- Email: [email protected]
source-rock correlations
Geochimrca n Cosmochimico Copyight 63 1989 Pc%pmon
#II&7037/89/$3.00 + .iM
Ada Vol. 53, pp. 2973-2983 PES pk. Printed in U.S.A.
Application of a new preparative pyrolysis technique for the determination of source-rock types and oil / source-rock correlations Institut Fran&
E. LAFARGUEand F. BEHAR du P&role, BP 3 II,92506 Rue&Malmaison Cedex, France
(Received January 24, 1989; accepted in revised form September 6, 1989)
Abstract-A new preparative pyrolysis technique enabling the recovery and fmctionation (into saturated hydr~ar~ns, unsaturated hydrocarbons, and aromatic hydrocarbons) of the total G+ hydrocarbon fraction (instead of the C&+ fraction usually recovered) has been applied to different types of sourcerocks. The composition of the C&is hydrocarbon fraction in the pyrolysate, particularly the amount of aromatic hydrocarbons as compared to alkanes, was found to be characteristic of each type of kerogen, with the alkane/aromatic ratio consistently decreasing in the progression from Type I to Type III kerogens. While the Ct3+ fraction is useful in kerogen typing, it was found that the C,-& hydrocarbon fraction, which represents 40 to 50% of the total recovered pyrolysate, was the most sign&ant in emph~i~ng differences between kerogen types, allowing a rapid and precise estimation of the source-rock type. This new technique was applied to potential source-rocks of the Viking Graben, North Sea (Draupne formation, Heather formation, Brent coals, and Dunlin group). In each case, the pyrolysates allowed us to determine whether the organic matter was Type II, Type III, or a mixture of both. Pyrolysis of asphaltenes from crude oils from the various regions was conducted and potential applications of our technique to studies of oil/source-rock correlations were examined. INTRODUCTION THE CHARA~RIZATION OF organic matter from sedimentary rocks is now widely recognized as a useful tool for petroleum exploration. Elementally based kerogen typing established from the work of Tissot and coworkers f TISSOT et al., I.974 ) is routinely used for this characterization. Since this classification is based on bulk chemical analysis, similar atomic ratios may correspond to very different chemical structures. Therefore, this technique gives access to bulk chemical properties of the kerogen, but it is not appropriate for the determination of detailed molecular con~~mtion of the kerogen, which is another important part of organic matter characterization ( LARTER and SENTFLE, 1985; VANDENBROUCKE et al., 1988). In addition, strict adherence to an elemental classification scheme has been reported to cause problems, especially in terrigenous organic matter provinces ( SNOWWN and POWELL, 1982; HORSRELD, 1984; LARTER, 1985 ). Analytical pyroIysis has become common in petroleum exploration for improving elemental-based kerogen typing. This technique was first applied in coal studies ( ROMOVACEK and KUBAT, 1968) and has more recently been extended to kerogens from a large range of sedimentary facies ( LARTER and DOUGLAS, 1980; HORSRELD, 1984; LARTER, 1985; BEHAR and PELET, 1985). Numerous analytical pyrolysis techniques are used (pyroIy&GC, pyr-GC-MS, hydrous pyrolysis, preparative pyrolysis, etc.) they all have specific applications in petroleum exploration (see LARTERand DOUGLAS, 1982; LARTER, 1984; VANDENBROUCKEet al., 1988). Here, we describe a new preparative pyrolysis technique which enables the recovery and quanti~tive fractionation (into saturated, unsaturated, and aromatic hydrocarbons) of the total Cg+ hydrocarbon fraction of the pyrolysate instead of the C,J+ fraction usually recovered and analyzed by this technique ( BEHAR AND PELET, 1985). We report the appli2973
cation to reference series of Type I, Type II, and Type III kerogens and then apply the technique to source-rock type characterization and to oil/source-rock correlation in a North Sea case study. EXPERIME~AL
SECHON
Methodology Pyrolysisis conducted on kerogens isolated from source-rocks according to the analytical procedure described in the literature ( DURand NKniSE, 1980). Prior to pyroiysis, kewgens are ChfOrOfO~extracted in order to avoid ~n~mination of the p~o~y~te by free hydrocarbons trapped in the kerogen structure (BEIIARand VANDENBROUCKE,1988). Pyrolysis is performed in a minifurnace designed at IFP ( BEHAR and PELET, 1985). The apparatus is sketched in Fig. I together with an overview of the analytical procedure. A preiiminary heating at 320°C for 2 min vaporizes any residual low molecular-weight material. The oven temperature is then raised to 550°C (at approximately ~‘C~rnin~ and heid there for 5 min. Hydrocarbons releasedthrough thermal cracking of the kerogen are carried away by argon gas to a trap cooled by liquid nitrogen. When pyrolysis is complete, the trap is disconnected and removed from the liquid nitrogen. After vaporization of argon out of the trap, the pyrolysate is dissolved in 100 rl of n-pentane. The C,-C5 hydrocarbons are lost by evaporation at this stage. The pyroIysate is fractionated by microcolumn liquid chromatography into saturated plus unsaturated hydrocarbons (HCSI) and light aromatic hydrocarbons plus thiophenes ( HCA) . The microcolumn is a Pasteur pipette of 0.5 cm diameter and 10 cm height filled with silica (on 5 cm). The saturated and unsaturated hydrocarbons are eluted with 2500 ~1 of n-pentane; the light aromatic hydrocarbons and thiophenes are eluted with 3000 ~1 of a 90~10 mixture of npentane and dichloromethane. The heavy aromatic hydrocarbons and thiophenes are not recovered in the 90: 10 mixture of n -permute and dichloromethane. Each fraction is analyzed and quantified separately by gas chromatography (GC) before recombination into a single fraction (HCSIA) to give the final chromatogram of the pyrolysate. The GC, with on-column injector, was fitted with a silica capillary coiumn of AND
E. Lafargue and F. Behar
2974
PYROLYSISDEVICE
TRAPPING DEVICE
/
\
\
I
Sample pasltlon dung analysis kerogen or ospholtenes (1 fa100mg)
Teflon valve /
i Argon ,_
sweep1 ^^__, IT--W’CI
Gloss,‘pyrex”
I 1‘rod Golc
Liquid mlrogen
KEROGEN ASPH::TENES I
Pyrolysis: 320°C- 550°C GO'C/min 550°C:5min I C6+EFFLUENTS
+n-C5 I
LIQUIDCHROMATOGRAPHY SiO2 I
100% n-C5r ALKJNES ALKENES (HCSI)
90% n-C5 1'10% CH,Q
LIGHT AROMATICS*THIOPHENES (HCA)
GC
I
GC
I I
MIXTURE HCSl+ HCA I
HCSIA
GC
FIG. 1. Pyrolysis device and analytical procedure.
30 m length and 0.32 mm inner diameter coated with a CPSil 5 film 0.25pm thick. Helium was the carrier gas. The initial temperature was 0°C with a heating rate of 3”C/min, taking the column to a maximum temperature of 300°C.
Samples
Kerogens obtained from classical Type I, Type II, and Type III evolution series ( TISSOTet al., 1974; Green River Shales, Paris Basin,
2915
Determining source-rock types and oiI/source-rock correlations and Mahakam Delta, respectively) were selected (Table 1) . Several studies of these serieshave been repotted in the literature (ROBINSON, 1969; YOUNGand YEN, 1977; TISSOTet al., 1978; ESPITALIE et al.. 1973; COMB,&? and DE MATHAREL,1978; DURANDand MONIN, 1980). Each series is characterized by its specific chemical structure. For Type II and Type III kerogens, it was fairly easy to select samples with increasing maturity. However, for Type I organic matter, due to the narrowness of its catagenetic zone (TISSOT et al., 1978) and to the heterogeneity of the Green River Shale Formation, it was not possible to get the same kind of evolution series. For these samples, Hydrogen Index (HI) values were used to assess their maturity level. The first four fall into the diagenesis zone and the last two into the catagenesis zone. North Sea samples are listed in Table 2. The stratigraphic horizons corresponding to these formations are presented in Fig. 2, with the position of the samples in a Van Krevelen diagram shown in Fig. 3. RESULTS
AND DISCUSSION
SAMPLE
FORNATION
NUMBER
DEPTR
PREP.
(ml
Tmax
"I
('C)
bg/ga
C
H/C
1%)
latom.~
o/c ,atom.,
70891
DRAUPNE
4500
KER
446
36,
19.0
.,5
.06
70895
DRWPNE
3675
KER
446
319
31.0
.a2
.06
71266
HEATHER
3316
KER
442
110
28.0
.74
.11
7126,
HEATHER
3317
KER
440
115
53.0
.68
.10
71276
HEATHER
3490
KER
445
217
24.0
.a5
.08
14812
BRENT
3328
KER
44,
134
61.0
.84
.09
70936
BRENT
3649
KER
453
202
71.0
.,O
.05
,I275
BRENT
3539
KER
452
248
73.0
.,4
.06
10953
DUNLIN
3682
KER
44,
131
23.0
.81
.O,
70955
DUNLIN
3691
KER
441
264
14.0
.Bl
.08
65145
DUNLIN
2687
KER
434
145
75.0
.,,
.I2
isolated
by
Application of the technique to reference series KER
Figure 4 compares the Cg+ and Ci3+ pyrolysis-derived chromatograms of the saturated plus unsaturated hydrocarbons (HCSI), together with the aromatic hydrocarbons (HCA) of one kerogen sample from the Toarcian of the Paris Basin. The advantages of the new analytical procedure, which allows the recovery of the Cg+ hydrocarbon fraction instead of the CiJ+ fraction usually recovered, are clearly illustrated in Fig. 4. Much more information is gained for the saturated and unsaturated hydrocarbons. In addition, the danger of partial loss of n-C,, and n-Cls by evaporation during fractionation is eliminated. Therefore, prist- 1-ene / n-C, and prist- 1-ene / n -Cis ratios are substantially more reliable. For the aromatic hydrocarbons, the precise analysis and quantification of the light aromatics and thiophenes more than compensates for the loss of the heavy ends in this fraction.
Table 1. SAMPLE
Geochamical
ORIGIN
NUMBER
data
on
DEPTN
0.n.
cm)
TYPE
kerogens
PREP.
TmaX ('C)
of
the
reference
HI lmglgcl
series.
C
H/C
o/c
(%I
,atom.)
(atom.)
15760
Uinta
B.
0ut.c.
I
KER
456
846
80.0
1.64
.05
15774
Uinta
B.
O"r_C.
I
KER
446
722
73.0
1.44
.08
15755
Uinta
B.
outc.
I
KER
452
726
75.0
1.51
.O6
15784
Uinea
B.
255,
I
KER
450
79,
67.0
1.41
.04
21562
Uinta
B.
2862
1
KER
452
37,
79.0
1.06
.03
15797
uinta
B.
3752
I
KER
451
86
63.0
0.71
.05 .08
10968
Paris
8.
O"tC.
11
KER
416
67,
57.0
1.29
38931
Paris
8.
1010
II
KER
429
666
62.0
1.33
.06
32893
Paris
B.
2040
II
KER
432
547
50.0
1.20
.O5
52092
Paris
B.
2253
IX
KLR
438
489
46.0
1.11
.04
20506
Parrs
8.
578
II
KIER
445
218
52.0
1.02
.03
20507
Paris
8.
