Hydrous and anhydrous pyrolysis of DSDP Leg 75 kerogens—A comparative study using a biological marker approach

Org. Geochem. Vol. 9, No. 4, pp. 171-182, 1986 Printed in Great Britain

0146-6380/86 $3.00+ 0.00 PergamonJournals Ltd

Hydrous and anhydrous pyrolysis of DSDP Leg 75 kerogens--A comparative study using a biological marker approach P. A. COIV[ET1, J. McEvoY 2, W. G1GER2 and A. G. DOUGLAS~ ~Organic Geochemistry Unit, Department of Geology, Drummond Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K. 2Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), 8600-Diibendorf, Switzerland (Received 31 May 1985; accepted 17 January 1986) Abstract--The aliphatic hydrocarbon distributions obtained from the natural bitumens of three Leg 75 sediments were compared using computerised gas chromatography-mass spectrometry (C-GC-MS), The kerogens isolated from these sediments were heated in sealed tubes at 330°C using the techniques of hydrous (i.e. heating kerogen in the presence of water) and anhydrous pyrolysis (i.e. heating dry kerogen alone). These experiments were then repeated at a lower temperature (28ff'C). At 330°C, under anhydrous conditions, considerable destruction of biomarkers in the ancient kerogens (i.e. pre-Tertiary) occurred, whereas with water present significant amounts of hopanes were obtained. However, with more recent kerogens (which contain larger amounts of chemically bound water), both anhydrous and hydrous pyrolysis gave a similar suite of biological markers, in which long chain acyclic isoprenoids (C40) are significantcomponents. Lowering the temperature of pyrolysis to 280°C yielded biological markers under both hydrous and anhydrous conditions for all kerogens, n-Alkenes were not detected in any of the pyrolysates; however, a single unknown triterpene was discovered in several of the hydrous and anhydrous pyrolysates. The results tentatively indicate that the chief value to petroleum research of kerogen hydrous pyrolysis lies in its ability to increase the yield of pyrolysate. High temperature hydrous pyrolysis (280-330°C), under high pressure (2000psi), does not appear to mimic natural conditions of oil generation. However, this study does not take into account whole rock pyrolysis.

Key words: hydrous pyrolysis, anhydrous pyrolysis, biomarkers, simulation of petroleum generation

INTRODUCTION Pyrolysis as a technique for simulating oil generation during basinal subsidence has been discussed by Saxby and Riley (1984), who heated kerogens under anhydrous conditions for several years in an effort to duplicate oil generation. Gas chromatography (GC) showed oil-like products were generated. Lewan and co-workers (Lewan et al., 1979; Winters et al., 1983; Lewan, 1983, 1985) have shown that the heating of whole rocks in the presence of water generates oil-like substances whose chemical properties closely mimic those of crude oils. In particular, the presence of alkenes reported in anhydrous pyrolysis of kerogens (Lewan et al., 1979) were absent in the hydrous pyrolysates. Because hydrous pyrolysis would appear to correspond more closely with natural conditions of oil generation, this technique has been commonly adopted by various oil companies, e.g. Lewan (1985). Rullk6tter et al. (1984) compared hydrous and anhydrous pyrolytic techniques. They heated three kerogens anhydrously and a whole rock hydrously and compared the evolved biomarker distributions. In the hydrous-whole rock system, they noticed the generation of rearranged steranes and neohopanes and concluded that the addition of water to the pyrolysis system enhanced isomerisation. Seifert (1978) used anhydrous kerogen pyrolysis and biomarker analysis to correlate oils and source rocks and

noted that the ratio of hopanes to steranes in the extract was different from that in the pyrolysate. Hoering (1984) conducted a series of experiments in which water and heavy water were used during the pyrolysis of extracted Messel shale. With heavy water, multiple deuteration of the generated biomarkers was noted. The positions of deuteration within the terpenoid skeleton were tentatively suggested to mark the site of biomarker attachment to the kerogen. This site appeared to correspond to the 3 position in the sterane series and to the side chain of the hopane series. The present study has compared the aliphatic hydrocarbon distribution of bitumen from Leg 75 samples with their corresponding hydrous and anhydrous kerogen pyrolysates.

EXPERIMENTAL

A Pleistocene (532-7-3) and two Cretaceous black shale (530A-97-3 and 530A-103-3) sediments from the Angola Basin, Deep Sea Drilling Project Leg 75 were used for this pyrolysis study. Their lithological descriptions, total organic carbon contents and vitrinite reflectance values are given in Table 1. The samples were dried at room temperature, Soxhlet extracted with dichloromethane and the hydrocarbons isolated from the extracted bitumens by the

171

P. m. COMET et al.

172

Table 1. Angolan Basin (DSDP leg 75) sediment description, vitrinite reflectance data, organic carbon

Sample

Age

Lithology

Kerogen description

75-532-7-3

Early Pleistocene

Bioturbated foraminifera nannofossil ooze

Amorphous kerogen--some fluorescence

75-530A-97-3

Cretaceous (Cenomanian)

Laminated carbonaceous pyrite-rich black shale

Amorphous kerogen--some fluorescence

75-530A-103-3

Cretaceous (Late Albian Early Cenomanian)

Laminated carbonaceous pyrite-rich black shale

Amorphous kerogen some fluorescence

TOC a (%)

Bitumen yield (rag/g)~

330 C Hydrous pyrolysis' yieldJ

0.22

2.0

0.6

0,11 g

0.34

7.0

0.6

0,11 g

0.38

7.0

0.3

0.08 g

Vitrinite reflectance (% Ro)

"Total organic carbon, approximate values--DSDP shipboard data. ~Dry weight of sediment. 'Kerogen heated in a steel tube for 72 hr. JYield of pyrolysate from 1 g of kerogen. "Amount of hydrocarbons in 1 g of pyrolysate.

