16. GEOCHEMICAL ANALYSES OF POTENTIAL PETROLEUM SOURCE BEDS

Paul J. Cernock, Texaco Inc., New Orleans, Louisiana

INTRODUCTION

Of the eight sites drilled during Leg 24, only oneprovided a cored interval which was visually definable as apotential petroleum source bed. The lithologic descriptionfor Site 231 shows two thin layers (upper Miocene age) at89-91 cm and 103-105 cm in Core 44, Section 1 as bitumenstreaks containing approximately 5% bitumen. Threesamples (Table 1) were sealed in plastic bags and latergeochemically analyzed for organic content and heavyhydrocarbons by a commercial laboratory.1 Light hydro-carbon analyses, i.e., C^ through Cη, were not performedbecause of the sampling procedure. The samples were sealedwithout benefit of distilled water and bactericide; there-fore, bacterial action upon the light hydrocarbons pre-cluded their measurement. Likewise, the plastic bag offeredan escape route for the light volatile hydrocarbons.

TABLE 1Sample Identification, Site 231

Sample

12(dk)3 (It)

Core

444444

Sample Identification

Section

111

Sample Depth(cm)

89-91103-105103-105

The geochemical analyses carried out comprised thestandard percent organic carbon (Table 2); total C 1 5 +

soxhlet extraction, deasphalting, and liquid chromato-graphic separation (Table 3); C 1 5 + normal paraffin analysisby gas chromatography (Table 4, Figure 3); and visualkerogen assessment (Table 5).

Before progressing into the specifics of this study, it isappropriate to present a very simplified scheme of sequencefor hydrocarbon genesis. The top diagram in Figure 1illustrates the development of hydrocarbons produced bythe progressive thermal degradation of organic matter,which is finely disseminated throughout the source rocksand/or sediments. An envelope of interest is definedbetween geothermal gradients of 3°F/100 feet and l°F/100feet, which represent the range of most geothermalgradients in the world. Initial thermal degradation oforganic matter originally produces primarily a heavy oil.Continued thermal degradation produces oil and gas, lightoil and gas, and finally the end product of pyrolysis—onlydry methane gas (Klemme, 1972).

1 GeoChem Laboratories, Inc., Houston, Texas.

The bottom diagram in Figure 1 is a general illustrationof the synthesis of hydrocarbons by bacterial activity.There is no evidence for the generation of oil as such bybiochemical processes, although certain hydrocarbons areproduced metabolically by numerous organisms (Stevens,1956). Fermentation of plant debris yields abundantquantities of methane, which is the source of marsh gas.Minute quantities of ethane are also produced in thismanner. The aerobic and anaerobic bacterial action islimited to temperatures generally less than 212°F. How-ever, literature shows that bacterial action predominates atshallow depths and becomes insignificant with increasingdepth (depicted by the width of the arrows in the bottomdiagram of Figure 1). Biogenically-derived methane gaswould be produced mainly at depths less than a couple ofthousand feet.

This study is oriented primarily toward defining thecharacter of potential liquid petroleum source beds — sincethis is a thermal process. The degree of maturation of apotential source bed herein will be defined by its normalparaffin distribution, pristane/phytane ratio, saturate/atomatic ratio, and kerogen analysis.

NORMAL PARAFFIN DISTRIBUTION ANDPRISTANE/PHYTANE RATIO

Figure 2 presents a chromatogram of the C15+ paraffin-naphthene fractions of a typical immature shale. The insert"A" illustrates the method of determining the peak heightsof the normal paraffins. The chart at the Top of Figure 2demonstrates the calculation of the average ratio of odd-toeven-numbered n-paraffins (CP index). Two differentdenominators are used (C26-C32 a n ^ C24-C30) so as not topreference the ratio to the higher or lower even-numberedn- paraffins. The CP index of 3.12 identified a typicalimmature shale.

Studies presented in the literature show that recent andimmature sediments have high values of CP index, rangingfrom 2.5 to 5.5 with an overall average of 3.6. These highvalues are due to the contribution of land derived organicmatter. The CP index approaches 1.0 with increasingmaturity (Bray and Evans, 1961). A low CP index may alsooccur if the organic matter is purely marine in origin, suchas phytoplankton.