1306
II
KER
453
96
39.0
0.69
36926
Hahakam
1010
III
KER
419
95
56.0
.89
.21
36933
Piahakam
1440
III
KER
422
151
70.0
.46
.11
36939
"ahakam
1910
III
KER
425
124
57.0
.90
.18
36945
“ahakam
2260
III
KER
432
164
70.0
.,,
.I3
52158
Mahakam
3980
III
KER
452
9,
77.0
.-I4
.10
KER on
-
chloroform
chloroform
extracted extracted
organic rock.
matter
iJo1at.d
by
HF/HCl
.05
attack
on
- chloroform chloroform
extracted extracted
organic
matter
W/KC1
attack
rack.
Table 3 and Figs. 5-7 summarize the results of kerogen pyrolyses for the Type I, Type II, and Type III reference series. It is apparent from Figs. 5-7 that the chromatograms of the Cg+ HCSIA (recombined saturated + unsaturated + light aromatic hydrocarbons and thiophene fractions) are characteristic for each type of organic matter. We see in particular that the amount of aromatics in the C,-C,, hydrocarbon fraction regularly increase when we go from Type I to Type III kerogen, where the aromatic hydrocarbons become predominant over saturated and unsaturated hydrocarbons. Therefore, the relative amount of saturated and unsaturated hydrocarbons versus aromatics in the Cs-C,4 hydrocarbons appears to be a good tool for kerogen typing. The concept of kerogen typing by careful examination of pyrolysate composition is not new and has already been extensively reported in the literature. ROMOVACEK and KUBAT ( 1968) and GIRAUD ( 1970) were among the first to use pyrolysis-gas chromatography (PYR-GC) techniques to characterize coals and kerogens. More recent studies by LARTER ( 1984, 1985), LARTER and DOUGLAS (1978), SCHENCK et al.(1981),H0~sn~~~(1984),andHoRsnELDetal.(1983) using PYR-GC, pyrolysis-mass spectrometry (PYR-MS) and pyrolysis-gas chromatography-mass spectrometry (PYR-GCMS) also indicate that the relative abundances of selected aromatic and aliphatic pyrolysate components permit the discrimination of kerogens from one another. With the technique we developed, we can precisely quantify the light aromatic and thiophene hydrocarbons ( C&i4 fraction) and the total saturated plus unsaturated hydrocarbons. We decided to study the evolution of the overall ratio (Q) of saturated plus unsaturated hydrocarbons to the aromatic hydrocarbons in the C&i4 fraction. It is apparent that Q roughly reflects the difference of composition between pyrolysates. For Type I kerogens, Q is greater than 2; whereas for Type II kerogens, it varies between 1 and 2, and for Type III kerogens, it is less than 1. Concurrently with this difference in pyrolysate composition, the results presented in Table 3 clearly indicate that the
E. Lafargueand F.Behar
2976
GAMMA I
DEPTH
SONIC
RAY
ImKBI 100
4, 3100
3200
3300
E I
L
-.-. 3FF
?_
--A
N
O0'
3400
60
01
03
ATOMIC
*
O/C
04
FIG. 3. Van Krevelen diagram (H/C vs. O/C) for the North Sea samples.
3500
-----
--.-._ -
3600
ported by VAN GRAAS et al. ( 198 1) and LARTER ( 1985 ), indicating that the qualitative pyrogram fingerprints of a given kerogen type are more or less rank independent over much of the oil window. Therefore, it is possible with this technique to compare kerogens from different types independent of their maturity level.
3700
3800
3900 FIG. 2.Stratigraphy of the Jurassic formations in the North Sea (from JOHNSONand EYSSAUTIER,1987)
carbon-normalized yields of hydrocarbons released by pyrolysis are also characteristic of each organic matter type. As expected and previously reported in the literature ( ROMOVACEKand KUBAT, 1968; TKSOT et al., 1978; LARTER and DOUGLAS, 1982; HORSF~ELDet al., 1983), the amount of pyrolysate is largest for Type I kerogens and then regularly decreases for Type II and Type III kerogens. This illustrates the fact that absolute yields of kerogen pyrolysis products of all types are simply a function of the overall aliphatic carbon and hdyrogen content of the kerogen ( LARTER and HOR.SFIELD, 1989). The evolution of the pyrolysate composition with kerogen maturity is also very important. We note in particular that the criteria we previously proposed for kerogen determination (overall chromatogram, value of Q) are essentially independent of maturity. This is in agreement with observations re-
64
269
4.17
63
137
2.22
67
131
1.97
71
186
3.63
12
110
3.68
12
21
2.21
II
65
0.97
46
76
1.10
41
66
1.08
38
63
1.11
45
85
1.63
59
127
2.27
38
80
2.13
14
25
2.00
5
13
0.56
10
21
0.93
11
28
0.76
10
20
0.72
lo
21
0.81
12
1.11
8
**
Q- HCSI
KS-Cldl/
“CA
Kg-C14,
Determining source-rock types and oil/source-rock
correlations
2977
KEROGEN PYROLYSIS 550°C c6+ WCS
C,3+ HCSI
cfj+ HCA
C13+
/;7/
--_
t-K%
Jl__y-----------__ /
/
j .10lu0n0
*
pm Xyhno
0
IHmothylttonxono INCREASING
RETENTION
TIME
b
FIG. 4. Comparison between ‘*C6+”(left) and “C IS+)’(right) preparative pyrolyses (see Fig. 1 for definitions of fraction abbreviations).
KEROGEN ~OL~lS55O*C
KEROGEN ~OL~lS5~C
TYPE I GREEN RIVER SHALES Cg+
TYPE II TOARCIAN
HCSIA
PARIS BASIN
C,3+ HCSIA .
7max4 I**
c
Qd. t 7 L-T_..-
__.
j
‘
Nhli$mijtgc
(1.2.27 16784
Q=
~‘%(cZj-C141
I
.
‘
520921
HCAQ-Cl41
a=
WCSHCG.Cj41 HCAQ-041
0.2.43 l
FIG. 5. Pyrograms of the Green River Shales samples.
Tokmw
l
pm
.
lth.thyl
*
Methyl Naphthalone
Xyiono
FIG. 6. Pyrograms of the Paris Basin samples.
8.nz.n.
E. Lafargue and F. Bihar KEROGEN PYROLYSIS550°C TYPE Ill MAHAKAM CS+
chains released by artificial thermal cracking will not carry functional groups anymore. Therefore, the pyrolysate is enriched in HCSI, whereas the yield of aromatic hydrocarbons remains relatively constant. The decrease of Q observed for the most mature Paris Basin sample ( T,,,, = 453°C) can be explained by a diminution of the HCSI available at this stage due to their expulsion from the kerogen. The decrease could also be due to thermal cracking of this hydrocarbon fraction into gas. For the Mahakam Delta sample, at the same maturity level, we
DELTA
HCSIA
HCSIQC,,,) o= HCA(C6-C14)
do not observe a similar decrease. For Type I organic matter, because of its low amount of heteroatoms ( TISSOT and WELTE, 1984 ), HCSI are rapidly released in the pyrolysate through thermal cracking, and, therefore, there is no significant evolution of Q with maturity in these series. For the three organic matter types, it is interesting to note the systematic decrease of the amount of prist-1-ene with maturity. VAN GRAAS et al. ( 1981), GOOSENSet al. ( 1984, 1988), and CURRY and SIMPLER( 1988) observed the same phenomenon. It is therefore probable, as suggested by these authors, that the precursor of pristane in the subsurface yields prist- l-ene as a pyrolysis product. Since the isoprenoid/alkane ratio decreases with maturity, it is not surprising that the amount of prist- 1-ene follows the same trend.
Application of the method to kerogen typing in the North Sea
FIG. 7. Pyrograms of the Mahakam Delta samples.
For Type I kerogens, there is no significant evolution of Q with maturity, whereas for Type II and Type III kerogens, there is a slight increase of this ratio (from 1 to 2.5 and from 0.5 to 1, respectively). LARTER and DOUGLAS( 1980), when studying the evolution with maturity of the m + p-xylene to n-octene ratio, also saw no significant trend for alginite samples, but, in contrast, there was an increase of this ratio with maturity for vitrinite and sporinite samples. The increase of Q with maturity observed within the Type II (from 1 to 2.5) and Type III series (from 0.5 to 1) is indicative of changes occurring during thermal kerogen cracking. In both evolution series, we consider kerogen samples at different levels of maturity, from diagenesis to catagenesis. The natural thermal evolution of the samples in the diagenesis zone did not proceed very far; mainly heteroatomic bonds were broken and labile carbonyl and carboxyl groups removed ( TISSOT and WELTE, 1984). Therefore. these samples still have some functional groups as well as hydrocarbon chains and small aromatic moieties. During pyrolysis, the majority of the chains released will carry a functional group. Consequently, they will be included in the heteroatomic fraction of the pyrolysate, and few HCSI will be released in the pyrolysate as such. The thermal evolution of the samples in the catagenesis zone is more advanced, and almost all the functional groups have been eliminated. Consequently, during pyrolysis the
We applied the preparative pyrolysis method to elucidate the source-rock type of samples from the “Greater Alwyn” area, North Sea. The potential source-rocks in this area are located in the shales of the Draupne, Heather, and Dunlin Formations and in the coals of the Brent Formation ( OUDIN, 1976; THOMAS et al., 1985; FIELD, 1985). As is obvious from Table 2 and Fig. 3, the samples studied are rather mature, with T,,,, values around 445°C. On a Van Krevelen diagram, they fall well into the catagenesis zone (H/C less than 0.85). Therefore, at this maturity level, it is rather difficult to assess organic matter types with only these bulk geochemical parameters. This illustrates the deficiencies of elementally based kerogen typing when dealing with mature samples or with provinces where terrestrial input is important ( SNOWDONand POWELL, 1982; LARTER and HORSL~ELD, 1989 ). It is clear that in such cases a multiparameter characterization approach as proposed by LARTER (“Integrated kerogen typing,” 1985 ) is necessary. The combination of routine geochemical analysis with more sophisticated and time consuming techniques (preparative pyrolysis, PYR-GC, PYR-GC-MS) applied on a limited number of samples properly selected is a very powerful tool for source-rock characterization in sedimentary basins. When available, the combination of geochemical data with optical petrology results gives even better information ( SENFTLE et al., 1987; HORSFIELD et al., 1988; ESPITALIE et al., 1989). A comprehensive characterization of organic matter goes beyond the “type” concept: it gives information on the environment of deposition of the source-rock, on the structure of the kerogen itself, on the hydrocarbons it can produce (quality and quantity), and it should also give some indi-
2919
Determining source-rock types and oil/source-rock correlations cations of the ability of the potential source-rock to expel hydrocarbons, which is not included in any of the presently applied kerogen classifications. For the application presented herein, optical data were not available. The kerogens we selected are from source-rock cores. To assess kerogen type, we compared the overall pyrogram composition, Q values, and pyrolysis yields with results obtained from the reference series. The pyrograms of two shale samples from the Draupne Formation (Fig. 8) show composition of C&i., hydrocarbons and Q values characteristic of Type II organic matter similar to the Paris Basin series. Two types of pyrograms were obtained from the shales of the Heather Formation (Fig. 9). The pyrogram of sample 7 1276 is similar to those from the Draupne Formation, being characteristic of Type II organic matter. The pyrograms of samples 7 1266 and 7 1267 show an enrichment in light aromatics ( C&i4 HCA) relative to light saturated plus unsaturated hydrocarbons ( C6-Ci4 HCSI). This is uncharacteristic of Type II organic matter. In addition, it is not typical of Type 111pyrograms, as illustrated by the Mahakam Delta series (Fig. 7), even though Q values for these two samples ( 1.00 and 1.08, respectively) are in the same range. To explain these observations, we suggest the presence of either an altered Type II or a “mixed Type II/III” (by mixed we imply a mixing of terrestrial and marine organic matter). Figure 10 shows the pyrograms of two Paris Basin samples (Type II kerogen) which are at the same maturity level but have different Hydrogen Index (HI) values (74 1 mg/gTOC and 574 mg/gTGC, respectively). The reduced petroleum
KEROGEN PYROLYSIS 550°C NORTH SEA SAMPLES HEATHER FORMATION C6+ HCSIA
‘II, .