method of Barnes et al. (1979). Kerogens were isolated from the extracted sediments by acid digestion (Saxby, 1970). Aliquots (0.5 g) of the three, dried kerogens were then separately pyrolysed under both hydrous and anhydrous conditions by the following methods. In the first experiment the kerogens were placed in clean, stainless steel tubes (bomblets) with and without the addition of distilled, extracted water (0.5 ml) for each sample. The six bomblets were purged with nitrogen and sealed with tightly-fitting screw caps. They were then heated for three days (72 hr) at 330"C in a Parr pressure reactor. The experiment was repeated at 2 8 0 C under identical conditions except that the kerogens were pyrolysed in silica glass tubes. To investigate any catalytic effects that the metal or silica tubes might have had on the biomarkers, a third experiment was conducted in which kerogen 530A-97-3 (0.5 g) was heated for three days at 330°C in both metal and silica tubes under both hydrous and anhydrous conditions. After cooling, the tubes were opened and their contents removed by scooping out and washing and extracting with dichloromethane and methanol. The low molecular weight gases were not investigated. The extracts and solid residues were dried and weighed. The aliphatic and aromatic hydrocarbons were isolated by thin layer chromatography on silica plates (Barnes et al., 1979). GC of the pyrolysate

hydrocarbons was performed with a Carlo Erba 4160 using a glass capillary column (OV-54; 25 m x 0.31 mm i.d.) with helium as the carrier gas (50-275°C; 4°C/rain). Gas chromatography/mass spectrometry (GC-MS) was carried out on a Carlo Erba 4160 coupled to a Finnigan 4021G mass spectrometer using similar conditions to that described above. Spectra were collected every second and the data processed with an Incos 2000 computer. The bitumen extracts were analysed by GC-MS in a similar manner using Carlo Erba mega 5160 coupled to a Finnigan 4000 mass spectrometer and an Incos 2300 data system. RESULTS AND DISCUSSION

The weights of the extracted bitumens from the three sediments and of their respective kerogen pyrolysates are given in Table 1. The compositions of the hydrocarbons generated by pyrolysis are given in Table 2. Details of the aliphatic hydrocarbons are reported here, the aromatic distributions will be the subject of a further publication. 1. Comparison o f Biomarker Distributions Between the Bitumen Fractions Sample 532-7-3 n-Alkanes were the principal components of the bitumen hydrocarbons. They were present in an

Table 2. Percentage yield of aliphatic and aromatic hydrocarbons in pyrolysates 330'C experiments

280'C experiments

Sample

Hydrous

Anhydrous

Hydrous

532-7-3

18 aliphatics 82 aromatics

21 aliphatics 79 aromatics

17 aliphatics 83 aromatics

ND

530A-97-3

14 aliphatics 85 aromatics

38 aliphatics 62 aromatics

19 aliphatics 81 aromatics

21 aliphatics 79 aromatics

530A-103-3

17 aliphatics 83 aromatics

50 aliphatics 50 aromatics

40 aliphatics 60 aromatics

ND

ND--not determined.

Anhydrous

173

Hydrous and anhydrous pyrolysis of kerogens contents and yields of bitumen, kerogen pyrolysates and hydrocarbons in kerogen pyrolysates Yield" of hydrocarbons from 330 C hydrous pyrolysates

Yield~ of hydrocarbons from 330°C anhydrous pyrolysis

330C Anhydrous pyrolysis~ yieldd

Yield" of hydrocarbons from 280"C hydrous pyrolysis

280'C Hydrous pyrolysis/ yielda

Yield' of hydrocarbons from 280 C anhydrous pyrolysis

280 C Anhydrous pyrolysis/ yieldd

0.32 g

0.065 g

0.30 g

0.024 g

0.21 g

0.009 g

ND

0.31 g

0.070 g

0.16 g

0.014 g

0.23 g

0.010 g

0.26 g

0.40 g

0.067 g

0.37 g

0.002 g~

0.38 g

0.0054 g

N D ~'

/Kerogen heated in a silica glass tube for 72 hr. ~Sample caked during heating and could not be fully extracted from silica tube. hOnly aliphatic hydrocarbons recovered. ND--not determined. unimodal distribution which maximised at n-C31 (Fig. 1A) and showed a pronounced odd-over-even predominance (OEP) which is characteristic of a higher plant source (Eglinton and Hamilton, 1967). Pristane (Pr), phytane (Ph) and norpristane were relatively minor components with P r / P h ~ 0 . 7 . Series of A2-sterenes and of other sterenes were present in low concentrations and were not further characterised. Series of 20R and S (C27 to C29) 5~(H),I4~(H), 17~ (H)and 5ct(H),I4fi (H), 178 (H)-steranes (~and z~fl-steranes respectively) were also present in trace amounts, the mature distribution of

which testified to a minor pollution of this sample by oil. The distribution of hopanes and hopenes (Fig. 2; Table 3) was dominated by 22R 17fl(H),21fi(H)hopanes (fifl-hopanes; C27 to C3~ with C2s absent) with /!//-C31 being the predominant cycloalkane extracted. Significant amounts of 22R C:9, C~0, C3] and C32 17~(H),21/~(H)-hopanes ( ~ - h o p a n e s ) were observed as were minor amounts of C:9 and C30 17fl(H),21~(H)-hopanes (fl~-hopanes: i.e. moretanes). Hopenes were also significant (Fig. 2), particularly neohop-13(18)-ene and neohop-12-ene. This

C

1001 i

,,, ii

'°°1

I:

Io

15

20

25

30

T~

20

25

30

It 3~

i!i . 15 2O 25 30

:i

ie 2O

25

3()

