29. ORGANIC GEOCHEMISTRY OF SOME CRETACEOUS BLACK SHALESFROM SITES 367 AND 368; LEG 41, EASTERN NORTH ATLANTIC

G. Deroo, J.P. Herbin, J. Roucache, and B. Tissot,Institut Francais du Petrole, 1 et 4 avenue de Bois Preau, 92502 Rueil-Malmaison, France

andP. Albrecht, J. Schaeffle, Institut de Chimie, Strasbourg, France

ABSTRACTThe black shale encountered in Cretaceous cores of the Cape

Verde area during the DSDP Leg 41 are of marine origin and corre-spond to excellent potential oil source rocks. They have a lowcontent of humic compounds.

Pyrolysis assays, chloroformic extracts, and kerogen data attest toa relatively low stage of evolution for samples at Site 367 (CapeVerde Basin). The samples from Site 368 (Cape Verde Rise) aremore evolved, and the deeper ones would be located at thebeginning of the principal zone of oil formation.

INTRODUCTION

The aim of this paper is to define, from ageochemical point of view, the black shale recovered inCretaceous cores of the Cape Verde area, during DSDPLeg 41.

A pyrolysis assay for the detection of hydrocarbonswas run on raw samples and chloroformic extractionswere performed. These are useful to characterize thetype of organic matter in sediments. Also, a study wasmade of the insoluble fraction including the humiccompounds and the kerogen fraction. The elementalcomposition of kerogen and the proportion of humiccompounds in organic matter yield valuable indicationsabout the marine autochthonous versus continentalorganic material. Furthermore, humic compounds aremore abundant in immature than in mature sediments,whereas the kerogen fraction is present in bothimmature and mature sediments.

SAMPLINGSeven samples of 50 g each were studied from Sites

367 and 368 (Figure 1). Five samples are black shale ofLate Cretaceous to Albian age and two are shale ofLate and Early Cretaceous age. Their stratigraphy andlithological description are given in Figure 2. The twoanalyzed samples from Site 368 are from either side of abasalt sill.

ANALYTICAL METHODSThe samples were first freeze-dried. Then they were

ground in an AUREC pulverizer (size grain 90 µm).The succession of analytical processes is shown onFigure 3. Organic carbon was analyzed with a LECOapparatus. A pyrolysis assay was made on raw samplesfor rapid source rock and maturation characterization.The method is deduced from kerogen experimentalstudies (Durand and Espitalie, 1973; Espitalie et al.,1973; Tissot et al., 1974). Equivalent parts of the

30° 25° 20° 15° 10° 5° 0'

Figure 1. Location of DSDP Sites 367 and 368.

ground samples were then extracted with chloroformand the extracts fractionated according to a methoddescribed in Hue et al. (1976). The total humiccompounds were extracted from the insoluble part of

865

G. DEROO ET AL.

EASTERN NORTH ATLANTICCAPE

VERDEBASIN

CAPEVERDERISE

OLIG

U.EOC

A

|-t- -)-| Foraml-t- -i-l Nanno Ooze (or Marl)

I -L Nanno Ooze (or Marl)

I I I Limestone

—~— Clay and Claystone

I'} •'".| Silt or Siltstone

U A | Chert

| | Porcellanite

fCXC] Basalt

• Cored interval

(18) Sampled cores

T 1200-1

Figure 2. Stratigraphic location of samples.

rock and humic acids were separated from fulvic acids(Hue, 1973). The new insoluble part corresponds tohumic and the nonhydrolysable fraction to kerogen(Durand et al., 1972; Robin et al., 1976). Analysis oforganic content from total humic compounds and fromthe fulvic fraction was performed with a CarmographWosthoff apparatus. Elemental analyses of C, H, O, N,S, and Fe plus ash content were measured on kerogenand calculations were made on a mineral-free basis.

DETAILED RESULTS

Carbonate Carbon (Table 1)All samples show a low content of carbonates (1% to

23%) and the richest was an Albian black shale fromSection 367-22-6.