NORTH SEA DRAUPNE
Tmax
FORMATION
I
,i
. 10
-
CARBONNUMBEROF
445’
C
I I
I
10
25
CARBON
NUMBER
OF ” ALKANES
7’276 !Q4.97 +
9. Pyrograms of the Heather Formation samples.
SAMPLES
Tmax 446’ C
-
C
7 1267
FIG.
CS+ HCSIA
‘ill
442’
I
-
KEROGEN PYROLYSIS 550°C
Tmax
25 nALKANES
b
FIG. 8. Pyrograms of the Draupne Formation samples.
potential has been ascribed to alteration of the organic matter either by bacteria or by oxidation (B. DURAND, pers. commun., 1988). It is clear from Figure 10 that even for an altered Type II kerogen, the composition of the C6-C14 hydrocarbon fraction remains characteristic of the Type II series. Therefore, we believe that for samples 7 1266 and 7 1267 from the Heather Formation the difference in organic matter composition reflected by the different pyrograms can be ascribed to a mixing of terrestrial and marine organic matter input. For the Brent samples, the pyrograms shows no ambiguity (Fig. 11). The composition of the C&i4 hydrocarbons, with the predominance of the light aromatics, is typical of Type III organic matter. To stress the advantages of our new technique as compared to the previous preparative pyrolysis technique (C,,,), in Fig. 12 we present results obtained on the samples from the Brent formation with the latter technique. Whereas samples 7 1275 and 70936 yield pyrograms typical of Type- III organic matter, sample 148 12 is different and could have been wrongly interpreted as representative of Type II organic matter. Therefore, it is clear that the complementary information we got in the C&i4 hydrocarbon fraction is very important for assessing organic matter type. This has also been shown in previous work using PYR-GC techniques ( LARTER and DOUGLAS, 1982).
E. L.&argueand F. Behar
2980
KEROGENPYROLYSIS 550°C PARIS BASIN SAMPLES C6+ HCSIA
50093
106961 -..-. _
* 10
-
Totwn~
Hk574mQ/fi 1
IIIIIII
L-.--
.
CARBON
25 NUMBER
OF nALKANES
Draupne Formation, as reported by these authors. The quality of the shales from this formation is believed to be due to good preservation of organic matter in an anaerobic environment {THOMASet al., 1985; CORNFORDet a1., 1986). The inhomogeneity of the Heather Formation illustrated by the pyrolysis results has been observed by many others before (RELD, 1985; Hut et al., 1985; SCHOU et al., 1985). This has been ascribed to conditions within the water column varying from oxidizing to mildly reducing during deposition (THOMAS et al., 1985), together with varying input of terrestrial organic matter (HUG et al., 1985), as suggested by the pyrolysis results. The Brent formation containing terrestrial organic matter shows a lower petroleum potential. However, due to the large amount of organic matter, its contribution to the hydrocarbons produced in this area is probably not negligible. Within the Dunlin Formation, organic matter quality is related to the environment of deposition. The shales of the Drake Group (Upper Dunlin) are similar to Draupne sourcerocks and are good potential oil sources. In contrast, the shales from the Cook Group underneath have a lower petroleum potential due to the p~ominance of terrestrial input. The problem of oil/source-rock correlations in this area has been investigated by applying the pyrolysis technique to asphaltenes from North Sea crude oils (for a review of asphaltene pyrolysis as a tool for oil/source-rock correlations,
Y
FIG. IO.Comparison of the gyrograms of “normal” Type II (upper) and “altered” Type II kerogens flower) from the Paris Basin.
KEROGEN PYROLYSIS 550°C NORTH SEA SAMPLES
Finally, the pyrograms of samples from the Dunlin formation (Fig. 13), using criteria established earlier, are ascribed to Type II organic matter (samples 70953 and 70955 from the Drake Group, Upper Dunlin) whereas sample 65145 (Cook Group) belongs to Type III organic matter. It is interesting to note that samples 148 12 and 70953-which have the same T,,,, values (447’C), similar HI values ( 134 and 131 mg/gTOC), and similar H/C and O/C atomic ratioswere clearly separated into Type III and Type II kerogens on the basis of very different pyrograms and Q values. We indicated in the first part ofthis paper that the amount of saturated and unsaturated hydrocarbons produced by kerogen pyrolysis was greater for Type II than for Type III sourcerocks. Pyrolysis yields from the North Sea samples, for each class of hydrocarbons (Table 4), can indicate organic matter types. Kerogen typing inferred from comparison with pyrograms of the reference series is in good agreement with observed pyrolysis yields. For Type II samples (Draupne Formation, samples 71276 from Heather and 70953 and 70955 from Dunlin), yields of saturated + unsaturated hydrocarbons are high, varying between 30 and 50 mg/gTOC. In contrast, when terrestrial input is important (Type III samples and “mixed” Type II/III samples), these yields are much lower, around 10 mg/gTOC. The results obtained on these samples are in good agreement with the data on source-rock analysis published for this sector of the North Sea ( BARNARDand COOPER,198 1; GOFF, 1983; THOMAS et al., 1985; FIELD, 1985). In particular, we confirm the good quality of the source-rocks from the
BRENT FORMATION Cg, HCSIA ~~
__._.~._.
~---._
r-~
14*‘2.!
CM.14
.
ill /:
p+m Xykn.
AuJr.:..
1msx 452-c
FIG. 11. Pyrograms of the Brent Formation samples (G+).
2981
Determining source-rock types and oil/source-rock correlations
KEROGEN PYROLYSIS 550°C
KEROGEN PYROLYSIS 550°C
NORTH SEA SAMPLES
NORTH SEA SAMPLES
DUNLIN FORMATION
BRENT FORMATION
c6+ HCSIA
C13+ HCSI I
I/ Tmax 447%
Tmax
447O
C
I!
li
i
‘
)
7og53/ kM.03
1
.
65’46
10
25
Tmax 453OC
t 70936 l
I II
Tmax
452V
Il...... I
-
i FIG.
CARBON
712751 .
‘
15
25
CAFiEiON
NUMBER
OF nALKANES
-+
12. Pyrograms of the Brent Formation samples (C,,,).
et al., 1982; BANDURSKY, 1982; BEHAR and PELET, 1985; TELNAESet al., 1985; JONES et al., 1988). The whole-oil gas chromatograms of two types of North Sea crude oils are presented in Fig. 14 together with the pyrograms issued from pyrolysis of their respective asphaltenes. Both oils come from the Viking Graben. Type A crude oil is typical of those encountered in the North Sea (OUDIN, 1976; THOMASet al., 1985; SCHOLIet al., 1985). Its gas chromatogram shows a regularly decreasing distribution of saturated hydrocarbons with few Czs+ compounds. Type B crude oil has a similar Pr/Ph ratio, but the saturated hydrocarbon distribution is enhanced in the heavy normal alkanes ( CzsCj,) region. This difference cannot be due to maturity since both oils have similar alkane/isoprenoid ratios and similar biomarkers distribution (J. L. OUDIN, pers. commun., 1988). Pyrolysates from asphaltene pyrolysis are almost identical for both oils, and exhibit hydrocarbon distribution similar to type A crude oil. The asphaltene pyrograms are also similar to Type II kerogen pyrograms, which confirms the genetic relationship between kerogens and asphaltenes, and demonstrates the usefulness of asphaltene pyrolysis for correlation purposes. The pyrolysis results also indicate a common ma-
see AREFEV
!
l
FIG.
NUWEROF
nALKAM3
I Q=0.74
*
13. Pyrograms of the Dunlin Formation samples.
rine origin for both crudes which is contradicted by the different saturated hydrocarbons distributions. Therefore we believe that type A oil originates from one of the marine source-rocks of the sector, presumably the Draupne Formation, due to its thickness in this area. On the other hand,
SANPLE
FOWTION
THAX
“I
HCSI
NUMBER
‘6-‘14 (‘C)
Im3/ga
HCSI
C
6+
(mgfgc)
lmg/9cl
“0.
HCI
'6-'lA mg/gcl
Q
'6+ mg/gc)
70891
DPA”PNE
446
367
45
80
25
50
1.79
70895
DRAUPNE
446
319
51
93
26
71
1.96
71266
HEATHER
442
110
11
16
11
24
1.00
71261
HEATHER
440
115
12
23
11
19
1.08
71276
HEATHER
445
217
29
48
15
35
1.97
14812
BRENT
447
134
9
18
8
16
1.14
70936
BRENT
453
202
9
15
13
26
0.70
BRENT
452
248
11
18
10
19
1.10
70953
DUNLIN
447
131
32
61
16
40
2.00
70955
DUNLJN
441
264
40
73
23
51
1.75
65146
DUNIJN
434
145
6
10
8
12
0.75
71275
*
a-
HCSI
(C6X14)/
“CA
(C
6
-c
14'
E. Lafargue and F. Behar
2982
OIL / SOURCE-F?OCK CORRELATION NORTH SEA CRUDE OILS TYPE A
TYPE B crude
crude
oil
L
10
oil
.
25
ASPHALTENES PYROLYSIS C(j+ HCSIA
10
25 CAFISCN M_h5ER
10
25 b
OF nAJ_KANES
FIG. 14. Oil/ source-rock correlation for two North Sea crude oils.
we interpret type B oil as being mixed marine/ terrestrial crude oil. This would explain the importance of the heavy end in the whole-oil chromatogram and its low level in the asphaltene pyrograms, because it is known that low amounts of asphaltenes are released from terrestrial organic matter ( TISSOT and WELTE, 1984). The juxtaposition of mature terrestrial organic matter (Brent coals) with formations containing marine organic matter makes such a mixing in this part of the North Sea possible. Another argument in favor of the contribution of coals in the formation of type B oil is found in the studies of THoMPsoN et al. ( 1985) and HORSFIELD et al. ( 1988 ), who demonstrated the production of waxy crudes from coals. CONCLUSIONS
The development of analytical pyrolysis provides the explorer with information on bulk-chemical and molecular structure of organic matter. This is of paramount importance for precisely assessing kerogen types in sedimentary basins. Through kerogen and asphaltene pyrolysis, it also proves to be very useful for oil/oil and oil/source-rock correlations. Preparative pyrolysis allows the determination of absolute quantities ofeach hydrocarbon fraction released in the pyrolysate. This is important for understanding kerogen structure and the thermal cracking of kerogen. It is also very useful for estimating source-rock petroleum potential. In doing so, the technique is complementary to PYR-GC, which gives the GOR of the pyrolysate. Acknowledgmmts-Special thanks are due to C. Comford, B. Durand, B. Horsfield, S. R. Larter, R. Pelet, and M. Vandenbroucke for the
critical readings of this manuscript and for their helpful suggestions. Technical assistance by C. Leblond is gratefully acknowledged. Editorial handling: J. RullkGtter
REFERENCES AREFEVO., MAKUSHINA., and PETROVA. ( 1982) Asphaltenes as indicators of the geochemical history of oil. Infl. Geol. Rev. 24, 723-738.
