-4-enes V, ster-5-βnes VI, 24-methylcholesta-4, 22-diene

18/

VII, 24-methylcholesta-5, 22-diene VI I I, steranes IX, neohop-13 (18)-enes

X, hop-17 (21)-enes XI, 17α (H)-hopanes X I I , 17j3 (H)-hopanes

XIII, 30-normoretane XIV, fern-8-ene XV, fern-9(11)-ene

XVI, lycopane

1032