7. PETROLEUM-GENERATING POTENTIAL OF SEDIMENTS FROM LEG 40,DEEP SEA DRILLING PROJECT

J. W. Kendrick, A. Hood, and J. R. Castano, Shell Development Company,Bellaire Research Center, Houston, Texas

INTRODUCTION AND SUMMARY

Leg 40 of the Deep Sea Drilling Project (DSDP),along the southwestern margin of Africa, recoverednumerous cores of sediment containing greater than 1%organic carbon. We have studied several samples fromthese cores in order to evaluate their petroleum-generating potential. In addition, data on the levels oforganic metamorphism of these sediments were used todetermine whether any of the sediments ha,ve beenheated sufficiently to generate petroleum.

The results of this study indicate that many Leg 40sediments contain sufficient amounts of effectiveorganic carbon (Ceff) to be considered potentialpetroleum source rocks. Probably none of the samples,however, have been buried deeply enough to reach thezone of significant petroleum generation (seeForesman, this volume).

Comparison of effective organic carbon with thetotal organic carbon indicates that a large fraction ofthe organic matter in many samples is not thermallyconvertible to hydrocarbons. Visual observationsfurther indicate that some sediments contain significantamounts of reworked organic matter.

ANALYTICAL METHODSOrganic richness and temperature history are major

parameters for identifying petroleum source rocks. Theorganic richness is an indicator of the petroleum-generating potential of a rock, and therefore organic-rich rocks may be referred to as "potential sourcerocks." The thermal history determines whether thepotential source rock has reached the stage at whichpetroleum has been generated and expelled, and thuswhether it has become an actual source rock.

Probably the most commonly used measure oforganic richness is total organic carbon (%Corg) which isthe acid-insoluble carbon in the sample. In addition, itis important to have a measure of a sample's effectiveorganic carbon content (C^), i.e., that portion of theorganic matter which can be converted to petroleumduring burial at greater depths and temperatures. Asmeasures of the effective carbon content, we have usedtwo laboratory pyrolysis methods. The first pyrolysismethod—pyrolysis fluorescence (PF)—is usedprimarily as a rapid screening tool which measures theamount of fluorescing bitumen (in arbitrary PF units)formed by pyrolysis. PF values in rocks can range fromzero to several thousand units. The second pyrolysismethod—pyrolysis-FID (P-FID)—measures the

amount of volatile hydrocarbon-like compoundsgenerated in the laboratory temperature range of 300-650°C, and it approximates the amount ofhydrocarbons generated at lower temperatures in thesubsurface. The effective carbon content of a sample iscalculated as 85% of the pyrolysis hydrocarbon content.

To determine whether the sediments had beensubjected to temperatures sufficient for the thermalconversion of kerogen to petroleum, we measured thereflectance (in oil) of vitrinite, a coal maceral which isdisseminated in many sediments. Vitrinite reflectanceprovides a measure of the level of organicmetamorphism (LOM) (Hood et al., 1975) and isapplicable over a wide range of coal rank andconditions during which oil and gas are formed (Figure1).

A more complete description of the analyticaltechniques has been given by Hood et al. (1976).

RESULTS AND DISCUSSION

Level of Organic MetamorphismThe results of the vitrinite reflectance measurements

are summarized in Table 1. In converting vitrinitereflectance (Ro) to LOM, all Ro values less than ~0.4%were assigned the LOM value <7 because of thedifficulty in resolving the LOM 0-7 range by means ofvitrinite reflectance. In addition, the effect of lithologyon vitrinite reflectance (Bostick and Foster, 1973) raisessome questions about the relationship of reflectance toLOM in deep-sea sediments with Ro less than 0.8%-1.0%.

Several of the samples from Leg 40 exhibit a broaddistribution of vitrinite reflectance values. Thereflectance values for vitrinite in a humic coalcommonly fall into a narrow range. The occurrence ofbroader reflectance distributions in several Leg 40samples (see comparison in Figure 2) implies that somevitrinite has either been partially oxidized or recycledfrom older sedimentary units with a prior thermalhistory. Consequently, the mean value of Ro for coresamples with reworked, or secondary, vitrinite will begreater than that for the primary vitrinite. For thisreason Table 1 includes two values of Ro and LOM foreach sample which appears to contain secondaryvitrinite. The "X" value is the mean of all Ro observa-tions for the sample, and it represents a maximumestimate of LOM. The "A" value is an estimate of themean Ro of primary vitrinite, obtained by omittingsample observations which appear to be attributable to

671

J. W. KENDRICK, A. HOOD, J. R. CASTANO

LOM

n

2 —

4 —

6 —

8 —

10 —

14 —

16 —

18 —

20 —

COALRANK

PEATAND

LIGNITE

CSUB-

B

C

HIGHVOL. BBIT.

A

M V BIT.