BANDURSKYE. ( 1982) Structural similarities between oil generating kerogens and petroleum asphaltenes. Energy Sources 6,47-66. BARNARDP. C. and COOPERB. S. ( 198 1) Oils and source-rocks of the North Sea area. In Petroleum Geology of the Conrinental Shelf of North West Europe. (eds. L. V. ILLING and G. D. HOBSON), pp. 169-175. Inst. of Petroleum, London. BEHARF. and PELETR. ( 1985) Pyrolysis-gas chromatography applied to organic geochemistry. Structural similarities between kerogens and asphaltenes from related source-rocks extracts and oils. J. Anal. Appl. Pyrolysis 8, 173- 187.
BEHARF. and VANDENBROUCKE M. ( 1988) Characterisation and quantification of saturates trapped inside kerogen: implications for pyrolysate composition. In Advances in Organic Geochemistry 1987 feds. L. MATTAVELLIand L. NOVELLI); Org. Geochem. 13, pp. 927-938. Pergamon Press, Oxford. COMBAZA. and DE MATHAREL M. ( 1978) Organic sedimentation and genesis of petroleum in Mahakam Delta, Borneo. Bull. Amer. Assoc. Petrol. Geol. 62, 1684-1695. CORNFORD C., NEEDHAM C. E. J., and DE WALQUEL. ( 1986) Geochemical habitat of North Sea oils and gases. In Habitat of Hydrocarbons on the Norwegian Continental Shelf (eds. B. M. THOMASet al.), pp. 39-54. Graham and Trotman, London. CURRY D. J. and SIMPLERT. K. ( 1988) Isoprenoid constituents in kerogens as a function of depositional environment and catagenesis. In Advances in Organic Geochemistry 1987 (eds. L. MAI-~AVELLI and L. NOVELLI); Org. Geochem. 13. pp. 995-1001. Pergamon Press, Oxford. DURAND B. and MONINJ. C. ( 1980) Elemental analysis of kerogens. In Kerogen (ed. B. DURAND), pp. 113-142. Technip, Paris.
Determining source-rock types and oil/source-rock correlations DURANDB. and NICAI~EG. ( 1980) Procedures for kerogen isolation. In Kerogen (ed. B. DURAND), pp. 35-53. Technip, Paris. ESPITALIEJ., DURAND B., ROUSSELJ. C., and SOURON C. ( 1973) Etude de la mat&e organique insoluble des argiles du Toarcien du Bassin de Paris. Rev. Inst. Fr. Petrol. 28, 37-66. ESPITALIEJ., LAFARGUE E., and EGGEN S. ( 1989) Differences between petroleum potential of marine and terrestrial organic matter in the Jurassic formations of the North Sea. Proc. Slst E.A.E.G. Meeting. Berlin, June I989 (in press). FIELD J. D. (1985) Organic geochemistry in exploration of the Northern North Sea. In Petroleum Geochemistry in Exploration of the Norwegian Shelf (eds. B. M. THOMAS et al.), pp. 39-57. Graham and Trotman, London. GlRAUD A. ( 1970) Application of pyrolysis and pyrolysis-gas chromatography to the geochemical characterisation of kerogen in sedimentary rocks. Bull. Amer. Assoc. Petrol. Geol. 54, 439-455. GOFF J. C. ( 1983) Hydrocarbon generation and migration from Jurassic source-m&s in the East Shetland Basin and in Viking Graben of the Northern North Sea. J. Geol. Sot. London 140,445-474. G~OSSENS H., DE LEEUW J. W., SCHENCK P. A., and BRASSELL S. C. ( 1984) Tocopherols as likely precursors of pristane in ancient sediments. Nature 312,440-442. G~~SSENS H., DE LANGE F., DE LEEUW J. W., and SCHENCK P. A. ( 1988) The Pristane Formation Index, a molecular maturity parameter. Confirmation in samples from the Paris Basin. Geochim. Cosmochim. Acta 52,2439-2444. VAN GRAAS G., DE LEEUW J. W., and SCHENCK P. A. ( I98 1) Kerogens of Toarcian shales of the Paris Basin: a study of its maturation by flash pyrolysis techniques. Geochim. Cosmochim. Acta 45,24652474. HORSFIELDB. ( 1984) Pyrolysis studies and petroleum exploration. In Advances in Petroleum Geochemistry (eds. J. Broods and D. WELTE), Vol. I, pp. 247-298. Academic Press, New York. HOR~FIELDB., DEMBKXI H., and Ho T. T. Y. ( 1983) Some potential applications of pyrolysis to basin studies. J. Geol. Sot., London 140,431-443. HORSFIELDB., YORDY K. L., and CRELLINGJ. C. (1988) Determining the petroleum-generating potential of coals using organic geochemistry and organic petrology. In Advances in Organic Gee chemistry I987 (eds. L. MATTAVELLIand L. NOVELLI);Org. Geothem. f3, pp. 12 1- 129. Pergamon Press, Oxford. HUC A., IRWINH., and SCHOELLM. ( 1985) Organic matter quality changes in an Upper Jurassic shale sequence from the Viking Graben. In Petroleum Geochemistry in Exploration ofthe Norwegian Shel/(eds. B. M. THOMASet al.), pp. 179-185. Graham and Trotman, London. JONESD., DOUGLASA. G., and CONNANJ. ( 1988) Hydrous pyrolysis of asphaltenes from biodegraded oils. In Advances in Organic Geochemistry 1987 (eds. L. MATTAVELLIand L. NOVELLI);Org. Geothem. 13, pp. 981-993. Pergamon Press, Oxford. JOHNSONA. and EYSSAUTIERM. ( 1987) Alwyn North field and its regional geological context. In Petroleum Geology ofNorth West Europe (eds. J. BROOKSand K. GLENNIE),pp. 963-977. Graham and Trotman, London. LARTERS. R. ( 1984) Application of analytical pyrolysis techniques to kerogen characterisation and fossil fuel exploration, exploitation. In Analytical Pyrolysis (ed. J. VCKIRHEES),pp. 2 12-275. Butterworths, London. LARTERS. R. ( 1985) Integrated kerogen typing in the recognition and quantitative assessment of petroleum source-rocks. In Petro leum Geochemistry in Exploration of the Norwegian Shelf (eds. B. M. THOMAS et al.), pp. 269-286. Graham and Trotman, London. LARTERS. R. and DOUGLASA. G. (1978) Low molecular weight aromatic hydrocarbons in coal maceral pyrolysates as indicators
2983
of diagenesis and organic matter type. In Environmental Biogeochemistry and Geomicrobiology (ed. W. KRUMBEIN),Vol. 1, pp. 373-386. Univ. Michigan Press, Ann Arbor. LARTER S. R. and DOUGLASA. G. ( 1980) A pyrolysis-gas chromatographic method for kerogen typing. In Advances in Organic Geochemistry 1979 (eds. A. G. DOUGLASand J. R. MAXWELL). pp. 579-584. Pergamon Press, Oxford. LARTER S. R. and DOUGLASA. G. ( 1982) Pyrolysis methods in organic geochemistry: an overview. J. Anal. Appl. Pyrol. 4, 1-19. LARTERS. R. and HORSFIELDB. ( 1989) Determination of structural components of kerogen using analytical pyrolysis methods. In Organic Geochemistry (eds. M. ENGEL and S. MACKO), Chap. 16 (in press). LARTERS. R. and SENTFLEJ. ( 1985) Improved kerogen typing for petroleum source-rock analysis. Nature 318, 277-280. OUDINJ. L. ( 1976) Etude geochimique du bassin de la Mer du Nord Bull. Centre Rech. Pau 10,339-358. ROBINSONW. E. ( 1969) Kerogen of the Green River Formation. In Organic Geochemistry (eds. G. EGLI~ON and M. T. J. MUWHY ). pp. 619-637. Springer-Verlag, Berlin. ROMOVACEKJ. and KUBATJ. ( 1968) Characterisation of coal substances by PYR-GC. Anal. Chem. 40, 1 I 19-I 126. SCHENCKP. A., DE LEEUWJ., VANGRAASJ. W., HAVERKAMPJ.. and BOUMANM. ( 198 1) Analysis of recent spores and pollen and of thermally-altered Sporopollenin by flash pyrolysis-mass spectrometry and flash pyrolysis-gas chromatography-mass spectrometry. In Organic Maturation Studies and Fossil Fuel Exploration (ed. J. BROOKS), DD. 225-237. Academic Press. New York. SC~OU L., EGGEN’S., and SCHOELLM. ( 1985 ) Oil-oil and oil-source rock correlation, Northern North Sea. In Petroleum Geochemrsrry in Exploration of the Norwegian Shelf (eds. B. M. THOMASet al.), pp. 101-l 17. Graham and Trotman, London. SENFTLEJ. T., BROWN,J. B., and LARTERS. R. ( 1987) Refinement of organic petrographic methods for kerogen characterisation. Ml. J. Coal Geol. 7, 105-I 17. SNOWDONL. R. and POWELLJ. G. ( 1982) Immature oil and condensate-modification of hydrocarbon generation model for terrestrial organic matter. Bull. Amer. Assoc. Petrol. Geol. 66, 775-788. TELNAESN.. SPEERSG. C., STEENA., and DOUGLASA. G. ( 1985 ) Hydrous pyrolysis of asphaltenes. In Petroleum Geochemistry rn Exploration of the Norwegian Shelf(eds. B. M. THOMASet al.). pp. 287-292. Graham and Trotman, London. THOMASB. M., MOLLER-PEDERSEN P., WHITAKERM. F.. and SHAW N. D. ( 1985) Organic facies and hydrocarbon distributions in the Norwegian North Sea. In Petroleum Geochemistry in Exploration of the Norwegian Shelf (eds. B. M. THOMASet al.), pp. 3-26. Graham and Trotman, London. THOMPSONS., COOPERB. S., MORLEYR. J.. and BARNARDP. C. ( 1985 ) Oil-generating coals. In Petroleum Geochemistr), rn E.xp/oration of the Norwegian Shelf(eds. B. M. THOMASet al.). pp. 5% 75. Graham and Trotman, London. TISSOTB. and WELTE D. ( 1984) Petroleum Formation and Occurrence, 2nd ed. Springer-Verlag, Berlin. TISSOTB., DURANDB., ESPITALIEJ., and COMBAZA. ( 1974) lnfluence of nature and diagenesis of organic matter in formation of petroleum. Bull. Amer. Assoc. Petrol. Geol. 58, 499-506. TISSOTB., DEROOG., and HOODSA. ( 1978) Geochemical study of the Uinta Basin: formation of petroleum from the Green River formation. Geochim. Cosmochim. Acta 42, 1469-1485. VANDENBROUCKE M., BEHARF., and ESPITALIEJ. ( 1988) Characterisation of sedimentary organic matter by preparative pyrolysis: comparison with Rock-Eva1 and PY-GC techniques. Energy and Fuel 2,252-258. YOUNGD. and YEN T. ( 1977) The nature of straight chain aliphatic structures in Green River kerogens. Geochim. Cosmochim. Acta 41, 141 I-1417.