The pristane/phytane ratio of 0.63 in Figure 2 indicatesimmaturity. This ratio becomes much greater than 1.0 withincreasing maturity.

The C 1 5 + paraffin-naphthene analyses by gas chromato-graphy for the three sediment samples are presented inTable 4 and Figure 3. The gas chromatographic patterns(Figure 3) clearly show that Samples 2 and 3 are closely

791

P. J. CERNOCK

TABLE 2Geochemical Analytical Data

Sample

123

SampleDepth(cm)

89-91103-105103-105

OrganicCarbonContent

1.651.83, 1.840.97

TotalC15 + Extract

(ppm)

554439303709

TotalHC's

(ppm)

300350172

Kerogen Data

Type

Am, H, WAm, H, WAn,-, H

Alteration

11 - 1 +1

TABLE 3Summary of C15+ Soxhlet Extraction Deasphaltening, and Liquid Chromatography

Sample

123

Sample

123

TotalExtract(ppm)

554439303709

P - N(%)

2.814.833.19

P - N(ppm)

155190118

Hydro-carbonsAROM

(%)

2.604.071.45

Concentration of Extracted Materials in Rock

AROM(ppm)

144160

53

PN/AROM

1.081:192.20

Hydrocarbons NonhydrocarbonsTotal Precipitated Eluted

(ppm) Asphaltene NSO's

300 4566 633350 3000 460172 3172 322

Composition of Extracts

Eluted Noneluted Precipitated AsphNSO's NSO's Asphaltene NSO

11.4 0.80 82.4 6.7411.7 3.05 76.3 5.17

8.7 1.16 85.5 8.63

NonelutedNSO.'s

44120

43

HCs

(%)

5.418.914.64

[ Total(ppm)

524435803537

NC/non-HC

0.0570.0980.049

TABLE 4Saturate Hydrocarbon Analyses

Normalized Paraffin Distribution

Hydrocarbons

«Cj.5

«Ci7Φ-C19

Φ-C20nC\g

"C20«C2i"C22« C 2 3

« C 2 5

"C26

"C28

"C30/1C31« C 3 2

"C33

Sample 1

0.81.6

2.82.45.54.34.45.37.9

13.315.611.2

5.95.15.11.73.31.22.30.30.2

Paraffin Distribution (percent)ParaffinIsoprenoidNaphthene

CP index

14.01.0

85.0

1.44

Pristane/Phytane ratio

(iP-Cl9/ip-C20) 0.57

Sample 2

0.91.4

4.83.68.17.86.36.67.1

10.19.56.85.85.25.61.64.01.23.00.30.3

8.61.1

90.3

1.75

0.46

Sample 3

0.50.9

3.32.07.55.27.06.27.0

10.410.9

7.36.54.76.32.34.91.54.50.40.9

9.50.7

89.8

1.95

0.39

similar, but that Sample 1 differs in the paraffin distribu-tion. There is a very definite preference for n-paraffinswith odd numbers of carbon atoms in the higher (C24+)molecular weight paraffins for all three samples. "TheCP index for the three samples ranges from 1.44 to1.95. Normally, this might be interpreted as due todifferences in maturity; however, all three samples are soclose in the stratigraphic column that they could not havebeen exposed to any significant differences in temperature.Consequently, the differences in CP index are more likelydue to different contributions of land and marine derivedorganic matter. Another possible, although less understoodfactor, is microbial reworking at the time of sedimentation.This has a tendency to depress CP indices. The low valuesof the pristane/phytane ratio (0.39-0.57) suggest that thesediment is rather immature".

The samples all contain predominant amounts ofnaphthene components as evidenced by the naphtheneenvelope (Figure 1) and the computed compositional data(Table 4).

It is interesting to compare the C 1 5 + paraffin-naphthenedata of the sediment samples with that from a mature oil ofTertiary age (Figure 4). Surprisingly, the chromatogramsare very similar in distribution of naphthenes and com-parable in «-paraffin content. The pristane/phytane ratio of1.30 and CP index of 1.13 both identify this oil as beingmature. The author believes that the organic shales fromSite 231 will, under increased thermal maturation, producean oil similar to that in Figure 4.