Organic Carbon (Table 1)

The organic carbon content varied from 36% to 6%for the black shales, 4% for the Lower Cretaceous shaleof Site 367, and 3% for the Upper Cretaceous sandyshale of Site 368.

Pyrolysis Assay and Organic MatterCharacterization (Table 2, Figure 4)

The pyrolysis assay method (Espitalié et al., in press)gives a rough estimate of the elemental analysis method

on kerogen (Tissot et al., 1974). The combined oxygenand hydrogen indexes related to organic carbon areused for pyrolysis assays. H/C and O/C atomic ratioswere used for kerogen. They allow three types oforganic matter to be defined; types 1, 2, and 3 (Figure4). The hydrogen index is expressed in milligrams ofhydrocarbon compounds, related to a gram of organiccarbon, and the oxygen index in milligrams of carbondioxide, related to a gram of organic carbon. Thetemperature of the maximum hydrocarbon productionduring pyrolysis assays corresponds to the immaturezone for the 400°-400°C range, to the main oil zone ormature zone from 440°-460°C, and up to 460°C to gasissued from cracking, i.e., the metamorphosed zone.

The maximum of hydrocarbon production duringpyrolysis was reached within a low range of tempera-tures, between 396° and 436°C. Even for the oldestsamples from Sites 367 and 368, the temperatures didnot reach the main zone of oil formation.

The six black shale samples (Table 1) belong to highpotential source rocks of petroleum (Table 2); the threeUpper Cretaceous samples (367-18-2, and 368-63-3) areclose to type 1 and the two others of Cenomanian andAlbian ages (367-20-3 and 367-22-6) are close to type 2.They can be compared to immature material of type 1where the hydrogen index value is around 600 or moreand to type 2 with values of 450-500 (Table 2). Oxygenindexes for the two types are lower than 50.

Sections 368-59-1 and 367-23-2 have intermediatevalues between reference types 2 and 3. However,Section 368-59-1 is close to the type 2 and Section 367-23-2 is similar to type 3.

Type 3 kerogens correspond mainly to higher plantdetritus and humic continental matter. Their potentialfor oil is usually lower than for types 1 and 2 even iftheir gas yield is of the same importance. Type 2kerogens are common in marine sediments withautochthonous organic matter and type 1 correspondsto algal material. Both of them are rich in saturated C-H groups and their potential for oil is good to excellent.

Study of Chloroformic Extracts (Table 3, Figures 5,6,7,and 8)

All the studied samples (Table 3) provided large con-tents of extract (100 to 1700 mg). Their fractionationwas performed with the thin layer chromatography(TLC) method. The extract/organic carbon ratio ofSite 367 samples was medium (0.04 to 0.09) with a largefraction (Figure 5) of NSO compounds (78%-88%). Theextract/organic carbon ratio was higher (0.11 and 0.24)for the two samples from Site 368 and the NSOcompounds content was lower (63% and 68%). The gaschromatography analysis of the saturated fraction forthe two groups of extracts revealed a C15 to C30normal alkanes distribution plus the relatedisoprenoids, commonly present in the saturatedfraction of ancient sediments (Figure 6). The normalalkanes distribution is characterized by a high odd-to-even carbon number predominance between C15 andC29 for Site 367 samples with an increasing distributionfrom C15 to C29. The «-alkanes distribution from Site368 samples is relatively flat except for thepredominance of odd-to-even carbon number «-alkanesin the C25-C29 range.

866

ORGANIC GEOCHEMISTRY OF SOME CRETACEOUS BLACK SHALES

GRINDFREEZE-DRIED

ROCK

Pyrolysis

I II Mineral and |

OrganicCarbons '

I I

HCCI3

soluble

(Thin layer or liquid

chromatography)

HCI

and

HCI + HF

KEROGEN

1HUMICACIDS

(HA)

FULVICACIDS

(FA)

SATURATEDHYDRO-CARBONS

UN-SATURATEDHYDRO-CARBONS

Elemental 'Analysis I

I Organic 'I Carbon |

I II II Gas 1 1 Gas |

Chromato- Chromato-πranhw nranhw

I I I

Figure 3. Schematic analytical processes.