#II&7037/89/$3.00 + .iM
Ada Vol. 53, pp. 2973-2983 PES pk. Printed in U.S.A.
Application of a new preparative pyrolysis technique for the determination of source-rock types and oil / source-rock correlations Institut Fran&
E. LAFARGUEand F. BEHAR du P&role, BP 3 II,92506 Rue&Malmaison Cedex, France
(Received January 24, 1989; accepted in revised form September 6, 1989)
Abstract-A new preparative pyrolysis technique enabling the recovery and fmctionation (into saturated hydr~ar~ns, unsaturated hydrocarbons, and aromatic hydrocarbons) of the total G+ hydrocarbon fraction (instead of the C&+ fraction usually recovered) has been applied to different types of sourcerocks. The composition of the C&is hydrocarbon fraction in the pyrolysate, particularly the amount of aromatic hydrocarbons as compared to alkanes, was found to be characteristic of each type of kerogen, with the alkane/aromatic ratio consistently decreasing in the progression from Type I to Type III kerogens. While the Ct3+ fraction is useful in kerogen typing, it was found that the C,-& hydrocarbon fraction, which represents 40 to 50% of the total recovered pyrolysate, was the most sign&ant in emph~i~ng differences between kerogen types, allowing a rapid and precise estimation of the source-rock type. This new technique was applied to potential source-rocks of the Viking Graben, North Sea (Draupne formation, Heather formation, Brent coals, and Dunlin group). In each case, the pyrolysates allowed us to determine whether the organic matter was Type II, Type III, or a mixture of both. Pyrolysis of asphaltenes from crude oils from the various regions was conducted and potential applications of our technique to studies of oil/source-rock correlations were examined. INTRODUCTION THE CHARA~RIZATION OF organic matter from sedimentary rocks is now widely recognized as a useful tool for petroleum exploration. Elementally based kerogen typing established from the work of Tissot and coworkers f TISSOT et al., I.974 ) is routinely used for this characterization. Since this classification is based on bulk chemical analysis, similar atomic ratios may correspond to very different chemical structures. Therefore, this technique gives access to bulk chemical properties of the kerogen, but it is not appropriate for the determination of detailed molecular con~~mtion of the kerogen, which is another important part of organic matter characterization ( LARTER and SENTFLE, 1985; VANDENBROUCKE et al., 1988). In addition, strict adherence to an elemental classification scheme has been reported to cause problems, especially in terrigenous organic matter provinces ( SNOWWN and POWELL, 1982; HORSRELD, 1984; LARTER, 1985 ). Analytical pyroIysis has become common in petroleum exploration for improving elemental-based kerogen typing. This technique was first applied in coal studies ( ROMOVACEK and KUBAT, 1968) and has more recently been extended to kerogens from a large range of sedimentary facies ( LARTER and DOUGLAS, 1980; HORSRELD, 1984; LARTER, 1985; BEHAR and PELET, 1985). Numerous analytical pyrolysis techniques are used (pyroIy&GC, pyr-GC-MS, hydrous pyrolysis, preparative pyrolysis, etc.) they all have specific applications in petroleum exploration (see LARTERand DOUGLAS, 1982; LARTER, 1984; VANDENBROUCKEet al., 1988). Here, we describe a new preparative pyrolysis technique which enables the recovery and quanti~tive fractionation (into saturated, unsaturated, and aromatic hydrocarbons) of the total Cg+ hydrocarbon fraction of the pyrolysate instead of the C,J+ fraction usually recovered and analyzed by this technique ( BEHAR AND PELET, 1985). We report the appli2973
cation to reference series of Type I, Type II, and Type III kerogens and then apply the technique to source-rock type characterization and to oil/source-rock correlation in a North Sea case study. EXPERIME~AL
SECHON
Methodology Pyrolysisis conducted on kerogens isolated from source-rocks according to the analytical procedure described in the literature ( DURand NKniSE, 1980). Prior to pyroiysis, kewgens are ChfOrOfO~extracted in order to avoid ~n~mination of the p~o~y~te by free hydrocarbons trapped in the kerogen structure (BEIIARand VANDENBROUCKE,1988). Pyrolysis is performed in a minifurnace designed at IFP ( BEHAR and PELET, 1985). The apparatus is sketched in Fig. I together with an overview of the analytical procedure. A preiiminary heating at 320°C for 2 min vaporizes any residual low molecular-weight material. The oven temperature is then raised to 550°C (at approximately ~‘C~rnin~ and heid there for 5 min. Hydrocarbons releasedthrough thermal cracking of the kerogen are carried away by argon gas to a trap cooled by liquid nitrogen. When pyrolysis is complete, the trap is disconnected and removed from the liquid nitrogen. After vaporization of argon out of the trap, the pyrolysate is dissolved in 100 rl of n-pentane. The C,-C5 hydrocarbons are lost by evaporation at this stage. The pyroIysate is fractionated by microcolumn liquid chromatography into saturated plus unsaturated hydrocarbons (HCSI) and light aromatic hydrocarbons plus thiophenes ( HCA) . The microcolumn is a Pasteur pipette of 0.5 cm diameter and 10 cm height filled with silica (on 5 cm). The saturated and unsaturated hydrocarbons are eluted with 2500 ~1 of n-pentane; the light aromatic hydrocarbons and thiophenes are eluted with 3000 ~1 of a 90~10 mixture of npentane and dichloromethane. The heavy aromatic hydrocarbons and thiophenes are not recovered in the 90: 10 mixture of n -permute and dichloromethane. Each fraction is analyzed and quantified separately by gas chromatography (GC) before recombination into a single fraction (HCSIA) to give the final chromatogram of the pyrolysate. The GC, with on-column injector, was fitted with a silica capillary coiumn of AND
E. Lafargue and F. Behar
2974
PYROLYSISDEVICE
TRAPPING DEVICE
/
\
\
I
Sample pasltlon dung analysis kerogen or ospholtenes (1 fa100mg)
Teflon valve /
i Argon ,_
sweep1 ^^__, IT--W’CI
Gloss,‘pyrex”
I 1‘rod Golc
Liquid mlrogen
KEROGEN ASPH::TENES I
Pyrolysis: 320°C- 550°C GO'C/min 550°C:5min I C6+EFFLUENTS
+n-C5 I
LIQUIDCHROMATOGRAPHY SiO2 I
100% n-C5r ALKJNES ALKENES (HCSI)
90% n-C5 1'10% CH,Q
LIGHT AROMATICS*THIOPHENES (HCA)
GC
I
GC
I I
MIXTURE HCSl+ HCA I
HCSIA
GC
FIG. 1. Pyrolysis device and analytical procedure.
30 m length and 0.32 mm inner diameter coated with a CPSil 5 film 0.25pm thick. Helium was the carrier gas. The initial temperature was 0°C with a heating rate of 3”C/min, taking the column to a maximum temperature of 300°C.
Samples
Kerogens obtained from classical Type I, Type II, and Type III evolution series ( TISSOTet al., 1974; Green River Shales, Paris Basin,
2915
Determining source-rock types and oiI/source-rock correlations and Mahakam Delta, respectively) were selected (Table 1) . Several studies of these serieshave been repotted in the literature (ROBINSON, 1969; YOUNGand YEN, 1977; TISSOTet al., 1978; ESPITALIE et al.. 1973; COMB,&? and DE MATHAREL,1978; DURANDand MONIN, 1980). Each series is characterized by its specific chemical structure. For Type II and Type III kerogens, it was fairly easy to select samples with increasing maturity. However, for Type I organic matter, due to the narrowness of its catagenetic zone (TISSOT et al., 1978) and to the heterogeneity of the Green River Shale Formation, it was not possible to get the same kind of evolution series. For these samples, Hydrogen Index (HI) values were used to assess their maturity level. The first four fall into the diagenesis zone and the last two into the catagenesis zone. North Sea samples are listed in Table 2. The stratigraphic horizons corresponding to these formations are presented in Fig. 2, with the position of the samples in a Van Krevelen diagram shown in Fig. 3. RESULTS
AND DISCUSSION
SAMPLE
FORNATION
NUMBER
DEPTR
PREP.
(ml
Tmax
"I
('C)
bg/ga
C
H/C
1%)
latom.~
o/c ,atom.,
70891
DRAUPNE
4500
KER
446
36,
19.0
.,5
.06
70895
DRWPNE
3675
KER
446
319
31.0
.a2
.06
71266
HEATHER
3316
KER
442
110
28.0
.74
.11
7126,
HEATHER
3317
KER
440
115
53.0
.68
.10
71276
HEATHER
3490
KER
445
217
24.0
.a5
.08
14812
BRENT
3328
KER
44,
134
61.0
.84
.09
70936
BRENT
3649
KER
453
202
71.0
.,O
.05
,I275
BRENT
3539
KER
452
248
73.0
.,4
.06
10953
DUNLIN
3682
KER
44,
131
23.0
.81
.O,
70955
DUNLIN
3691
KER
441
264
14.0
.Bl
.08
65145
DUNLIN
2687
KER
434
145
75.0
.,,
.I2
isolated
by
Application of the technique to reference series KER
Figure 4 compares the Cg+ and Ci3+ pyrolysis-derived chromatograms of the saturated plus unsaturated hydrocarbons (HCSI), together with the aromatic hydrocarbons (HCA) of one kerogen sample from the Toarcian of the Paris Basin. The advantages of the new analytical procedure, which allows the recovery of the Cg+ hydrocarbon fraction instead of the CiJ+ fraction usually recovered, are clearly illustrated in Fig. 4. Much more information is gained for the saturated and unsaturated hydrocarbons. In addition, the danger of partial loss of n-C,, and n-Cls by evaporation during fractionation is eliminated. Therefore, prist- 1-ene / n-C, and prist- 1-ene / n -Cis ratios are substantially more reliable. For the aromatic hydrocarbons, the precise analysis and quantification of the light aromatics and thiophenes more than compensates for the loss of the heavy ends in this fraction.
Table 1. SAMPLE
Geochamical
ORIGIN
NUMBER
data
on
DEPTN
0.n.
cm)
TYPE
kerogens
PREP.
TmaX ('C)
of
the
reference
HI lmglgcl
series.
C
H/C
o/c
(%I
,atom.)
(atom.)
15760
Uinta
B.
0ut.c.
I
KER
456
846
80.0
1.64
.05
15774
Uinta
B.
O"r_C.
I
KER
446
722
73.0
1.44
.08
15755
Uinta
B.
outc.
I
KER
452
726
75.0
1.51
.O6
15784
Uinea
B.
255,
I
KER
450
79,
67.0
1.41
.04
21562
Uinta
B.
2862
1
KER
452
37,
79.0
1.06
.03
15797
uinta
B.
3752
I
KER
451
86
63.0
0.71
.05 .08
10968
Paris
8.
O"tC.
11
KER
416
67,
57.0
1.29
38931
Paris
8.
1010
II
KER
429
666
62.0
1.33
.06
32893
Paris
B.
2040
II
KER
432
547
50.0
1.20
.O5
52092
Paris
B.
2253
IX
KLR
438
489
46.0
1.11
.04
20506
Parrs
8.
578
II
KIER
445
218
52.0
1.02
.03
20507
Paris
8.