SATURATE/AROMATIC RATIOS

The saturate/aromatic ratio and the quantity of totalextracted hydrocarbons provide a clue to the maturity of a

792

GEOCHEMICAL ANALYSES OF POTENTIAL PETROLEUM SOURCE BEDS

Sample

12

3

Type of Organic Matte

alga

eR

eco

gniz

able

+

ipro

pel

Am

orp

ho

us-

s

•

Sp

ore

s-p

oll

ei

mi-+

5 d

ebri

soa

Cu

ticle

-mem

H-

ebri

stu

red

dW

oo

dy

-str

uc

$t-\J

Co

al d

ebri

s

4-•4-

1

Py

rob

itu

men

4-4-L

r

ris

ble

deb

No

nre

cogn

iza

44-_u

yel

low

Gre

enis

h l

igh

t

A

A

TABLE 5Visual Kerogen Assessment Data

Color of Organic Matter

Yell

ow

iV.

Ye

llo

w o

ran

g<

rO

ran

ge b

row

r

Lig

ht

bro

wn

Bro

wn

Dar

k b

row

n

Bla

ck

_

State of Organic Matter

Particle Size

lin

ated

Fin

ely

D

isse

n

Fin

e

•

Med

ium

•__

Co

arse

Preservation

Ex

cel

len

t

|

A

Go

od

E

Fan

-

Po

or

J

Maturation Index

Un

alte

red

1

I•

Sli

ghtl

y a

lter

e

1+

r

T3α>

Str

on

gly

alte

r

3

T3α>

Sev

erel

y a

lter

4

•a

Met

amo

rph

os

5

]

]

u

Off

sho

re m

an

X

XX

Oepositional

Environment

rin

eN

ears

ho

re m

a

-mar

shir

sho

re-

Res

tric

ted

ne,

Lac

ust

rin

e

Co

nti

nen

tal

tes

oo

rdin

aP

ho

togr

aph

c

61-2 x 124

62-2 x 12-561-1 x 12-4

potential source rock. The interpretation of the datapresented herein can best be understood when oneexamines what should occur in an advanced mature section.Such a section would contain a large quantity of extractedhydrocarbons making up the total extract, and the per-centage of aromatics would be decreased in favor of anincrease in saturates (paraffin-naphthene fraction). Thelatter is obtained by the thermal degradation of the heavieraromatics yielding the lighter naphthenes and paraffins.

Note that the saturate/aromatic ratio is moderatelymature (Table 3, 1.08 to 2.20) suggesting a preference forthe saturates. However, the total extracted hydrocarbonsonly constitute 4.64% to 8.91% of the total extract (Table3), suggesting immaturity.

ORGANIC CARBON CONTENT ANDKEROGEN ANALYSIS

All three samples have quite high organic carbon content(Table 2, 0.97% to 1.87%) with comparably high totalextractable bitumen (Table, 2, 3709-5544 ppm). Sincemoderate source rocks contain approximately 1.0% organiccarbon content, these samples may be rated as goodpotential source sediments.

The visual kerogen analyses are presented in Table 5.The kerogen is very typical of deep-water sediments beingamorphous-sapropel in type and being generally finelydisseminated throughout the sample. This organic matter isprobably derived exclusively from the water column andrepresents zoo- and phytoplanktonic debris. Somesecondary herbaceous plant and woody debris is alsopresent in Samples 1 and 2.

The maturity of the kerogen is defined by the color andmaturation index. The color range of the kerogen wasobtained by microscopically examining primarily the plantcuticles in each sample. The scale denotes immaturity tothe left (light greenish-yellow) and maximum maturity to

the right (black). The maturation index is also obtained bymicroscopically examining the kerogen. This empiricalindex is based on color, on visual interpretation ofalteration, and by experience in examining numeroussamples, ranging from recent "immature" sediments toancient "mature" known source rocks. Both parameterssuggest that the kerogen is totally immature in thesesediments. This would suggest that the paraffin-naphthenehydrocarbons previously mentioned are probably primarilyindigenous naturally occurring types. The fact that the totalhydrocarbons only constitute 4.64% to 8.91% of the totalextract further attests to this interpretation.

CONCLUSIONS

The upper Miocene age bitumen layers at Site 231 aredefined as being immature source beds. However, withcontinued thermal degradation, they would be goodpotential petroleum source beds. The product of pyrolysiswould probably be a mature, predominantly naphthenic oil,similar to that shown in Figure 4.