,, lilli fi II, l, Jl ]1[,,

tlll

15

35

20

25

3b

35

°°ll'l'lliillil, ii,,== i]lll, :G

f5

! II'i' ~l:llllllll 1 il t5 2o 25 30

fill:

i 3"5

H

15 20

25

30

°oI =1= 1

II

35

°°I,=lillli ,JJi II,,

15

20

25

30

15 20

25

ll, 11

3O

i 35

Fig. 1. Distributions of n-alkanes and acyclic isoprenoid hydrocarbons (broken lines) in bitumens and kerogen pyrolysates derived from DSDP Leg 75 (Angolan Basin) sediments. All components arc normalized to the predominant peak (100%): (A)Sample 532-7-3, bitumen extract; (B)sample 530A-97-3, bitumen extract; (C)sample 530A-I03-3, bitumen extract; (D)sample 532-7-3, 330C anhydrous pyrolysate; (E) sample 530A-97-3, 330°C hydrous pyrolysatc; (F) sample 530A-97-3, 330'C anhydrous pyrolysate; (G) sample 532-7-3, 280°C hydrous pyrolysate; (H) sample 530A-97-3, 280C hydrous pyrolysate; (1) sample 530A-97-3, 280°C anhydrous pyrolysate.

174

P.A. COMETet al. 100%. m / z 191

m,,

C

F

E

g i

1200 20:00

1400 25:20

1600 26:40

' 1800 50:00

2 o' O0 53:20

2 2 0' 0 Scan No. 5 6 : 4 0 Time

Fig. 2. Mass fragmentogram (m/z 19l) showing the distribution of hopanes and hopenes in the free lipids (bitumen) of sample 532-7-3 (see Table 3 for peak identifications). hopanoid distribution is characteristic of very immature sediments (e.g. Brassell et al., 1980; McEvoy et al., 1981) at an early stage of diagenesis. Fern-8-ene was also significant in this sample. Diterpanes and diterpenes were present at about half the concentration of the hopanes but were not further characterised. Samples 530A-97-3 and 530A-103-3

The n-alkane distributions in these Cretaceous black shales showed similar plant wax assemblages to that of 532-7-3 and maximised at n-C3u However, both black shale samples showed a smaller maximum in the region of n-C17 to n-Cl9 (Figs IB and 1C). Pristane and phytane, as well as polycyclic isoprenoids, were much more abundant in these shales than in 532-7-3. The Pr/Ph ratio was ,,~ 1.3 in both shales. Diterperoid hydrocarbons were not observed. Sterenes were abundant, varied and complex and only a cursory description is given here. Rearranged

Table 3. Major triterpenoid hydrocarbons identified in the bitumen extracts of samples 532-7-3, 530A-97-3 and 530A-103-3 Peak" A B C D E F G H I J K L M N O P Q R S T U

Assignment 22,29,30-Trisnorneohop-13(18)-ene 17~(H)-22,29,30-Trisnorhopaneh 17/3(H)-22,29,30-Trisnorhopane 30-Norneohop- 13(18)-ene 30-Nnrhop-17(21)-ene 17~(H),21,8(H)-30-Norhopane Hop-17(21)-ene 17/3(H),21~(H)-30-Norhopane 17c~(H),21,8(H)-Hopane Fern-8-ene Neohop- 13(18)-ene 17,8(H),21/3(H)-30-Norhopane 17,8(H),21 • (H)-Hopane Homohop-17(21)-ene (22R) Neohop-12-ene 17~(H),21/3 (H)-Homohopane (22R) 17/3(H),21/3(H)-Hopane (22R) 17/3(H),21 ~ (H)-Homohopane (22R) 17c~(H)21,8(H)-Bishomohopane (22R) 17,8(H),21,8 (H)-Homohopane (22R) 17/3(H),21,8(H)-Bishomohopane (22R)

~See Figs 2 and 3. ~Coelutes with an unknown component in samples 530A-97-3 and 530A-103-3.

sterenes (including 4-methyl components) were particularly abundant and were accompanied by series of A4- and AS-sterenes along with a series of unknown sterenes, mass spectral interpretation and retention data suggest that these components may be 19-normethyl-5fl-methylster-8-enes (McEvoy, 1983). Spirosterenes (Peakman et al., 1984) were also significant components of the steroid hydrocarbons. Only 20R 5~(H)- and 5fl(H),14~(H),-17~(H)steranes ( c ~ and f l ~ respectively) were detected. Diasteranes were not present. The steroidal hydrocarbon assemblages in these shales are characteristic of lipids which are at an early to medium stage of diagenesis (McEvoy and Maxwell, 1983). The distribution and abundances of hopanes and hopenes (Fig. 3, Table 3) was similar to 532-7-3 in that the 22R flfl-hopanes (C30 to C32) were major and again the C31 component predominated. The e/3-hopanes were minor components. Hopenes were generally more prominent in these black shales than in 532-7-3; e.g. 22,29,30-trisnorneohop-13(18)-ene, 30-norneohop-I 3(18)-ene, neohop-13(18)-ene, 30norhop-17(21)-ene, hop-17(21)-ene, and homohop17(21)-ene were abundant in 530A-97-3 and 530A-110-3 but were less abundant, minor or absent in 532-7-3. Neohop-12-ene which was present in 532-7-3 and is normally associated with very immature sediments was absent in the black shales. Fern-8-ene and fern-9(ll)-ene were minor components in both black shale bitumens. The differences in the biomarker distributions reflect, in the main, different degrees of maturity between the Pleistocene and Cretaceous samples which is also reflected by their vitrinite reflectance values (Table 1) which are approximately 0.2, and 0.3-0.4 respectively. Hence the molecular distributions are probably characteristic of these maturity levels. Analogous distributions have been noted in the bitumens of DSDP sediments of similar maturities from Blake Bahama (Wardroper, 1979), the Bay of Biscay (Barnes et al., 1979), the Japan Trench (Brassell et al., 1980) the Hess Rise (Comet et al., 1981) and the San Miguel Gap (McEvoy et al., 1981; McEvoy, 1983).