1306
II
KER
453
96
39.0
0.69
36926
Hahakam
1010
III
KER
419
95
56.0
.89
.21
36933
Piahakam
1440
III
KER
422
151
70.0
.46
.11
36939
"ahakam
1910
III
KER
425
124
57.0
.90
.18
36945
“ahakam
2260
III
KER
432
164
70.0
.,,
.I3
52158
Mahakam
3980
III
KER
452
9,
77.0
.-I4
.10
KER on
-
chloroform
chloroform
extracted extracted
organic rock.
matter
iJo1at.d
by
HF/HCl
.05
attack
on
- chloroform chloroform
extracted extracted
organic
matter
W/KC1
attack
rack.
Table 3 and Figs. 5-7 summarize the results of kerogen pyrolyses for the Type I, Type II, and Type III reference series. It is apparent from Figs. 5-7 that the chromatograms of the Cg+ HCSIA (recombined saturated + unsaturated + light aromatic hydrocarbons and thiophene fractions) are characteristic for each type of organic matter. We see in particular that the amount of aromatics in the C,-C,, hydrocarbon fraction regularly increase when we go from Type I to Type III kerogen, where the aromatic hydrocarbons become predominant over saturated and unsaturated hydrocarbons. Therefore, the relative amount of saturated and unsaturated hydrocarbons versus aromatics in the Cs-C,4 hydrocarbons appears to be a good tool for kerogen typing. The concept of kerogen typing by careful examination of pyrolysate composition is not new and has already been extensively reported in the literature. ROMOVACEK and KUBAT ( 1968) and GIRAUD ( 1970) were among the first to use pyrolysis-gas chromatography (PYR-GC) techniques to characterize coals and kerogens. More recent studies by LARTER ( 1984, 1985), LARTER and DOUGLAS (1978), SCHENCK et al.(1981),H0~sn~~~(1984),andHoRsnELDetal.(1983) using PYR-GC, pyrolysis-mass spectrometry (PYR-MS) and pyrolysis-gas chromatography-mass spectrometry (PYR-GCMS) also indicate that the relative abundances of selected aromatic and aliphatic pyrolysate components permit the discrimination of kerogens from one another. With the technique we developed, we can precisely quantify the light aromatic and thiophene hydrocarbons ( C&i4 fraction) and the total saturated plus unsaturated hydrocarbons. We decided to study the evolution of the overall ratio (Q) of saturated plus unsaturated hydrocarbons to the aromatic hydrocarbons in the C&i4 fraction. It is apparent that Q roughly reflects the difference of composition between pyrolysates. For Type I kerogens, Q is greater than 2; whereas for Type II kerogens, it varies between 1 and 2, and for Type III kerogens, it is less than 1. Concurrently with this difference in pyrolysate composition, the results presented in Table 3 clearly indicate that the
E. Lafargueand F.Behar
2976
GAMMA I
DEPTH
SONIC
RAY
ImKBI 100
4, 3100
3200
3300
E I
L
-.-. 3FF
?_
--A
N
O0'
3400
60
01
03
ATOMIC
*
O/C
04
FIG. 3. Van Krevelen diagram (H/C vs. O/C) for the North Sea samples.
3500
-----
--.-._ -
3600
ported by VAN GRAAS et al. ( 198 1) and LARTER ( 1985 ), indicating that the qualitative pyrogram fingerprints of a given kerogen type are more or less rank independent over much of the oil window. Therefore, it is possible with this technique to compare kerogens from different types independent of their maturity level.
3700
3800
3900 FIG. 2.Stratigraphy of the Jurassic formations in the North Sea (from JOHNSONand EYSSAUTIER,1987)
carbon-normalized yields of hydrocarbons released by pyrolysis are also characteristic of each organic matter type. As expected and previously reported in the literature ( ROMOVACEKand KUBAT, 1968; TKSOT et al., 1978; LARTER and DOUGLAS, 1982; HORSF~ELDet al., 1983), the amount of pyrolysate is largest for Type I kerogens and then regularly decreases for Type II and Type III kerogens. This illustrates the fact that absolute yields of kerogen pyrolysis products of all types are simply a function of the overall aliphatic carbon and hdyrogen content of the kerogen ( LARTER and HOR.SFIELD, 1989). The evolution of the pyrolysate composition with kerogen maturity is also very important. We note in particular that the criteria we previously proposed for kerogen determination (overall chromatogram, value of Q) are essentially independent of maturity. This is in agreement with observations re-
64
269
4.17
63
137
2.22
67
131
1.97
71
186
3.63
12
110
3.68
12
21
2.21
II
65
0.97
46
76
1.10
41
66
1.08
38
63
1.11
45
85
1.63
59
127
2.27
38
80
2.13
14
25
2.00
5
13
0.56
10
21
0.93
11
28
0.76
10
20
0.72
lo
21
0.81
12
1.11
8
**
Q- HCSI
KS-Cldl/
“CA
Kg-C14,
Determining source-rock types and oil/source-rock
correlations
2977
KEROGEN PYROLYSIS 550°C c6+ WCS
C,3+ HCSI
cfj+ HCA
C13+
/;7/
--_
t-K%
Jl__y-----------__ /
/
j .10lu0n0
*
pm Xyhno
0
IHmothylttonxono INCREASING
RETENTION
TIME
b
FIG. 4. Comparison between ‘*C6+”(left) and “C IS+)’(right) preparative pyrolyses (see Fig. 1 for definitions of fraction abbreviations).
KEROGEN ~OL~lS55O*C
KEROGEN ~OL~lS5~C
TYPE I GREEN RIVER SHALES Cg+
TYPE II TOARCIAN
HCSIA
PARIS BASIN
C,3+ HCSIA .
7max4 I**
c
Qd. t 7 L-T_..-
__.
j
‘
Nhli$mijtgc
(1.2.27 16784
Q=
~‘%(cZj-C141
I
.
‘
520921
HCAQ-Cl41
a=
WCSHCG.Cj41 HCAQ-041
0.2.43 l
FIG. 5. Pyrograms of the Green River Shales samples.
Tokmw
l
pm
.
lth.thyl
*
Methyl Naphthalone
Xyiono
FIG. 6. Pyrograms of the Paris Basin samples.
8.nz.n.
E. Lafargue and F. Bihar KEROGEN PYROLYSIS550°C TYPE Ill MAHAKAM CS+
chains released by artificial thermal cracking will not carry functional groups anymore. Therefore, the pyrolysate is enriched in HCSI, whereas the yield of aromatic hydrocarbons remains relatively constant. The decrease of Q observed for the most mature Paris Basin sample ( T,,,, = 453°C) can be explained by a diminution of the HCSI available at this stage due to their expulsion from the kerogen. The decrease could also be due to thermal cracking of this hydrocarbon fraction into gas. For the Mahakam Delta sample, at the same maturity level, we
DELTA
HCSIA
HCSIQC,,,) o= HCA(C6-C14)
do not observe a similar decrease. For Type I organic matter, because of its low amount of heteroatoms ( TISSOT and WELTE, 1984 ), HCSI are rapidly released in the pyrolysate through thermal cracking, and, therefore, there is no significant evolution of Q with maturity in these series. For the three organic matter types, it is interesting to note the systematic decrease of the amount of prist-1-ene with maturity. VAN GRAAS et al. ( 1981), GOOSENSet al. ( 1984, 1988), and CURRY and SIMPLER( 1988) observed the same phenomenon. It is therefore probable, as suggested by these authors, that the precursor of pristane in the subsurface yields prist- l-ene as a pyrolysis product. Since the isoprenoid/alkane ratio decreases with maturity, it is not surprising that the amount of prist- 1-ene follows the same trend.
Application of the method to kerogen typing in the North Sea
FIG. 7. Pyrograms of the Mahakam Delta samples.
For Type I kerogens, there is no significant evolution of Q with maturity, whereas for Type II and Type III kerogens, there is a slight increase of this ratio (from 1 to 2.5 and from 0.5 to 1, respectively). LARTER and DOUGLAS( 1980), when studying the evolution with maturity of the m + p-xylene to n-octene ratio, also saw no significant trend for alginite samples, but, in contrast, there was an increase of this ratio with maturity for vitrinite and sporinite samples. The increase of Q with maturity observed within the Type II (from 1 to 2.5) and Type III series (from 0.5 to 1) is indicative of changes occurring during thermal kerogen cracking. In both evolution series, we consider kerogen samples at different levels of maturity, from diagenesis to catagenesis. The natural thermal evolution of the samples in the diagenesis zone did not proceed very far; mainly heteroatomic bonds were broken and labile carbonyl and carboxyl groups removed ( TISSOT and WELTE, 1984). Therefore. these samples still have some functional groups as well as hydrocarbon chains and small aromatic moieties. During pyrolysis, the majority of the chains released will carry a functional group. Consequently, they will be included in the heteroatomic fraction of the pyrolysate, and few HCSI will be released in the pyrolysate as such. The thermal evolution of the samples in the catagenesis zone is more advanced, and almost all the functional groups have been eliminated. Consequently, during pyrolysis the
We applied the preparative pyrolysis method to elucidate the source-rock type of samples from the “Greater Alwyn” area, North Sea. The potential source-rocks in this area are located in the shales of the Draupne, Heather, and Dunlin Formations and in the coals of the Brent Formation ( OUDIN, 1976; THOMAS et al., 1985; FIELD, 1985). As is obvious from Table 2 and Fig. 3, the samples studied are rather mature, with T,,,, values around 445°C. On a Van Krevelen diagram, they fall well into the catagenesis zone (H/C less than 0.85). Therefore, at this maturity level, it is rather difficult to assess organic matter types with only these bulk geochemical parameters. This illustrates the deficiencies of elementally based kerogen typing when dealing with mature samples or with provinces where terrestrial input is important ( SNOWDONand POWELL, 1982; LARTER and HORSL~ELD, 1989 ). It is clear that in such cases a multiparameter characterization approach as proposed by LARTER (“Integrated kerogen typing,” 1985 ) is necessary. The combination of routine geochemical analysis with more sophisticated and time consuming techniques (preparative pyrolysis, PYR-GC, PYR-GC-MS) applied on a limited number of samples properly selected is a very powerful tool for source-rock characterization in sedimentary basins. When available, the combination of geochemical data with optical petrology results gives even better information ( SENFTLE et al., 1987; HORSFIELD et al., 1988; ESPITALIE et al., 1989). A comprehensive characterization of organic matter goes beyond the “type” concept: it gives information on the environment of deposition of the source-rock, on the structure of the kerogen itself, on the hydrocarbons it can produce (quality and quantity), and it should also give some indi-
2919
Determining source-rock types and oil/source-rock correlations cations of the ability of the potential source-rock to expel hydrocarbons, which is not included in any of the presently applied kerogen classifications. For the application presented herein, optical data were not available. The kerogens we selected are from source-rock cores. To assess kerogen type, we compared the overall pyrogram composition, Q values, and pyrolysis yields with results obtained from the reference series. The pyrograms of two shale samples from the Draupne Formation (Fig. 8) show composition of C&i., hydrocarbons and Q values characteristic of Type II organic matter similar to the Paris Basin series. Two types of pyrograms were obtained from the shales of the Heather Formation (Fig. 9). The pyrogram of sample 7 1276 is similar to those from the Draupne Formation, being characteristic of Type II organic matter. The pyrograms of samples 7 1266 and 7 1267 show an enrichment in light aromatics ( C&i4 HCA) relative to light saturated plus unsaturated hydrocarbons ( C6-Ci4 HCSI). This is uncharacteristic of Type II organic matter. In addition, it is not typical of Type 111pyrograms, as illustrated by the Mahakam Delta series (Fig. 7), even though Q values for these two samples ( 1.00 and 1.08, respectively) are in the same range. To explain these observations, we suggest the presence of either an altered Type II or a “mixed Type II/III” (by mixed we imply a mixing of terrestrial and marine organic matter). Figure 10 shows the pyrograms of two Paris Basin samples (Type II kerogen) which are at the same maturity level but have different Hydrogen Index (HI) values (74 1 mg/gTOC and 574 mg/gTGC, respectively). The reduced petroleum
KEROGEN PYROLYSIS 550°C NORTH SEA SAMPLES HEATHER FORMATION C6+ HCSIA
‘II, .