The significance of this short study is that moderatelyrich potential petroleum source sediments are present indeep-water sediments. With similar thicker layers presentand in proximity to coarse-grained turbidites acting aspotential reservoirs, the prospectiveness for plays in deep-water environments is greatly enhanced.

REFERENCES

Bray, E. E. and Evans, E. D., 1961. Distribution ofπ-paraffins as a clue to recognition of source beds:Geochim. Cosmochim. Acta, v. 22, p. 2-15.

Klemme, H. D., 1972. Heat influences size of oil giants: OilGas J., July 17, p. 136-144.

Stevens, N. P., 1956. Origin of petroleum—A review: Am.Assoc. Petrol..Geol. Bull., v. 40, p. 51-61.

793

P. J. CERNOCK

50

•– 5.0 -

50

u

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

22.5

-

-

-

-

-

•

-

-

100

\

\

150

HEAVY

OIL

x 1\i

TEMPERATURE IN

200 250

o lGAS

\

\

Φ

r i

LIGHT

OIL

&

GAS

1

•F

3001

A

MOST LY

GAS

1

\ \

350 40

-

-

-

GAS

ONLY

-

HYDROCARBONS PRODUCED BY THERMAL DISINTEGRATION

TEMPERATURE IN 'F

100 150 200 250 300

10.0 -

12.5

15.0 -

PREDOMINANTLY METHANE GAS

PRODUCED BY BACTERIAL ACTIVITY

Figure 1. Generalized illustrations showing depth/temperature relation for hydrocarbongenesis. Hydrocarbons produced by thermal disintegration (top); predominantly methanegas produced by bacterial activity (bottom).

794

GEOCHEMICAL ANALYSES OF POTENTIAL PETROLEUM SOURCE BEDS

PEAK HEIGHTS

C29 c30 C31 C32

14.5 25.2 14.0 28.8 6.0 26.7 4.3 13.1 0.1

C 2 5 * C27 + C29 * C31 25.2 + 28.8 + 26.7 + 13.1 _ 93.8C26 + C28 + C30 + C32 = 1 4 0 + 6 0 + 4 3 + 0 1 2 4 4

C25 + C27 + C29 + C31 25.2 + 28.8 + 26.7 + 13.1 _ 93.8

24 + C26 + C28 + C30 14 5 + 14 ° * 6 ° + 4 3 38 8

= 3.84

= 2.41

3.84 + 2.41= 13712

AVERAGE RATIO OF ODD TO EVENNUMBERED n-PARAFFINS

PRISTANE PEAK HEIGHT 6.4PHYTANE PEAK HEIGHT 10.1 = 5763]

BOTH VALUES INDICATE AN IMMATURE SHALE

ipC 1 9 (PRISTANE)

20ipC2 0 (PHYTANE)

2218 1 23 .NORMAL PARAFFINS

NAPHTHENES

C 2 6 PEAK HEIGHTC27 PEAK HEIGHT

Figure 2. Chromatogram of C15+ paraffin-naphthene hydrocarbon fractions for an immature shale.

795

βZO

n

SAMPLE 1

SATURATE HYDROCARBON ANALYSIS

SAMPLE 2

SATURATE HYDROCARBON ANALYSIS

SAMPLE 3

SATURATE HYDROCARBON ANALYSIS

NORMAL PARAFFIN STANDARD

ip-C..=a=Pristane

ip-C2Q=b=Phytane

fi

Figure 3. Chromatograms ofCj$+ paraffin-naphthene hydrocarbon fractions for three sediment samples from Section 1, Core 44 of Site 231.

GEOCHEMICAL ANALYSES OF POTENTIAL PETROLEUM SOURCE BEDS

P C 1 9 ( P R I S T A N E )

18

17

16

19| , 2 0

21

P C 2 0 ( P H Y T A N E )

CP Index = 1 . 1 3

P r i s t a n e / P h y t a n e R a t i o = 1 . 3 0

NAPHTHENES

N O R M A L P A R A F F I N

Figure 4. Chromatograms ofCj$+ paraffin-naphthene hydrocarbon fractions for a mature oil of Tertiary age.

797