Hydrous and anhydrous pyrolysis of kerogens

175

100 % -

m/z 191

v1 II)

D

IK

A

C

E

G

L

g

u

I

1300 21:40

I

1500 25:00

I

1

1700 28 : 20

1

I

1900 31 : 40

I

I

2100 Scan No. 3 5 : 0 0 Time

Fig. 3. Mass fragmentogram (m/z 191) showing the distribution of hopanes and hopenes in the free lipids (bitumen) of sample 530A-97-3 (see Table 3 for peak identifications). 2. Pyrolysis Yields The pyrolysates generated from the kerogen heating experiments are shown in Table 1. The Pleistocene kerogen 532-7-3 generated approximately twice as much pyrolysate under hydrous than under anhydrous conditions. This increased production of pyrolysate under hydrous conditions was also observed at 330°C for the two black shale kerogens (530A-97-3 and 530A-103-3), however, the relative abundances of hydrous and anhydrous generated pyrolysates differed less drastically than that of the Pleistocene sample. The kerogens generated between five and ten times more pyrolysate at 330°C than at 280°C under anhydrous conditions. The hydrocarbon yield by hydrous pyrolysis was always greater than that by anhydrous pyrolysis, particularly in the case of 530A-97-3 at 330°C. These general trends are in agreement with other results observed in the Newcastle laboratory (Rowland et al., 1986). The percentage yields of aliphatic and aromatic hydrocarbons in the pyrolysates are given in Table 2. 3. Comparison Between Pyrolysates Generated at 330 and 280°C Pyrolysates generated at 330°C Sample 532-7-3. The distribution of n-alkanes was very similar in both anhydrous (Fig. ID) and hydrous pyrolysates. Both showed a smooth unimodal distribution which maximised at n-C]9 with an OEP of unity. Acyclic isoprenoid hydrocarbons were significant components of both hydrous and anhydrous pyrolysates. Norpristane, pristane and phytane were present in significant concentrations, Pr/Ph ~ 0.8. A number of long chain isoprenoids containing head-to-head linkages, probably of archaebacterial origin (Moldowan and Seifert, 1979; Chappe et al., 1980) were present in both hydrous and anhydrous pyrolysates. These components eluted later than n-C33 , the major components in order of decreasing concentration (Fig. 1D) were biphytane (C40), norbiphytane (C39) and bisnorbiphytane (C38).

Two further C40 "biphytanes" occurred in minor amounts. These compounds eluted after biphytane and mass spectral evidence (Chappe et al., 1980) suggests that the earlier eluting component contains one pentacyclic ring and the later eluting component two, the positions of the pentacyclic rings are presently uncertain. Steranes were absent from the pyrolysates of 532-7-3 at 330°C but a full suite of aft- and flc(-hopanes was obtained from both hydrous and anhydrous pyrolysates (see Fig. 4 and Table 4). flfl-Hopanes were absent, as expected from earlier heating studies (Ensminger, 1977; Mackenzie et al., 1981). For the extended hopanes both 22R and S epimers were present. The 22R epimers were dominant over the 22S (22S/22S + 22R ratio = 0.35 for C31 ~tfl components), flc~-Hopanes were abundant but less so than cq%hopanes. The hopane distribution is essentially that of a very immature oil although the content of /%~-hopanes is higher than observed in such oils. C29 and C3~ components dominated the Table 4. Major hopanesidentifiedin the 330'C hydrous pyrolysates of kerogens 532-7-3, 530A-97-3and 530A-103-3 Peaka Assignment 17ct(H)22,29,30-Trisnorhopane A B 17#(H)22,29,30-Trisnorhopane 17a(H),21/3(H)-30-Norhopane C D 17/3(H),21~(H)-30 Norhopane 17a(H),21,8(H)-Hopane E F 17/3(H),21c((H)-Hopane 17c((H),21/3(H)-Homobopane(22S) G 17ct(H),21/3(H)-Homohopane(22R) H I 17~(H),21c((H)-Homohopane(22R and S) J 17a(H),21# (H)-Bishomohopane (22S) 17a(H),21,6(H)-Bisbomohopane(22R) K L 17/~(H),21a (H)-Bishomohopane (22S) M 17#(H),21ct(H)-Bishomohopane (22R) N 17~(H),21fl(H)-Trishomohopane(22S) 17c((H),21#(H)-Trishomohopane (22R) O P 17,8(H),21a (H)-Trishomohopane(22S) Q 17/J(H),21a(H)-Trishomobopane(22R) R 17a(H),21/~(H)-Tetrakishomohopane(22S) S 17:((H),21# (H)-Tetrakishomohopane(22R) T 17/3(H),21c~(H)-Tetrakishomohopane(22S) U 171/(H),21~(H)-Tetrakishomohopane(22R) V 17a(H),21[:~(H)-Pentakisbomohopane(22S) W 17:((H),21/~(H)-Pentakishomohopane(22R) °See Fig. 4.

176

P.A. COMETet al. 100" m / z 191

F ,

K

>,

E

g

I-4 O

S T

v',...._

16oo 26:40

~8oo

2&o

2doo

30;O0

33:20

36:40

z~oo

z&oo s~oo No.