NORTH SEA DRAUPNE
Tmax
FORMATION
I
,i
. 10
-
CARBONNUMBEROF
445’
C
I I
I
10
25
CARBON
NUMBER
OF ” ALKANES
7’276 !Q4.97 +
9. Pyrograms of the Heather Formation samples.
SAMPLES
Tmax 446’ C
-
C
7 1267
FIG.
CS+ HCSIA
‘ill
442’
I
-
KEROGEN PYROLYSIS 550°C
Tmax
25 nALKANES
b
FIG. 8. Pyrograms of the Draupne Formation samples.
potential has been ascribed to alteration of the organic matter either by bacteria or by oxidation (B. DURAND, pers. commun., 1988). It is clear from Figure 10 that even for an altered Type II kerogen, the composition of the C6-C14 hydrocarbon fraction remains characteristic of the Type II series. Therefore, we believe that for samples 7 1266 and 7 1267 from the Heather Formation the difference in organic matter composition reflected by the different pyrograms can be ascribed to a mixing of terrestrial and marine organic matter input. For the Brent samples, the pyrograms shows no ambiguity (Fig. 11). The composition of the C&i4 hydrocarbons, with the predominance of the light aromatics, is typical of Type III organic matter. To stress the advantages of our new technique as compared to the previous preparative pyrolysis technique (C,,,), in Fig. 12 we present results obtained on the samples from the Brent formation with the latter technique. Whereas samples 7 1275 and 70936 yield pyrograms typical of Type- III organic matter, sample 148 12 is different and could have been wrongly interpreted as representative of Type II organic matter. Therefore, it is clear that the complementary information we got in the C&i4 hydrocarbon fraction is very important for assessing organic matter type. This has also been shown in previous work using PYR-GC techniques ( LARTER and DOUGLAS, 1982).
E. L.&argueand F. Behar
2980
KEROGENPYROLYSIS 550°C PARIS BASIN SAMPLES C6+ HCSIA
50093
106961 -..-. _
* 10
-
Totwn~
Hk574mQ/fi 1
IIIIIII
L-.--
.
CARBON
25 NUMBER
OF nALKANES
Draupne Formation, as reported by these authors. The quality of the shales from this formation is believed to be due to good preservation of organic matter in an anaerobic environment {THOMASet al., 1985; CORNFORDet a1., 1986). The inhomogeneity of the Heather Formation illustrated by the pyrolysis results has been observed by many others before (RELD, 1985; Hut et al., 1985; SCHOU et al., 1985). This has been ascribed to conditions within the water column varying from oxidizing to mildly reducing during deposition (THOMAS et al., 1985), together with varying input of terrestrial organic matter (HUG et al., 1985), as suggested by the pyrolysis results. The Brent formation containing terrestrial organic matter shows a lower petroleum potential. However, due to the large amount of organic matter, its contribution to the hydrocarbons produced in this area is probably not negligible. Within the Dunlin Formation, organic matter quality is related to the environment of deposition. The shales of the Drake Group (Upper Dunlin) are similar to Draupne sourcerocks and are good potential oil sources. In contrast, the shales from the Cook Group underneath have a lower petroleum potential due to the p~ominance of terrestrial input. The problem of oil/source-rock correlations in this area has been investigated by applying the pyrolysis technique to asphaltenes from North Sea crude oils (for a review of asphaltene pyrolysis as a tool for oil/source-rock correlations,
Y
FIG. IO.Comparison of the gyrograms of “normal” Type II (upper) and “altered” Type II kerogens flower) from the Paris Basin.
KEROGEN PYROLYSIS 550°C NORTH SEA SAMPLES
Finally, the pyrograms of samples from the Dunlin formation (Fig. 13), using criteria established earlier, are ascribed to Type II organic matter (samples 70953 and 70955 from the Drake Group, Upper Dunlin) whereas sample 65145 (Cook Group) belongs to Type III organic matter. It is interesting to note that samples 148 12 and 70953-which have the same T,,,, values (447’C), similar HI values ( 134 and 131 mg/gTOC), and similar H/C and O/C atomic ratioswere clearly separated into Type III and Type II kerogens on the basis of very different pyrograms and Q values. We indicated in the first part ofthis paper that the amount of saturated and unsaturated hydrocarbons produced by kerogen pyrolysis was greater for Type II than for Type III sourcerocks. Pyrolysis yields from the North Sea samples, for each class of hydrocarbons (Table 4), can indicate organic matter types. Kerogen typing inferred from comparison with pyrograms of the reference series is in good agreement with observed pyrolysis yields. For Type II samples (Draupne Formation, samples 71276 from Heather and 70953 and 70955 from Dunlin), yields of saturated + unsaturated hydrocarbons are high, varying between 30 and 50 mg/gTOC. In contrast, when terrestrial input is important (Type III samples and “mixed” Type II/III samples), these yields are much lower, around 10 mg/gTOC. The results obtained on these samples are in good agreement with the data on source-rock analysis published for this sector of the North Sea ( BARNARDand COOPER,198 1; GOFF, 1983; THOMAS et al., 1985; FIELD, 1985). In particular, we confirm the good quality of the source-rocks from the
BRENT FORMATION Cg, HCSIA ~~
__._.~._.
~---._
r-~
14*‘2.!
CM.14
.
ill /:
p+m Xykn.
AuJr.:..
1msx 452-c
FIG. 11. Pyrograms of the Brent Formation samples (G+).
2981
Determining source-rock types and oil/source-rock correlations
KEROGEN PYROLYSIS 550°C
KEROGEN PYROLYSIS 550°C
NORTH SEA SAMPLES
NORTH SEA SAMPLES
DUNLIN FORMATION
BRENT FORMATION
c6+ HCSIA
C13+ HCSI I
I/ Tmax 447%
Tmax
447O
C
I!
li
i
‘
)
7og53/ kM.03
1
.
65’46
10
25
Tmax 453OC
t 70936 l
I II
Tmax
452V
Il...... I
-
i FIG.
CARBON
712751 .
‘
15
25
CAFiEiON
NUMBER
OF nALKANES
-+
12. Pyrograms of the Brent Formation samples (C,,,).
et al., 1982; BANDURSKY, 1982; BEHAR and PELET, 1985; TELNAESet al., 1985; JONES et al., 1988). The whole-oil gas chromatograms of two types of North Sea crude oils are presented in Fig. 14 together with the pyrograms issued from pyrolysis of their respective asphaltenes. Both oils come from the Viking Graben. Type A crude oil is typical of those encountered in the North Sea (OUDIN, 1976; THOMASet al., 1985; SCHOLIet al., 1985). Its gas chromatogram shows a regularly decreasing distribution of saturated hydrocarbons with few Czs+ compounds. Type B crude oil has a similar Pr/Ph ratio, but the saturated hydrocarbon distribution is enhanced in the heavy normal alkanes ( CzsCj,) region. This difference cannot be due to maturity since both oils have similar alkane/isoprenoid ratios and similar biomarkers distribution (J. L. OUDIN, pers. commun., 1988). Pyrolysates from asphaltene pyrolysis are almost identical for both oils, and exhibit hydrocarbon distribution similar to type A crude oil. The asphaltene pyrograms are also similar to Type II kerogen pyrograms, which confirms the genetic relationship between kerogens and asphaltenes, and demonstrates the usefulness of asphaltene pyrolysis for correlation purposes. The pyrolysis results also indicate a common ma-
see AREFEV
!
l
FIG.
NUWEROF
nALKAM3
I Q=0.74
*
13. Pyrograms of the Dunlin Formation samples.
rine origin for both crudes which is contradicted by the different saturated hydrocarbons distributions. Therefore we believe that type A oil originates from one of the marine source-rocks of the sector, presumably the Draupne Formation, due to its thickness in this area. On the other hand,
SANPLE
FOWTION
THAX
“I
HCSI
NUMBER
‘6-‘14 (‘C)
Im3/ga
HCSI
C
6+
(mgfgc)
lmg/9cl
“0.
HCI
'6-'lA mg/gcl
Q
'6+ mg/gc)
70891
DPA”PNE
446
367
45
80
25
50
1.79
70895
DRAUPNE
446
319
51
93
26
71
1.96
71266
HEATHER
442
110
11
16
11
24
1.00
71261
HEATHER
440
115
12
23
11
19
1.08
71276
HEATHER
445
217
29
48
15
35
1.97
14812
BRENT
447
134
9
18
8
16
1.14
70936
BRENT
453
202
9
15
13
26
0.70
BRENT
452
248
11
18
10
19
1.10
70953
DUNLIN
447
131
32
61
16
40
2.00
70955
DUNLJN
441
264
40
73
23
51
1.75
65146
DUNIJN
434
145
6
10
8
12
0.75
71275
*
a-
HCSI
(C6X14)/
“CA
(C
6
-c
14'
E. Lafargue and F. Behar
2982
OIL / SOURCE-F?OCK CORRELATION NORTH SEA CRUDE OILS TYPE A
TYPE B crude
crude
oil
L
10
oil
.
25
ASPHALTENES PYROLYSIS C(j+ HCSIA
10
25 CAFISCN M_h5ER
10
25 b
OF nAJ_KANES
FIG. 14. Oil/ source-rock correlation for two North Sea crude oils.
we interpret type B oil as being mixed marine/ terrestrial crude oil. This would explain the importance of the heavy end in the whole-oil chromatogram and its low level in the asphaltene pyrograms, because it is known that low amounts of asphaltenes are released from terrestrial organic matter ( TISSOT and WELTE, 1984). The juxtaposition of mature terrestrial organic matter (Brent coals) with formations containing marine organic matter makes such a mixing in this part of the North Sea possible. Another argument in favor of the contribution of coals in the formation of type B oil is found in the studies of THoMPsoN et al. ( 1985) and HORSFIELD et al. ( 1988 ), who demonstrated the production of waxy crudes from coals. CONCLUSIONS
The development of analytical pyrolysis provides the explorer with information on bulk-chemical and molecular structure of organic matter. This is of paramount importance for precisely assessing kerogen types in sedimentary basins. Through kerogen and asphaltene pyrolysis, it also proves to be very useful for oil/oil and oil/source-rock correlations. Preparative pyrolysis allows the determination of absolute quantities ofeach hydrocarbon fraction released in the pyrolysate. This is important for understanding kerogen structure and the thermal cracking of kerogen. It is also very useful for estimating source-rock petroleum potential. In doing so, the technique is complementary to PYR-GC, which gives the GOR of the pyrolysate. Acknowledgmmts-Special thanks are due to C. Comford, B. Durand, B. Horsfield, S. R. Larter, R. Pelet, and M. Vandenbroucke for the
critical readings of this manuscript and for their helpful suggestions. Technical assistance by C. Leblond is gratefully acknowledged. Editorial handling: J. RullkGtter
REFERENCES AREFEVO., MAKUSHINA., and PETROVA. ( 1982) Asphaltenes as indicators of the geochemical history of oil. Infl. Geol. Rev. 24, 723-738.