graphyI I

graphy

TABLE 1Location of Samples, Lithology and Age, and Carbon Data

Sample(Interval in cm)

Site 367

18-2, 80-8518-2, 104-10920-3, 38-4822-6, 0-1023-2, 0-10

Site 368

59-1,97-107

63-3, 17-27

Depth BelowSea Floor

(m)

638.30638.54687.88728.00777.50

939.47

978.17

Lithology

Black shaleBlack shaleBlack shaleBlack shaleInterbeddedshale and clay

Sandy blackshaleBlack shale

Age

Upper CretaceousUpper CretaceousCenomanianAlbianLower Cretaceous

Upper Cretaceous

Turonian-Albian

%CaCO3

61

132313

4

10

Organic Carbon

BeforeHCCI3

Extraction

36.0936.09

9.936.104.08

3.30

7.60

AfterHCCI3

Extraction

33.4033.40

9.495.833.96

3.00

6.12

Phytane and pristane isoprenoids are also present inabundance and phytane is the most important (17.35%-26.86% of total C14-C30 n-alkanes) isoprenoid (Figure

6). A large hump in the C29-C33 range of n-alkanes(Figures 7a, b) is present, except for the older section(368-63-3) (Figure 7c). It corresponds to unsaturated

867

G. DEROO ET AL.

-g 800-c

iO 7 0 0 -o

"§. 600-

á>

500-

×111O— 400-

LUαO 300-

oX

Type 1

/

IfII

13/3»r

!

1111l|•59/1

! !IIl|

TABLE 2Data of Pyrolysis Essays

Sample(Interval in cm)

Depth BelowSea Floor

(m)

OrganicCarbon(wt %)

Hydrogen Index(mg Hydroc.Compounds/g. Org. Carb.)

Oxygen Index(mg CO2/

g. Org. Carb.)

PyrolysisTemperature (°C)

Site 367

18-1, 80-8518-2, 104-10920-3, 38-4822-6, 0-1023-2, 0-10

638.30638.54687.88728.00777.50

30.8634.199.716.134.15

655604477453181

2320445886

403396411419431

Site 368

59-1,97-10763-3, 17-27

939.47978.17

3.146.92

313496

2412

423436

TABLE 3Chloroformic Extracts

Thin-LayerChromatography

Sample(Interval in cm)

Depth BelowSea Floor

(m)

HCC13

Extract(wt mg)

ExtractRock

(wt %)

ExtractOrg. Carb.

(wt)

NSOCompounds

Hydrocarb.Fractions

Aro. (%) Sat. (%)

Site 367

18-2,80-10920-3, 384822-6, 0-1023-2, 0-10

Site 368

59-1,97-10763-8, 17-27

638.30687.88728.00777.50

939.47978.17

1680.9290.3265.4101.7

244.81424.3

3.360.540.330.15

0.371.85

0.0930.0550.0430.037

0.1140.243

88788188

6863

71211

7

1818

510

85

1419

•

18/2

tea

S i t

-

C o

Unit

β 367

368

β/section

rval, cm)

Type 3

^ OXYGEN INDEX (mg. CO2/g.org.carbon)

Figure 4. Pyrolysis essays. Hydrogen and oxygen index (da-ta related to organic carbon).

compounds probably of polycyclic structures as by gaschromatography of the unsaturated fraction (Figures7a, c, bottom).

The unsaturated compounds were obtained fromTLC fractionation upon layers coated with AgNCb

The odd predominance shown by n-alkanes, a largecontent of isoprenoids (compared to n-alkanes), andthe occurrence of unsaturated cyclic hydrocarbons areindicative of an early stage of evolution sometimebefore the beginning of oil genesis and expulsion. Asomewhat higher evolution is shown by the samples ofSite 368, especially for the oldest sample (Section 368-63-31). This sample shows a more flattened or slightlydecreasing distribution of the C14-C30 n-alkanes and amuch lower content of unsaturated hydrocarbons(Figure 7), compared to a larger content of extract andhydrocarbons in the samples from Site 367. Theimportant contribution of supposed polycyclics in thetotal saturated fraction is characteristic of the marinetype of organic matter usually found associated withimmature samples of the type 2 kerogen. The poly-cyclics correspond to possible precursors of sterane andtriterpane groups of compounds.