LV BIT.

SEMI-ANTH.

ANTH.

VITRINITEREFLECTANCE(CASTANO, IN

HOOD andCASTANO,

1974)

— 0.5

— 1.0

— 2.0

— 2.5

— 3.0

— 3.5

— 4.0

PRINCIPAL STAGESOF PETROLEUM

GENERATION(VASSOYEVICH et al., 1970)

EARLYDIAGENETIC

METHANE

OIL

CONDENSATEAND

WET GAS

THERMALCATAGENETIC

METHANE

Figure 1. Scale relating coal rank, vitrinite reflectance, andpetroleum generation to the level of organic meta-morphism (LOM), after Hood and Castano (1974) andHoodetal (1975).

secondary vitrinite. The vitrinite reflectances of sampleswith large proportions of secondary vitrinite areconsidered less-reliable estimates of LOM than arethose of samples with only primary vitrinite.

The majority of LOM values in Table 1 lie in therange of <7 corresponding to coal ranks no higher than

subbituminous B. Such samples probably have notbeen buried sufficiently to reach the LOM (~8) atwhich significant oil generation begins (Figure 1; Hoodet al., 1975; Vassoyevich et al., 1970). Although severalsamples from Site 361 exhibit LOM values greater than8, these samples contain large amounts of secondaryvitrinite. The interpreted, or "A," values of LOM forSite 361 are in good agreement with the low LOMvalues from comparable depths in Site 364, implyingthat the Site 361 sediments have indeed not yet reachedthe oil-generation stage.

Organic Richness

Numerous values of organic carbon content havebeen suggested as minimum requirements for potentialpetroleum source rocks. Ronov (1958) concluded thatthe critical CorK value for source rocks of economicpetroleum accumulations lies somewhere between theaverage values for clays of petroliferous (1.4%) andnonpetroliferous (0.4%) areas of the Russian Plat-form—and probably closer to the former. This suggestsa minimum value of about 1.0% CW(,. Schrayer andZarrella (1963) reported a value of about 1.5% C ^ as aminimum requirement for oil source rocks based onstudies of the Mowry Shales of Wyoming. The organiccarbon contents of the Leg 40 sediments (Table 2) rangefrom 0.1% to 13.8%. The above criteria indicate thatseveral of these samples are good potential sourcerocks.

The results of the pyrolysis-FID and pyrolysisfluorescence measurements (Table 2) place anadditional constraint on determining which samplesshould be considered potential source rocks. As ageneral rule, we do not consider samples with less than0.3% hydrocarbons (by P-FID) or less than 10 PF unitsto be potential source rock for economic oilaccumulations. Similarly, samples with less than 0.8%hydrocarbons or less than 30 PF units are considered tobe marginal source rocks at best. While generally thereis good agreement between organic carbon andpyrolysis-FID in the evaluation of source rockpotential, there are some exceptions. For example,Samples 361-36-3, 140-150 cm and 365-1-5, 130-133 cmcontain about 2% Corg, but less than 0.3% hydro-carbons, implying that a large fraction of the organicmaterial in these samples is not thermally convertible topetroleum. Consequently, some samples with moderateamounts of organic carbon may not contain sufficientamounts of reactive organic matter to be consideredpotential petroleum source rocks.

A comparison of effective carbon with organiccarbon provides information about the compositionand nature of the organic matter. Tissot et al. (1974)have demonstrated that differences in the elementalcomposition (especially hydrogen and oxygencontents) of the kerogen strongly influence the amountsof petroleum produced during heating. The graph ofeffective carbon versus organic carbon (Figure 3)suggests that consistent compositional differences existbetween sediments from different sites. In particular,the sediments at Site 362 contain greater amounts ofeffective carbon per unit of organic carbon than do the

672

PETROLEUM-GENERATING POTENTIAL OF SEDIMENTS

TABLE 1Vitrinite Reflectance and Level of Organic Metamorphism

Sample(Interval in cm)

Site 361

23-3, 143-150

25-3, 140-15027-3, 140-150

29-5, 140-150

31-2, 144-150

34-4, 134-136

36-3, 140-150

38-2, 130-133

40-3, 130-138

42-0, 11-13

44-3, 137-144

Site 362

3-4, 130-1405-4, 130-1357-5, 145-1509-5, 144-15011-5, 130-135

13-4, 130-14015-5, 130-150

19-5, 130-136

25-5, 142-150

Site 363

30-5, 130-136

34-2, 142-150Site 364

20-3, 130-136

22-2, 120-12339-5, 145-150

41-3, 144-150

43-1, 130-133Site 365

1-5, 130-133

3-1, 130-135

Depth BelowSea Floor

(m)

768

863958

1037

1051

1082

1099

1117

1147

1181

1223

618091

120138

156186

262

376

523

595

582

618975

1010

1035

233

397

Age

Cretaceous?

