Energy & Fuels 1988,2, 74-81
74
Programmed Pyrolysis-Gas Chromatography of Artificially Matured Green River Kerogen? B. J. Huizinga,*J Z. A. Aizenshtat, and K. E. Peters Chevron Oil Field Research Company, P.O.Box 446,La Habra, California 90631 Received May 19,1987. Revised Manuscript Received September 18,1987
Compositional changes in Green River kerogen during laboratory-simulated maturation by hydrous pyrolysis were studied by using programmed pyrolysis-gas chromatography (PP-GC) and Rock-Eval I1 pyrolysis. Hydrous pyrolysis of Green River marl up to 330 OC results in the thermal degradation of kerogen to form bitumen that is enriched in asphaltenes and resins. During this main stage of bitumen generation, the hydrogen index of the kerogen decreases slightly, n-alkane precursors are concentrated in the kerogen, and acyclic isoprenoids (especially prist-1-ene) are preferentially lost. A t hydrous-pyrolysis temperatures above 330 OC, the hydrogen index and concentration of n-alkane precursors within the kerogen decrease. Several PP-GC parameters from the kerogens, such as the prist-1-ene/ (n-heptadecane + n-heptadec-1-ene) ratio, the phytane/ (n-octadecane + n-octadec-1-ene) ratio, and the odd-even predominance (OEP) of n-alkanes or n-alk-1-enes, show systematic changes with increasing maturity. However, the 9"- of the Green River kerogen does not change significantly during thermal maturation.
Introduction Various methods of open-system pyrolysis, coupled with analysis by gas chromatography, have been used to investigate the structure and composition of kerogen. Many of the previous studies have focused on differences in kerogen composition that result from variations in source.l-ll Relatively few studies have reported on changes in kerogen composition caused by maturation.6J0J2 In the present investigation, we focused specifically on maturation-dependent changes in kerogen composition. Green River marl was artificially matured by using hydrous pyrolysis. This closed-system pyrolytic technique was utilized to simulate the catagenetic evolution of kerogen, as well as the attendant generation and expulsion of petr01eum.l~'~In our study, a single source rock was homogenized to produce a constant mineralogical and organic composition. Thus,our hydrous-pyrolysisexperiments had an advantage over natural basin studies in that they enabled us to examine changes caused solely by thermal maturation. Following hydrous pyrolysis, the residual kerogens were isolated and studied, by using programmed pyrolysis-gas chromatography (PP-GC) and Rock-Eval I1 pyrolysis, to elucidate any structural and compositional modifications that were induced by artificial maturation. It was important to work with isolated kerogens because certain minerals may significantly alter the composition of the kerogen Experimental Section An immature sample of Green River marl from the Red Point Mine in the Piceance Basin, Colorado, was obtained for laboratory-simulated maturation using hydrous pyrolysis. The sample contains type-I kerogen (H/C = 1.59)and displays a 10.6% total organic carbon content. About 400 g of preaieved, homogenized rock chips, ranging from 0.5 to 1.0 cm in size, were loaded into a 1-Lstainless-steel reactor vessel (Parr Instruments, Inc.). Distilled water was added to yield a rock to water ratio of 1.511 by weight. Prior to pyrolysis, the
'Presented at the Symposium on Pyrolysis in Petroleum Exploration Geochemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-6, 1987. *Present address: Chevron Overseas Petroleum Inc., 6001 Bollinger Canyon Road, San Ramon, CA 94583-0946.