The chromatograms obtained with the flame ioniza-tion detector (FID) were generally low, especially in theC30 + range because the yield of aromatic fraction waspoor for all the samples. However, a C15-C20 modewas detected. That mode, located at the beginning ofthe chromatogram, corresponds to mono- and

868

ORGANIC GEOCHEMISTRY OF SOME CRETACEOUS BLACK SHALES

AROMATICHYDROC.

100

• SITE 367

• // 368

63/3 core/section

/

AΔΔ100

SATURATEDHYDROC.

V

50

\Aso

100NSO

COMPOUNDS

(50) A 50

(40) 6 0

10050 40 30 20 10 0

Figure 5. Gross composition of the chloroformic extracts.

diaromatic molecules with a probable monoaromaticpredominance. The chromatogram from the flamephotometric detector (FPD) show in the older samples,such as 368-63-3 (Figure 8), benzothiophenes in theC15-C20 range (Castex et al., 1974).

Study of Humic Compounds (Table 4)

Five samples were analyzed for humic compounds(Table 4). The contents of humic material in cores fromSite 367 are generally low compared with their lowstage of evolution (< 14% of total organic carbon). Thelowest content is for the black shale with the highestorganic carbon content, Core 18, Section 2. Such a lowproportion of humic material is related to a relativelylow stage of the catagenesis and is indicative of amarine origin for the organic matter.

The only black shale sample from Site 368 (Core 63,Section 3) yielded a very poor content of humicmaterial (1.08% of total organic carbon) composed onlyof fulvic acids. This indicates marine organic matter.

Study of Kerogen (Table 5, Figure 9)

Six samples were analyzed for kerogen content. Theelemental composition of the kerogen fraction (Table 5)is interpreted using the H/C and O/C atomic ratios ona Van Krevelen diagram (Figure 9) in order to definethe types of organic matter (Tissot et al., 1974). Pyriterepresents a large proportion of the ashes (41%) in the

Upper Cretaceous Section 368-59-1. The black shale ofCore 18, Section 2, which has the highest content oforganic carbon, is located between path I and path IIbut close to the beginning of path II. All the othersamples from Site 367 are located close to the beginningof path II, except Core 23, Section 2, which is in anintermediate position between types II and III. Thissample corresponds to "shale and clay" material. Allthe samples show a very low evolution stage on thediagram.

Samples from Site 368 are also close to path II, butthey are farther from the beginning of path II than theother samples. That location indicates a somewhatmore advanced stage of evolution, and the older Core63, Section 3 is even farther from path II.

All the black shale can be compared with aliphaticand alicyclic-rich kerogens of type II which arecommonly found in organic matter from marine origindeposited in a reducing environment. The LowerCretaceous sample of Section 367-23-2 is attributed to amixture of continental and marine organic matter. All

22

23

59

63 ~

ORGANIC CARBON(%weight)

EXTRACTHCCI3

GC OF SATURATED HC

• —

π-»

10 15 20 25 30

CARBON ATOM NUMBER

Figure 6. Organic carbon content; gross composition ofchloroformic extracts; distribution of normal alkanesand isoprenoids in the C14-C30 range.

869

G. DEROO ET AL.

SITE 367

! I CORE 20/3{ M i l ; ! 'j Sot. i•l/nsσturσfeói tiydrocoròoπs

SITE 368

I i t I i i l l h l l i

CARBON ATOM NUMBER

Figure 7a. Extracts. Gas chromatography of saturated (topchromatogram) and unsaturated (bottom chromatogram)hydrocarbons. Site 367, Core 20, Section 3. Black shale,Cenomanian.