CretaceousAlbian-Aptian

Aptian?

L. Aptian?

L. Aptian

L. Cretaceous

L. Aptian?

_

-

_

PleistocenePleistocenePleistocenePleistoceneL. Pliocene

L. PlioceneU. Miocene

U. Miocene

U. Miocene

U. Albian

Albian-Aptian

Coniacian

TuronianAlbian-Aptian

Albian-Aptian

-

Miocene w/reworked Cret.Miocene w/reworked Cret.

.a

AX

AXAXAXAXAXAXAXAXAX

AX

AXAXAX

AX

AX

AXAX

Vitrinite Reflectance (in Oil)

No. ofObservations

2366

0125433554850266017542654376227594160

131721191718

656

19261014

730

9

733

920241819

3

24

17

Range of %RO

0.29-0.520.29-0.91Barren0.29-0.530.29-1.000.27-0.600.27-0.870.21-0.580.21-0.680.33-0.550.33-1.040.16-0.520.26-0.930.24-0.520.24-0.950.31-0.600.25-1.020.30-0.570.25-0.950.32-0.640.26-0.89

0.20-0.350.15-0.360.17-0.340.18-0.400.19-0.450.19-0.530.23-0.470.23-0.470.23-0.620.23-0.510.23-0.700.22-0.530.22-0.85

0.40-0.450.40-0.990.66-1.18

0.27-0.450.27-1.010.68-1.250.23-0.360.23-0.700.23-0.480.23-0.690.20-0.26

0.22-0.53

0.19-0.53

Mean %RO °

0.44 +0.030.61 ±0.04

0.44 ±0.050.66 ±0.050.47 ±0.040.57 ±0.040.40 ±0.030.41 ±0.040.47 ±0.030.62 ±0.050.42 ±0.040.64 ±0.050.40 ±0.040.53 ±0.050.46 ±0.040.57 ±0.050.46 ±0.040.59 ±0.050.47 ±0.040.55 ±0.05

0.26 ±0.030.24 ±0.030.23 ±0.020.27 ±0.040.31 ±0.050.32 ±0.050.33 ±0.110.36 ±0.130.41 ±0.150.40 ±0.040.46 ±0.050.35 ±0.080.45 ±0.11

0.42 ±0.020.65 ±0.070.98 ±0.13

0.33 ±0.060.66 ±0.080.89 ±0.150.28 ±0.020.34 ±0.060.32 ±0.040.34 ±0.060.24 ±0.08

0.32 ±0.04

0.35 ±0.06

LOMC

79

797.58.5

<777.5979

<787.58.57.58.57.58.5

<7<7<7<7<7< 7 A

N.D.d

N.D.N.D.

<7

<7N.D.

79N.D.

<79N.D.

<7<7<7<7<7

<7

<7

"X" represents the entire range of vitrinite observations; "A" represents the interpreted range of primary vitrinite observa-tions. Where "A" and "X" are not noted, they are the same.

b %RQ ± 95% confidence limit,c All Ro values less than 0.4% are assigned LOM<l because of the difficulty of resolving the LOM 0-7 range by means of

vitrinite reflectance. Ro values <0.4% are converted to LOM on the basis of Castaho's /?o-L0M relationship (Hood andCastano, 1974).

N.D. - Not determined, because variance of data was too great.

673

J. W. KENDRICK, A. HOOD, J. R. CASTANO

0.0 0.1

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0.7 0.8

PERCENT

(B)

X XX X X

361-40-3, 130-138 cm

X XX

1 L.0.0 0.1 0.2 0.3 0.7 0.8

PERCENT

0.9 1.0 1.1 1.2 1.3 1.4 1.5

" " A " ~ H VITRINITE REFLECTANCE HISTOGRAM

Figure 2. Histograms of vitrinite reflectance for samples with (A) primary vitrinite (362-5-4, 130-132 cm); (B) pri-mary and secondary vitrinite (361-40-3,130-138 cm). "A "denotes the range of interpreted primary reflectancevalues.

674

PETROLEUM-GENERATING POTENTIAL OF SEDIMENTS

TABLE 2Organic Richness

Sample(Interval in cm)

eff%C

org PF

% HC % Cfrom P-FID(300-650°C) (% HC × 0.85)

Site 361

23-3, 143-15025-3, 140-15027-3, 140-15029-5, 140-15031-2, 144-15034-4, 134-13636-3, 140-15038-2, 130-13340-3, 130-13842-0, 11-1344-3, 137-144

Site 362

3-4, 130-1355-4, 130-1357-5, 145-1509-5, 144-15011-5, 130-13513-4, 130-14015-5, 130-15019-5, 130-13625-5, 142-150