40:00

4 3 : 2 0 Time

Fig. 4. Mass fragmentogram (m/z 191) showing the distribution of hopanes derived from the hydrous pyrolysis at 330°C of kerogen 532-7-3 (see Table 4 for peak identifications). pyrolysate hopane distributions (Fig. 4) which is in contrast to that in the bitumens where C3~fl-hopane predominated (Figs 2 and 3). This observation has also been noted for bitumens and hydrous pyrolysates of kerogens isolated from Serpiano oil shale (McEvoy and Giger, 1986). There were no significant differences between the hydrous and anhydrous pyrolysates in the hopane distributions. Indeed, whenever hopane distributions were encountered amongst the various pyrolysates, whether produced under hydrous or anhnydrous conditions, the distributions were always very similar. The relative distributions of the aliphatic component types in this sample is given in Table 6A. A series of alkylcyclohexanes, identified by mass spectral interpretation (e.g. the diagnostic ion at m/z 83) and retention data was present in both pyrolysates. Their distributions were unimodal and ranged from C~, to C30 (anhydrous; maximising at Clg) and from C~7 to C30 (hydrous; maximising at C20). An unresolved "hump" was prominent in the region between rt-Ci6 to F/-C24 in both pyrolysates. Samples 530A-97-3 and 530A-I03-3. These Cretaceous black shale kerogens produced pyrolysates with very similar biomarker distributions which shall be considered together. Some differences were noted in the n-alkane distributions between the hydrous and anhydrous pyrolysates of sample 530A-97-3. The hydrous pyrolysate n-alkanes maximized at nC,s-CL,~ with a slight secondary maximum at t/-C24 (Fig. 1E). In the anhydrous case the n-alkanes maximised at rt-Ci9 (Fig. 1F). In general a fairly smooth envelope of n-alkanes was observed in both cases. In 530A-103-3 the n-alkane distributions of both hydrous and anhydrous pyrolysates were virtually iden-

tical, the unimodal distributions of n-alkanes maximised at//-C]9, The acyclic isoprenoids norpristane, pristane and phytane were present in high relative abundances in both pyrolysates from 530A-97-3 (Figs IE and IF) with Pr/Ph ~ 1.2 in both cases. A similar result was observed in the hydrous pyrolysate of 530A-103-3, however, all the isoprenoid biomarkers were destroyed during the anhydrous pyrolysis of this kerogen. Long chain isoprenoid hydrocarbons were not detected in the black shale pyrolystates. In contrast to their bitumens, series (C27 to C2~) of 20R and 20S c ~ - and of 20R flc~c~-steranes, in low abundance, were seen in the hydrous pyrolysates of both 530A-97-3 and 530A-103-3; minor amounts of 20R and 20S ~fifi-steranes were also noted (Fig. 5, Table 5). Rearranged steranes were not observed. The sterane distribution was essentially immature with the 20S/20S + 20R ratio ~ 0.2 for the C29 c ~ components. The hopane distributions generated by hydrous pyrolysis of these black shales were virtually C

100m / z 217

1

M

.k-_ g ---

1600 26:40

r

18'00 30:00

•

2(~)0 Scan No_ 3 3 : 2 0 Time

Fig. 5. Mass fragmentogram (m/z 217) showing the sterane distribution derived from the hydrous pyrolysis of kerogen 530A-97-3 at 330'C (see Table 5 for peak identifications).

pyrolysis of kerogens

H y d r o u s and a n h y d r o u s Table 5. Steranes in kerogen pyrolysates of samples'532-7-3, 530A-97-3 and 530A-103-3

Assignment

Peak" A

B C D E F G H

I J K k M

177

those generated by sample 532-7-3. Again, all samples revealed a "hump" in the region of n-C~6 to n -C24.

5p'(H), 14c~(H),I 7:~(H)-Cholestane (20R) 5zt (H), 14c~(H), 17~ (H)-Cholestane (20S) 57(H),14~ (H),I 7e (H)-Cholestane (20R) 24-Methyl-5~(H),l 4c~(H),I 7e (H)-cholestane (20S) 24-Methyl-5~ (H),I4~ (H),I 7,B(H)-cholestane (20R) 24- Methyl-5/](H),l 4~ (H), 17c~(H)-cholestane (20R) 24-Methyl-5~(H), 14/t(H),l 7/3(H)-cholestane (20S) 24- Methyl-5c~ (H), 14c~(H), 17~ (H)-cholestane (20R) 24- Ethyl-5z~ (H), 14z~(H). 17c~(H)-cholestane (20S) 24-Ethyl-5:~(H).14//(H), 17fl(H)-cholestane (20R) 24-Ethyl-5/~(H), 14~ (H), 17~ (H)-cholestane (20R) 24-Ethyl-5z~(H), 14/3(H),I 7/.¢(H)-cholestane (20S) 24-Ethyl-5~ (H),I4~ (H),l 7~ (H)-cholestane (20R)

"See Fig. 5.

identical with that of sample 532-7-3 (see Fig. 4), again the 22S/22S + 22R ratio was about 0.35 for the C3~ c~[]components. The higher rate of isomerisation observed at the C-22 position of extended hopanes compared to that at C-20 for c~c~-steranes is in agreement with earlier observations in both field and thermal simulation experiments (Mackenzie et al., 1980, 1981). Only traces of hopanes were found in the anhydrous pyrolysate of sample 530A-97-3. The relative distribution of the aliphatic component types present in samples 530A-97-3 and 530A-103-3 are given in Tables 6B and 6C respectively. Alkylcyclohexanes were observed in both hydrous and anhydrous pyrolysates in distributions similar to

Pyrolysates generated at 280~C Sample 532-7-3. Hydrocarbon data was obtained only from the hydrous pyrolysate (see Table I), due to losses in the work-up of the anhydrous pyrolysate; there was insufficient kerogen to repeat the latter experiment. The distribution of n-alkanes produced in the 280°C hydrous pyrolysate (Fig. IG) was very different to that produced at 330"C (Fig. I D). The dominant n-alkane was n-C2j at 280~C as opposed to n-C~9 at 330°C. Also the distribution at 280°C was less "mature" showing a significant even-over-odd predominance (EOP) in the region of n-C22 to n-C3t whereas carbon preference was absent at 330C. Norpristane, pristane and phytane were less prominent at 280°C than at 330°C and Pr/Ph was approximately unity as opposed to 0.8 at 330"C. The enhanced concentration of long chain isoprenoid hydrocarbons at 280°C (Fig. I G) compared to 330'C (Fig. 1D) was striking. As at 330"C the major components were biphytane, norbiphytane and bisnorbiphytane with biphytane being the predominant aliphatic hydrocarbon observed in the hydrous pyrolysate. The two C40 isoprenoids containing pentacyclic rings (see above) were also present in significant concentrations at 2 8 0 C (Fig. 1G).