SITE 368

ill 11 'i i i i HIM II i MI i I I i HI i I I i MI i n ITCARBON ATOM NUMBER

Figure 7b. Extracts. Gas chromatography of saturated (topchromatogram) and unsaturated (bottom chromatogram)hydrocarbons. Site 368, Core 59, Section 1, Sandy blackshale, Upper Cretaceous.

these characteristics were largely comparable with theprevious conclusions deduced from the pyrolysisassays.

DISCUSSION AND CONCLUSIONS

The organic matter of black shale of Cretaceous agefrom Leg 41 sediments is of marine origin, and theshales correspond to excellent potential oil sourcerocks. Their low content of humic compounds suggest a

CARBON ATOM NUMBER

Figure 7c. Extracts. Gas chromatography of saturated (topchromatogram) and unsaturated (bottom chromatogram)hydrocarbons. Site 368, Core 63, Section 3, Black shale,Turonian to Albion.

SITE 367

F P D . . . i . i i ; i i i u I I i I I I I I

M l ! ' ; ! i

i CORE 63/3

CARBON ATOM NUMBER

Figure 8. Extracts. Gas chromatography of aromatic frac-tion (FID) and thiophenic compounds (FPD).

870

ORGANIC GEOCHEMISTRY OF SOME CRETACEOUS BLACK SHALES

Sample(Interval in cm)

Site 367

18-2, 80-10920-3, 38-3822-6, 0-1023-2,0-10

TABLE 4Humic Compounds - Carbon Organic Content

Depth BelowSea Floor

(m)

638.30687.88728.00777.50

TotalOrganicCarbon(wt %)

32.529.716.134.15

Carbon Content(Percent Total Org. Carbon)

Humic Fulvic HumicCompounds Acids Acids(FA + HA) (FA) (HA)

0.5210.0011.5814.22

0.431.651.631.44

0.098.359.95

12.78

FA

HA

(4.89)0.200.160.11

Site 368

63-3, 17-27 978.17 6.92 1.08 1.08 0.00 indet.

TABLE 5Kerogens - Elemental Composition and Ash Content

Sample(Interval in cm)

Depth BelowSea Floor

(m)

Weight Percent, on Ash-Free Basis Q/CC H O S N H/C X102

Ashes(wt %)

Site 36718-2, 80-10920-3, 38-4822-6, 0-1023-2, 0-10

Site 36859-1,97-10763-3, 17-27

638.30687.88728.00777.50

939.49978.17

67.1267.8165.7869.10

77.6178.47

7.907.027.256.37

7.537.65

11.3314.9713.7516.80

9.615.75

11.447.36

10.854.60

2.795.27

2.212.842.373.13

2.472.86

1.411.241.321.11

1.161.17

12.6616.5515.6718.24

9.286.49

7.9417.0019.8831.22

43.0422.46

O/C0,4 0,5

(ATOMIC RATIO)

Figure 9. Kerogens. H/C and O/C diagram.

low original content of continental organic matter asconfirmed with pyrolysis assays. However, somecontinental input is present in the Lower Cretaceousshale of Section 367-23-2. The relatively low stage ofevolution of sediments from Site 367 is shown frompyrolysis indices, chloroformic extracts, and kerogendata. A somewhat higher evolution is apparent for thesamples from Site 368, especially for the oldest sample,from beneath a volcanic sill. This latter sample

corresponds to the beginning of the principal zone ofoil formation.

REFERENCESCastex, H., Roucache, J., and Boulet, R., 1974. Le soufre

thiophénique dans les pétroles et les extraits de roche.Analyse par spectrométrie de masse et chromatographie enphase gazeuse: Rev. Inst. Franc. Pétrole, v. 24, p. 3-40.

Durand, B. and Espitalie, J., 1973. Etude de la matièreorganique au cours de 1'enfouissement des sediments: C.R.Acad. Sci., Paris, D-276, p. 2253-2256.

Durand, B., Espitalie, J., Niccuse, G., and Combaz, A., 1972.Etude de la matière organique insoluble (kérogène) desargiles du Toarcien du Bassin de Paris. Premierepartie—Etude par les procédés optiques. Analyseélémentaire. Etude en microscopie et diffractionélectroniques: Rev. Inst. Franc. Pétrole, v. 27, p. 865-884.