Site 363

30-5, 130-13634-2, 142-150

Site 364

20-3, 130-13622-2, 120-12339-5, 145-15041-3, 144-15043-1, 130-133

Site 365

1-5, 130-1333-1, 130-135

0.860.250.674.232.681.761.883.540.953.882.80

3.573.752.333.352.442.462.351.060.70

0.140.19

0.100.101.586.1213.8

2.488.56

05483350284192

24032

21024012018015012098102

22

13

1202401500

9300

0.020.000.090.850.470.370.210.390.220.920.29

1.581.891.441.090.700.800.570.200.14

0.000.00

0.020.000.311.70

11.0

0.284.305

0.010.000.080.730.400.310.180.330.190.780.25

1.351.611.220.930.600.680.480.170.12

0.000.00

0.020.000.261.449.35

0.243.66

sediments at Site 361, implying that the organic matterat Site 361 contains a greater percentage of eitheroxygenated carbon compounds or recycled, thermallyless reactive organic matter.

During the measurement of vitrinite reflectance, wemade visual estimates of the types of organic matter inthe sediments (Table 3). From these qualitative visualestimates, it is apparent that the samples from Site 362contain relatively large amounts of amorphous(generally hydrogen-rich) organic matter, whereas thesamples from Site 361 contain significant amounts ofhumic material as well as some amorphous organicmatter. The greater proportions of humic matter in thekerogens of Site 361 sediments help to explain theirlower values of effective carbon per unit of organiccarbon.

REFERENCESBostick, N.H. and Foster, J.N., 1973. Comparison of vitrinite

reflectance in coal seams and in kerogen of sandstones,shales, and limestones in the same part of the seiminentarysection: In Alpern, B. (Ed.), Pétrographie de la MatiéreOrgaπique des Sediments, Relations avec la Paléotem-pérature et le Potentiel Pétrolier: Paris, (Centre Nationalde la Recherche Scientifique), p. 13-25.

TABLE 3Relative Abundance of Types of Organic Matter

by Visual Kerogen AnalysisReworked

Primary Humic andHumic Inert

Sample(Interval in cm) Amorphous Exinitic

Site 361

23-3, 143-15025-3, 140-15027-3, 140-15029-5, 140-15031-2, 144-15034-4, 134-13636-3, 140-15038-2, 130-13340-3, 130-13842-0, 11-1344-3, 137-144

Site 362

3-4, 130-1405-4, 130-1357-5, 145-1509-5, 144-15011-5, 130-13513-4, 130-14015-5, 130-15019-5, 130-13625-5, 142-150

Site 363

30-5, 130-13634-2, 142-150

Site 364

20-3, 130-13622-2, 120-12339-5, 145-15041-3, 144-15043-1, 130-133

Site 365

1-5, 130-1333-1, 130-135

2J22

222222

22

222211

11

11

21

21

21234424344

222222211

66

41225

62

Note: Numerical abundance scale and percentages (by area):1=0-1%; 2=2-5%; 3=6-10%; 4=11-25%; 5=26-50%; 6=51-75%; 7=76-100%.

Hood, A. and Castafio, J.R., 1974. Organic metamorphism:Its relationship to petroleum generation and application tostudies of authigenic minerals: United Nations ESCAP,CCOP Tech. Bull., v. 8, p. 85-118.

Hood, A., Gutjahr, C.C.M., and Heacock, R.L., 1975.Organic metamorphism and the generation of petroleum:Am. Assoc. Petrol. Geol. Bull., v. 59, p. 986-996.

Hood, A., Castario, J.R., and Kendrick, J.W., 1976.Petroleum-generating potential and thermal history ofDSDP Leg 38 sediments. In Talwani, M., Udintsev, G., etal., Initial Reports of the Deep Sea Drilling Project, 38,Washington (U.S. Government Printing Office),p.801-803.

Ronov, A.B., 1958. Organic carbon in sedimentary rocks (inrelation to the presence of petroleum). Geochemistry (atranslation of Geokhimiya) No. 5, p. 510-536.

Schrayer, G.J. and Zarrella, W.M., 1963. Organic geo-chemistry of shales—I. distribution of organic matter inthe siliceous Mowry Shale of Wyoming: Geochim. Cosmo-chim. Acta, v. 27, p. 1033-1046.

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

Vassoyevich, N.B., Korchagina, Yu. I., Lopatin, N.V., andChernyshev, V.V., 1970. Principal phase of oil formation:Internatl. Geol. Rev., v. 12, p. 1276-1296.

675

J. W. KENDRICK, A. HOOD, J. R. CASTANO

4.0

3.0

o 2.0

1.0

w^

VDO

— SITE 361— SITE 362

SITE 363SITE 364SITE 365

13.8% Corg

8.56% Co

D

5.0 6.0 7.0

.Figure 3. Graph of effective organic carbon (Coff) versus total organic carbon (C' ) .

676