BANDURSKYE. ( 1982) Structural similarities between oil generating kerogens and petroleum asphaltenes. Energy Sources 6,47-66. BARNARDP. C. and COOPERB. S. ( 198 1) Oils and source-rocks of the North Sea area. In Petroleum Geology of the Conrinental Shelf of North West Europe. (eds. L. V. ILLING and G. D. HOBSON), pp. 169-175. Inst. of Petroleum, London. BEHARF. and PELETR. ( 1985) Pyrolysis-gas chromatography applied to organic geochemistry. Structural similarities between kerogens and asphaltenes from related source-rocks extracts and oils. J. Anal. Appl. Pyrolysis 8, 173- 187.
BEHARF. and VANDENBROUCKE M. ( 1988) Characterisation and quantification of saturates trapped inside kerogen: implications for pyrolysate composition. In Advances in Organic Geochemistry 1987 feds. L. MATTAVELLIand L. NOVELLI); Org. Geochem. 13, pp. 927-938. Pergamon Press, Oxford. COMBAZA. and DE MATHAREL M. ( 1978) Organic sedimentation and genesis of petroleum in Mahakam Delta, Borneo. Bull. Amer. Assoc. Petrol. Geol. 62, 1684-1695. CORNFORD C., NEEDHAM C. E. J., and DE WALQUEL. ( 1986) Geochemical habitat of North Sea oils and gases. In Habitat of Hydrocarbons on the Norwegian Continental Shelf (eds. B. M. THOMASet al.), pp. 39-54. Graham and Trotman, London. CURRY D. J. and SIMPLERT. K. ( 1988) Isoprenoid constituents in kerogens as a function of depositional environment and catagenesis. In Advances in Organic Geochemistry 1987 (eds. L. MAI-~AVELLI and L. NOVELLI); Org. Geochem. 13. pp. 995-1001. Pergamon Press, Oxford. DURAND B. and MONINJ. C. ( 1980) Elemental analysis of kerogens. In Kerogen (ed. B. DURAND), pp. 113-142. Technip, Paris.
Determining source-rock types and oil/source-rock correlations DURANDB. and NICAI~EG. ( 1980) Procedures for kerogen isolation. In Kerogen (ed. B. DURAND), pp. 35-53. Technip, Paris. ESPITALIEJ., DURAND B., ROUSSELJ. C., and SOURON C. ( 1973) Etude de la mat&e organique insoluble des argiles du Toarcien du Bassin de Paris. Rev. Inst. Fr. Petrol. 28, 37-66. ESPITALIEJ., LAFARGUE E., and EGGEN S. ( 1989) Differences between petroleum potential of marine and terrestrial organic matter in the Jurassic formations of the North Sea. Proc. Slst E.A.E.G. Meeting. Berlin, June I989 (in press). FIELD J. D. (1985) Organic geochemistry in exploration of the Northern North Sea. In Petroleum Geochemistry in Exploration of the Norwegian Shelf (eds. B. M. THOMAS et al.), pp. 39-57. Graham and Trotman, London. GlRAUD A. ( 1970) Application of pyrolysis and pyrolysis-gas chromatography to the geochemical characterisation of kerogen in sedimentary rocks. Bull. Amer. Assoc. Petrol. Geol. 54, 439-455. GOFF J. C. ( 1983) Hydrocarbon generation and migration from Jurassic source-m&s in the East Shetland Basin and in Viking Graben of the Northern North Sea. J. Geol. Sot. London 140,445-474. G~OSSENS H., DE LEEUW J. W., SCHENCK P. A., and BRASSELL S. C. ( 1984) Tocopherols as likely precursors of pristane in ancient sediments. Nature 312,440-442. G~~SSENS H., DE LANGE F., DE LEEUW J. W., and SCHENCK P. A. ( 1988) The Pristane Formation Index, a molecular maturity parameter. Confirmation in samples from the Paris Basin. Geochim. Cosmochim. Acta 52,2439-2444. VAN GRAAS G., DE LEEUW J. W., and SCHENCK P. A. ( I98 1) Kerogens of Toarcian shales of the Paris Basin: a study of its maturation by flash pyrolysis techniques. Geochim. Cosmochim. Acta 45,24652474. HORSFIELDB. ( 1984) Pyrolysis studies and petroleum exploration. In Advances in Petroleum Geochemistry (eds. J. Broods and D. WELTE), Vol. I, pp. 247-298. Academic Press, New York. HOR~FIELDB., DEMBKXI H., and Ho T. T. Y. ( 1983) Some potential applications of pyrolysis to basin studies. J. Geol. Sot., London 140,431-443. HORSFIELDB., YORDY K. L., and CRELLINGJ. C. (1988) Determining the petroleum-generating potential of coals using organic geochemistry and organic petrology. In Advances in Organic Gee chemistry I987 (eds. L. MATTAVELLIand L. NOVELLI);Org. Geothem. f3, pp. 12 1- 129. Pergamon Press, Oxford. HUC A., IRWINH., and SCHOELLM. ( 1985) Organic matter quality changes in an Upper Jurassic shale sequence from the Viking Graben. In Petroleum Geochemistry in Exploration ofthe Norwegian Shel/(eds. B. M. THOMASet al.), pp. 179-185. Graham and Trotman, London. JONESD., DOUGLASA. G., and CONNANJ. ( 1988) Hydrous pyrolysis of asphaltenes from biodegraded oils. In Advances in Organic Geochemistry 1987 (eds. L. MATTAVELLIand L. NOVELLI);Org. Geothem. 13, pp. 981-993. Pergamon Press, Oxford. JOHNSONA. and EYSSAUTIERM. ( 1987) Alwyn North field and its regional geological context. In Petroleum Geology ofNorth West Europe (eds. J. BROOKSand K. GLENNIE),pp. 963-977. Graham and Trotman, London. LARTERS. R. ( 1984) Application of analytical pyrolysis techniques to kerogen characterisation and fossil fuel exploration, exploitation. In Analytical Pyrolysis (ed. J. VCKIRHEES),pp. 2 12-275. Butterworths, London. LARTERS. R. ( 1985) Integrated kerogen typing in the recognition and quantitative assessment of petroleum source-rocks. In Petro leum Geochemistry in Exploration of the Norwegian Shelf (eds. B. M. THOMAS et al.), pp. 269-286. Graham and Trotman, London. LARTERS. R. and DOUGLASA. G. (1978) Low molecular weight aromatic hydrocarbons in coal maceral pyrolysates as indicators
2983
of diagenesis and organic matter type. In Environmental Biogeochemistry and Geomicrobiology (ed. W. KRUMBEIN),Vol. 1, pp. 373-386. Univ. Michigan Press, Ann Arbor. LARTER S. R. and DOUGLASA. G. ( 1980) A pyrolysis-gas chromatographic method for kerogen typing. In Advances in Organic Geochemistry 1979 (eds. A. G. DOUGLASand J. R. MAXWELL). pp. 579-584. Pergamon Press, Oxford. LARTER S. R. and DOUGLASA. G. ( 1982) Pyrolysis methods in organic geochemistry: an overview. J. Anal. Appl. Pyrol. 4, 1-19. LARTERS. R. and HORSFIELDB. ( 1989) Determination of structural components of kerogen using analytical pyrolysis methods. In Organic Geochemistry (eds. M. ENGEL and S. MACKO), Chap. 16 (in press). LARTERS. R. and SENTFLEJ. ( 1985) Improved kerogen typing for petroleum source-rock analysis. Nature 318, 277-280. OUDINJ. L. ( 1976) Etude geochimique du bassin de la Mer du Nord Bull. Centre Rech. Pau 10,339-358. ROBINSONW. E. ( 1969) Kerogen of the Green River Formation. In Organic Geochemistry (eds. G. EGLI~ON and M. T. J. MUWHY ). pp. 619-637. Springer-Verlag, Berlin. ROMOVACEKJ. and KUBATJ. ( 1968) Characterisation of coal substances by PYR-GC. Anal. Chem. 40, 1 I 19-I 126. SCHENCKP. A., DE LEEUWJ., VANGRAASJ. W., HAVERKAMPJ.. and BOUMANM. ( 198 1) Analysis of recent spores and pollen and of thermally-altered Sporopollenin by flash pyrolysis-mass spectrometry and flash pyrolysis-gas chromatography-mass spectrometry. In Organic Maturation Studies and Fossil Fuel Exploration (ed. J. BROOKS), DD. 225-237. Academic Press. New York. SC~OU L., EGGEN’S., and SCHOELLM. ( 1985 ) Oil-oil and oil-source rock correlation, Northern North Sea. In Petroleum Geochemrsrry in Exploration of the Norwegian Shelf (eds. B. M. THOMASet al.), pp. 101-l 17. Graham and Trotman, London. SENFTLEJ. T., BROWN,J. B., and LARTERS. R. ( 1987) Refinement of organic petrographic methods for kerogen characterisation. Ml. J. Coal Geol. 7, 105-I 17. SNOWDONL. R. and POWELLJ. G. ( 1982) Immature oil and condensate-modification of hydrocarbon generation model for terrestrial organic matter. Bull. Amer. Assoc. Petrol. Geol. 66, 775-788. TELNAESN.. SPEERSG. C., STEENA., and DOUGLASA. G. ( 1985 ) Hydrous pyrolysis of asphaltenes. In Petroleum Geochemistry rn Exploration of the Norwegian Shelf(eds. B. M. THOMASet al.). pp. 287-292. Graham and Trotman, London. THOMASB. M., MOLLER-PEDERSEN P., WHITAKERM. F.. and SHAW N. D. ( 1985) Organic facies and hydrocarbon distributions in the Norwegian North Sea. In Petroleum Geochemistry in Exploration of the Norwegian Shelf (eds. B. M. THOMASet al.), pp. 3-26. Graham and Trotman, London. THOMPSONS., COOPERB. S., MORLEYR. J.. and BARNARDP. C. ( 1985 ) Oil-generating coals. In Petroleum Geochemistr), rn E.xp/oration of the Norwegian Shelf(eds. B. M. THOMASet al.). pp. 5% 75. Graham and Trotman, London. TISSOTB. and WELTE D. ( 1984) Petroleum Formation and Occurrence, 2nd ed. Springer-Verlag, Berlin. TISSOTB., DURANDB., ESPITALIEJ., and COMBAZA. ( 1974) lnfluence of nature and diagenesis of organic matter in formation of petroleum. Bull. Amer. Assoc. Petrol. Geol. 58, 499-506. TISSOTB., DEROOG., and HOODSA. ( 1978) Geochemical study of the Uinta Basin: formation of petroleum from the Green River formation. Geochim. Cosmochim. Acta 42, 1469-1485. VANDENBROUCKE M., BEHARF., and ESPITALIEJ. ( 1988) Characterisation of sedimentary organic matter by preparative pyrolysis: comparison with Rock-Eva1 and PY-GC techniques. Energy and Fuel 2,252-258. YOUNGD. and YEN T. ( 1977) The nature of straight chain aliphatic structures in Green River kerogens. Geochim. Cosmochim. Acta 41, 141 I-1417.