Espitalie, J., Durand, B., Roussel, J.C., and Souron, C, 1973.Etude de la matière organique insoluble (kérogène) desargiles du Toarcien du Bassin de Paris. II—Etudes enspectrométrie infrarouge, en analyse thermique différen-tielle et en analyse thermogravimétrique: Rev. Inst. Franc.Pétrole, v. 28, p. 37-66.

Espitalie, J., Laporte, J.L., Madec, M., Marquis, F., Leplat,P., Paulet, J., and Boutefeu, A., in press. Méthode rapidede caracterisation des roches mères, de leur potentielpétrolier et de leur degré devolution. Rev. Inst. Franc.Pétrole, v. 32.

Hue, A.Y., 1973. Contribution à 1'étude de 1'humus marin etde ses relations avec les kérogènes: Thesis, University ofNancy, France.

871

G. DEROO ET AL.

300 650

Figure 10. Mass chromatogram of Sample 367-18-2, 80-109 cm, corresponding to peaks 191, 217, and 231.

200

Figure 11. Mass chromatogram of Sample 367-23-2, 6-10 cm corresponding to peaks 191, 217, and 231.

300

C 2 7(5αH) C 2 8 C 2 9

600

Figure 12. Mass chromatogram of Sample 368-59-1, 97-107 cm, corresponding to peaks 191, 217, and 231.

872

ORGANIC GEOCHEMISTRY OF SOME CRETACEOUS BLACK SHALES

'27

C29

300 400m/1

500 550

Figure 13. Mass chromatogram of Sample 367-23-2, 10-10 cm, corresponding to peak 257 (sterenes).

Hue, A.Y., Roucache, J., Bernon, M., Caulet, G., and DaSilva, M., 1976. Application de la chromatographie surcouche mince à 1'étude quantitative et qualitative desextraits de roche et des huiles. Rev. Inst. Franc. Pétrole,v. 31, p. 67-98.

Robin, P.L., Rouxhet, P.G., and Durand, B., 1976.Caractérisation des kérogènes et de leur evolution parspectroscopie infra rouge: Advances in organic geo-chemistry, Madrid.

Tissot, B., Durand, B., Espitalie, J., and Combaz, A., 1974.Influence of the nature and diagenesis of organic matter inthe formation of petroleum: Am. Assoc. Petrol. Geol.Bull., v. 58, p. 499-506.

APPENDIXAnalysis by Gas Chromatography-Mass Spectrometry

of Polycyclic Hydrocarbons

The hopane, steranes, and methyl steranes have been investigatedby computerized GC-MS on Samples 367-18-2, 80-109 cm; 367-23-2,

0-10 cm; and 368-59-1, 97-107 cm. The mass chromatogramscorresponding to peaks 191, 217, and 231 are shown on Figures 10,11, and 12. They correspond, respectively, to the hopane, steranes,and methylsteranes series.

Steroids are present in all samples, and are relatively abundant.They occur mostly as steranes in Sections 368-59-1, and mostly asunsaturated sterenes in Sections 367-18-2 and 367-23-2. The sterenescorrespond to two different series, which could be the Δ4 and Δ5

sterenes (Figure 13).Steranes of Section 368-59-1 also correspond to a different series:

the 5 α H steranes (C27, C28, C29) are the major series and the sub-ordinate one is probably the 5 ß H steranes (C27, d», C29).Methylsteranes show a low abundance in all sediments compared tosteranes or sterenes.

Triterpanes of the hopane type are particularly abundant in Section367-18-2. The naturally occurring and less stable 17 (/?H) hopaneseries is predominant in Sections 367-18-2 and 367-23-2 (up to C35,with a single diastereomer in position 22). However, the more stable17 (αH) hopane series occurs in subordinate amounts. In Section 368-59-1, the 17 (αH) hopanes are predominant.

The occurrence of unsaturated compounds and of the 17 (ßH)hopane series indicates a low stage of diagenesis. However, Section368-59-1 may correspond to a more advanced stage of diagenesis.

•

873