Table 6. Relative abundance of major structural types of hydrocarbons in each leg 75 kerogen pyrolysate obtained after heating for 72 hr Unheated bitumen

330'C Hydrous

330 C Anhydrous

280 ~C Hydrous

280 C Anhydrous

+ + + + + + + + + + + --

+ + + + + + --+

+ +-F + + + --+

+ + + + + + --+ ~

ND ND ND ND ND

+ + + + + --

+ + + + + + + Tr . .

+ + ++ + + --

+ + + + + + + Tr

+ + + + + + + Tr

(A) 532-7-3

n-Alkanes Hopanoid Steroid Diterpenoid C4~~ Acyclic

isoprenoid (B) 530A-97-3

n-Alkanes Hopanoid Steroid Diterpenoid C40 Acyclic

+ + ++ + + + + + + . .

+ ++++ + + Tr . . . .

. .

. .

+

isoprcnoid (C) 530A-103-3

n-Alkanes Hopanoid Steroid Diterpenoid C40 Acyclic

+ + + + + + + + + + . .

+ + + + + + + + . . .

+ + + + + -.

. .

.

isoprenoid (D) 530A-97-3

Steep

Steel b

Silica"

Silica h

n-Alkanes Hopanes Steranes

+ + + + + + + Tr --

+ + + + + ----

+ + + + + ----

ND ND ND ND

C~0 Acyclic

isoprenoid "330C hydrous. ~330 C anhydrous. N D - - n o data. Number ofcrosses give a crude estimate ofrelative abundances, i.e. + + ~ + + very abundant, + minor amounts, Tr trace,--absent.

178

P.A. COMETet al.

Series of 20R and 20S ~ and of 20R /3ct~-steranes (C27 to C29) were present in trace quantities at 280~'C, although they were absent at 330°C. The distribution was similar to that observed in the black shale hydrous pyrolysates at 330°C although the isomerisation at C-20 was slightly less marked (20S/20S + 20R ratio ~ 0.15). For 532-7-3 the distribution of hopanes in the 280~C hydrous pyrolysate was again similar to that of the 330°C hydrous pyrolysate, however, a significant difference was the presence of a C30 triterpene in the 280°C pyrolysate. This unidentified triterpene coeluted with n-C28 and gave prominent ions at m / z 177 (base peak, i.e. intensity= 100%), 410 (M.+, 5%), 395 (15%), 259 (10%), 231 (60%), 218 (20%), 215 (10%), 191 (90%) and 189 (25%). The presence of this compound is significant as it indicates that alkenes can be generated under hydrous conditions. This alkene was not present in the 330:C hydrous pyrolysate, but was observed in both the hydrous and anhydrous black shale pyrolysates which were generated at 280°C. The relative distributions of the aliphatic hydrocarbons in this pyrolysate are given in Table 6A. Alkylcyclohexanes were less prominent in the 280°C hydrous pyrolysate than in the 330°C pyrolysates. C~7 was predominant but other maxima occurred at C2~ and C24. An unresolved "hump" was observed, which maximised in the n-Ci7 region. Samples 530A-97-3 and 530A-103-3. Under hydrous conditions at 2 8 0 C the n-alkanes of both 530A-97-3 (Fig. 1H) and 530A-103-3 maximised at n-C21 and showed a slight EOP in the n-C24 to n-C28 region. This EOP was particularly prominent in 530A-103-3 showing a secondary maximum at n-C28. Under anhydrous conditions at 280<'C, the n-alkanes of sample 530A-97-3 were less uniform (Fig. lI) with a maximum at n-C2s and a secondary maximum at n-C2~. A slight EOP was evident in the n-C25 to n-C30 region. The n-alkane distribution of the 530A-103-3 anhydrous pyrolysate maximised at n-C~9 and nalkanes above n-C24 were not observed. Phytane, pristane and norpristane were the major isoprenoids detected, long chain isoprenoids were not observed in the black shale pyrolysates. Pristane and phytane were more abundant in the 530A-97-3 anhydrous pyrolysate than the hydrous pyrolysate at 280°C. Pr/Ph varied from 0.8 (hydrous) to 1.3 (anhydrous). Series of 20R and 20S ~ and of 20R /3~-steranes (C27 to C29; 20S/20S + 20R ~ 0.15 for C29 ~ components) were present in similar amounts in both hydrous and anhydrous pyrolysates of 530A-97-3 at 280c~C. The distribution was similar to that obtained at 33ffC. Negligible amounts of steranes were present in the 530A-103-3 pyrolysates. These results contrast somewhat with the 330°C anhydrous pyrolysates where the polycyclic biomarkers were almost totally destroyed (Tables 6B and 6C). Series of ~/~- and ~ - h o p a n e s were noted in both hydrous and anhydrous pyrolysates of the black shale

kerogens at 280°C. The distributions were similar to that described for the 330°C hydrous pyrolysates. Again the presence of hopanes in the anhydrous pyrolysates at 280°C is significant considering their absence or trace presence in the anhydrous pyrolysates at 330°C (Tables 6B and 6C). The unknown C30 triterpene, noted in the hydrous pyrolysate at 2 8 0 C for sample 532-7-3, was also present in all of the black shale pyrolysates at 280C. 4. Pyrolysis in Stainless Steel vs Silica Ghzss Reaction Tubes

The kerogen from 530A-97-3 which was heated at 330°C in both hydrous and anhydrous modes using both stainless steel and silica tubes showed several differences. First of all the pyrolysate yield, using a silica tube under anhydrous conditions, was too low for an aliphatic fraction to be isolated. Aliphatic fractions were isolated from the remaining three pyrolysates. The aliphatic hydrocarbons and their respective distributions generated from pyrolysates produced in the metal tubes coincided with the results from the first experiment in metal tubes at 330C (cf. Tables 6B and 6D). The only difference was that the traces of hopanes seen under anhydrous conditions in the first experiment (Table 6B) were absent in the anhydrous pyrolysate produced in this second experiment at 330°C (Table 6D). The hydrocarbons generated under anhydrous conditions in the steel tube consisted of n-alkanes, pristane and phytane only. A similar result was obtained from the hydrous pyrolysate in the silica tube (Table 6D). Hence biomarkers, mainly hopanes with traces of steranes, were observed only in the metal tube under hydrous conditions at 330°C. The presence of polycyclic biomarkers only in the steel tube containing water and kerogen, was interpreted as being partly caused by the affinity of the metal walls for catalytically active sulphur released from the kerogen. The steel tubes, after reaction, were always coated with sparkling yellow sulphides of iron, presumably pyrite/marcasite. Elemental sulphur is known to be very destructive to biomarkers during heating (personal communication A. Lewis, Bristol OGU). Presumably, the metal walls may remove catalytically destructive sulphur from the reaction mixture which would help to preserve the generated biomarkers. The interior surface of the silica tube employed under hydrous conditions was covered in a white, frost-like coating which indicated considerable corrosion had occurred. The silica tube employed in an anhydrous mode had a clean interior surface. Other factors must play an important role in preservation or destruction of biomarkers, such as pressure (Sajg6 et al., 1986). A lower degree of molecular vibration probably results under hydrous condilions; hence, the greater molecular movement under anhydrous conditions may result in enhanced biomarker destruction. The main observations from this cxperi-

Hydrous and anhydrous pyrolysis of kerogens ment is that the choice of glass or metal reactor vessels will influence the quantity and quality of the biomarkers produced. Also, when considering the use of glass tubes, much higher yields of biomarkers were found in the pyrolysates at 280°C than at 330°C (cf. Tables 1, 6B and 6D). These observations point to the catalytic effect of the mineral matrix on hydrocarbon production (Het6nyi, 1983; Horsfield and Douglas, 1980).

5. Hydrous Pyrolysis as a Technique for Simulating Oil Formations

The review by Lewan (1985) and in particular the discussion section held at this meeting (Geochemistry of buried sediments--Royal Society, August, 1984) evaluated hydrous pyrolysis as a technique for assessing natural petroleum formation. An important point raised was the formation of alkenes in pyrolysis. The absence of n-alkenes was shown to be a feature of closing the system during pyrolysis, rather than any water-mediated inhibition of alkene formation. The absence of pyrolytic artifacts, i.e. alkenes, was previously believed to show that hydrous pyrolysis more closely simulated natural oil formation than other pyrolysis techniques. However, the present study has shown that fl~-hopanes are also generated in significant abundance, both by hydrous and anhydrous methods. The concentration of these compounds would appear to be much higher than in most petroleums. It would appear that the formation of artifacts is inescapable during both hydrous and anhydrous pyrolysis. Although n-alkenes were not found in either hydrous or anhydrous pyrolysates, an unknown triterpene was generated under these cor/ditions at 280°C, however, it was not observed in the pyrolysates generated at 330°C. It thus appears to be possible to generate alkenes under conditions of closed system pyrolysis. Monthioux et al. (1985) also noted the absence of n-alkenes in closed system kerogen pyrolysis; their results compared closely with information discussed by Lewan (1985). They account for the lack of n-alkenes during closed system pyrolysis in terms of free radical deactivation either by water present or by the generated hydrocarbons. It would appear that the significance of n-alkenes in understanding petroleum formation, and in selecting the most "natural" conditions of pyrolysis, is chimaeric. Presumably alkenes form under natural conditions of generation and are subsequently reduced although some oils do contain small concentrations of n-alkenes (e.g. Hoering, 1977). n-Alkenes formed at a given temperature in pyrolysis do disappear with increasing time of heating (Ishiwatari and Fukushima, 1979) or with increasing temperature (Sajg6 et al., 1986). The role of water in free radical suppression (Hoering, 1984) may be important in natural conditions of generation where significant amounts of

179

water will be generated from both the kerogen (Ishiwatari et al., 1977) and from the mineral matrix. Water does not appear to contribute to the net amount of organically bound hydrogen in hydrous pyrolysates (Lewan et al., 1979) but it certainly plays a role in hydrogen exchange reactions both with the kerogen prior to generation (Hoering, 1968) and with the products generated during pyrolysis (Hoering, 1984). Lewan (1985) also noted that anhydrous pyrolysis generated an oil with greater amounts of aromatic and polar components than hydrous pyrolysis. The present work tentatively indicated that these results do not hold good for some immature kerogens. Hydrous pyrolysis of black shale kerogens at 330"C yielded more aromatics and less aliphatics relative to anhydrous pyrolysis (Table 2). However, a striking feature, also noted by Lewan (1985), is the enhanced hydrocarbon yields when using water as a pyrolytic medium. The present work shows that hydrous pyrolysis, at 330°C for 3 days, liberated more hydrocarbon than anhydrous pyrolysis at the same conditions of time and temperature (Table 1). Furthermore the yield of pyrolysate under anhydrous conditions is 7-11 times greater at 330°C than at 280~C. For recent, immature kerogens hydrous and anhydrous pyrolysis may give similar results. 6. Bitumen / Kerogen Interactions

The composition of the aliphatic hydrocarbons in the bitumen fractions varies slightly between the samples. The Pleistocene sample 532-7-3 contains large amounts of steroid, hopanoid and diterpenoid hydrocarbons, particularly alkenes. The Cretaceous black shales 530A-97-3 and 530A-103-3 show similar bitumen hydrocarbons but did not contain diterpenoids, presumably due to the absence of nearby land plants during sedimentary deposition (Simoneit, 1977). The differences in the distributions of biological markers in the bitumens and in their respective kerogen pyrolysates were striking. For example, the marked OEP for the n-alkanes in all three bitumens was effectively absent in the pyrolysates. The nalkanes in the latter also showed the expected decrease in average carbon number which has been observed with increasing maturity in both sediments (Tissot et al., 1971) and in laboratory thermal experiments (e.g. Albrecht et al., 1976). Sterenes were the most abundant steroids of the bitumens, particularly rearranged species in the case of the black shales. However, only steranes were observed in the pyrolysates, flfl-Hopanes dominated the hopane distributions in the bitumens but were absent in the pyrolysates where ~/~- and #~-hopanes were present, dominated by C29 and C30 components compared to C3~ hopane in the bitumens. The aliphatic polycyclic hydrocarbons from the kerogen pyrolysates contained only hopanes in

t80

P.A. COMETet al.

significant abundance with steranes being minor, trace or absent (Table 6). A possible suggestion to explain the paucity of saturated steroids and diterpenoids from pyrolysis would be to infer their preferential aromatisation upon kerogen release. Similarly, during kerogen release, steroids and diterpenoid moieties could also become "rebound" into pyrobitumen and asphaltenes during pyrolysis. Preliminary data indicates that aromatic steroids (both mono- and triaromatic) are present within both the hydrous and anhydrous pyrolysates. The alkylcyclohexanes, not observed in the extracted bitumens but present in all of the pyrolysates, probably derive from alphatic carboxylic acids bound onto the kerogen. Rubinstein and Strausz (1979) have shown how these cyclisc during clay catalysed defunctionalization to give alkylcyclohexanes, however, the kerogens were clay free but some mineral matter undoubtedly survives the isolation procedure. Although no long chain acyclic isoprenoids were detected in the bitumens of 532-7-3 these components were major components of its kerogen pyrolysates+ both hydrous and anhydrous at 280 and 330 C. It is difficult to rationalize why these structures should be liberated from Pleistocene kerogens (highly immature), but not from Cenomanian-Albian kerogens (immature). Possibly kerogen cross linking is much more developed in the Cretaceous kerogens but the difference may be attributable to source input. It appears, from their aliphatic hydrocarbons, that the bitumens are much richer than the corresponding pyrolysates in a variety of biological markers, particularly steroids. This was previously noted by Seifert (1978). The prominence of hopanes in the pyrolysate hydrocarbons may reflect the greater binding affinity of the hopane tetrol (Ourisson et al., 1979) into the kerogen matrix as compared to sterols (see Seifert and Moldowan, 1980). Not only did the relative concentrations of the various biomarker groups show significant differences between the kerogen pyrolysates and the bitumens, but also between the different types of pyrolysates. In particular the concentration (relative to n-alkanes) of long chain (C38-C40) irregular isoprenoids, in 532-7-3, and hopanes and steranes in all three samples, were significantly higher in the 2 8 0 C hydrous pyrolysates than in the 330°C hydrous pyrolysates. The relatively enhanced biomarker concentrations at 280°C were interpreted as due partly to their increased preservation from thermal cracking reactions and also to the lower amounts of n-alkanes expelled at this lower temperature. At 330°C more n-alkanes were formed, and catalytic destruction of the biomarkers was accelerated. The alkylcyclohexanes were an exception to this trend, their concentration relative to n-alkanes increased at the higher temperature. This was interpreted as being due to an accelerated rate of formation of alkylcyclohexanes from kerogen decarboxylation reactions (Rubinstein and Strausz, 1979) at the higher temperature.

7. Effect o f Temperature and Pressure on Biomarker Distributions

Hopane distributions produced at 330°C in the presence of water showed similarities to those produced at 280°C anhydrously, thus the addition of water produces a similar result to lowering the effective temperature of pyrolysis. This may be due to the increased partial pressure of water vapour. Striking differences were exhibited between the 330°C hydrous and anhydrous pyrolysates in the two black shales 530A-97-3 and 530A-103-3. In the anhydrous case at 330°C all the biological markers were destroyed. In the hydrous case at 330°C the hopanes were abundant. However, in the Pleistocene sample hopanes were abundant in both hydrous and anhydrous pyrolysates at 330°C. This more immature kerogen may contain a significant amount of bound water which helps to preserve the generated biomarkers under anhydrous conditions. The bias towards hopanes in the pyrolysates may indicate that oils liberated from late stage kerogen cracking will be relatively richer in hopanes and poorer in steranes. Some oils sourced from young kerogens may possibly contain large amounts of long chain acyclic isoprenoids. No consistent change in isomeric ratios, within the hopane and sterane distributions, were found with increasing temperature of pyrolysis. Very little is known of the role pressure plays in pyrolysis studies but it may be an important factor in closedsystem pyrolysis (cf. Sajg6 et al., 1986) governed partially or fully by the volatile products of pyrolysis.

SUMMARY

Overall, at both 280 and 330°C, hydrous pyrolysis increases the yield of (C~3) hydrocarbons relative to anhydrous pyrolysis. Total pyrolysis yields are higher at 330°C than at 280°C. Generated polycyclic biomarkers appear to be better preserved in metal than glass reaction vessels, and under hydrous rather than anhydrous conditions. There are marked differences in the distributions of biomarkers found in the bitumens when compared to those in the respective kerogen pyrolysates. The presence of EOP of nalkanes at 280°C but not at 330°C suggests that generation by carbon-carbon cracking has not been completed at the lower temperature. The implications of the enhanced yield of hydrocarbons during hydrous pyrolysis may be that hydrostatic pressure of the sediment during oil formation may possibly influence both the amount and character of the generated hydrocarbon. Nevertheless, hydrous pyrolysis does not appear to confer any special advantages to industry in understanding and duplicating oil formation when compared with other methods of closed system pyrolysis, There is really no such thing as anhydrous pyrolysis where kerogen is concerned as water will always be generated, particularly with very immature ker-

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