0887-0624/88/2502-0074$01.50/0
vessel was checked for leaks under 1500 psi of helium and purged three times with helium to remove air. The final helium gas pressure was reduced to 100 psi. For hydrous-pyrolysis experimenta, the sealed vessels were heated for 72 h at selected temperatures in the 290-365 O C range. The temperatures were monitored to f l "C. Following hydrous pyrolysis, collection and quantification of the organic phases (i.e., expelled oil, generated bitumen, residual kerogen) were conducted by procedures that were similar to published methods.16 Expelled oil consisted of liquid pyrolyzate floating on the water or residing as a film on the surface of rock chips. The rock chips were recovered and ground to a fine powder. The generated bitumen :-:as removed from the sample by a
(1)Maters,W. L.; van de Meent, D.; Schuyl, P. J. W.; de Leeuw, J. W.; Schenck, P. A.; Meuzelaar, H. L. C. In Analytical Pyrolysis; Jones, C. E. R., Cramers, C. A., Eds.; Elsevier: New York, 1977;pp 203-216. (2)Klesment, 1. J. Anal. Appl. Pyrolysis 1980,2,63-77. (3)Larter, S.R.;Douglas, A. G.In Advances in Organic Geochemistry, 1979; Douglas, A. G.,Maxwell, J. R., Eds.; Pergamon: Oxford, England, 1980; pp 579-583. (4)Van de Meent, D.; Brown, S. C.; Philp, R. P.; Simoneit, B. R. T. Geochim. Cosmochim. Acta 1980,4.4, 999-1013. (5)Leventhal, J. S. Geochim. Cosmochim. Acta 1981,45, 883-889. (6) Burnham, A. K.; Clarkson, J. E.; Singleton, M. F.; Wong, C. M.; Crawford, R. W. Geochim. Cosmochim. Acta 1982,46,1243-1251. (7)Dembicki, H.; Horsfield, B.; Ho, T. T. Y! AAPG Bull. 1983,67, 1094-1103. (8)Larter, S.R.;Senftle, J. T. Nature (London) 1985,318,277-280. (9)Williams, P.F. V.; Douglas, A. G.Fuel 1986,64,1062-1069. (10)William, P.F. V.; Douglas, A. G.Fuel 1986,65,1728-1734. (11)Solli, H.; Leplat, P. Org. Geochem. 1986,IO, 313-329. (12)Van Graas, G.;de Leeuw, J. W.; Schenck, P. A.; Haverkamp, J. Geochim. Cosmochim. Acta 1981,45,2465-2474. (13)Lewan, M. D.; Winters, J. C.; McDonald, J. H. Science (Washington, D.C.)1979,203,897-899. (14)Winters, J. C.; Williams, J. A,; Lewan, M. D. In Aduances in Organic Geochemistry, 1981; Bjoroy, M., et al., Eds.; Wiley: Chichester, England, 1983;pp 524-533. (15)Lewan, M. D. Philos. Trans. R. SOC.London, A 1985, 315, 123-134. (16)EspitaliB, J.; Madec, M.; Tissot, B. AAPG Bull. 1980,64,59-66. (17)EspitaliB, J.; Senga Makadi, K.; Trichet, J. Org. Geochem. 1984, 6,365-382. (18)Horsfield, B.; Douglas, A. G. Geochim. Cosmochim. Acta 1980,44, 1119-1131. (19)Davis, J. B.; Stanley, J. P. J. Anal. Appl. Pyrolysis 1982,4, 227-240. (20) Huizinga, B. J.; Tannenbaum, E.; Kaplan, I. R. Geochim. Cosmochcm. Acta 1987,51, 1083-1097.
0 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 1, 1988 75
Programmed Pyrolysis-Gas Chromatography
Atomic H/C
fr\
P2
Chromatograph
Sample Inlet
c
310
5
320
0)
c
Inert Gas Splitter Valve
330
l l L k L 360
Figure 1. Simplified flow diagram of the CDS 820GS pyroanalyzer. Programmed pyrolysis-gas chromatography was used to study kerogens isolated from artificially matured Green River marl. flowthrough extraction technique using dichloromethane. The solvent-extracted rock powder was demineralized with 37.5% HC1 to remove carbonates, followed by 60% H F to remove silicate minerals. This demineralization procedure was repeated twice. The residual solid material was subsequently acidified with 37.5% HCl, washed with water three times, air-dried, and repeatedly washed with dichloromethane until freshly added solvent was colorless. The isolated kerogen concentrate was dried a t 60 "C under vacuum for 24 h. The residual kerogen obtained from each hydrous-pyrolysis experiment was analyzed by PP-GC using the CDS (Chemical Data Systems, Inc.) 820GS pyroanalyzer. A simplified flow diagram of the CDS 820GS pyroanalyzer is given in Figure 1. A weighed quantity of kerogen (in the 3-5 mg range) was placed inside a precleaned 2 mm i.d. X 15 mm quartz tube and held in place a t one end of the quartz tube (purge gas exit) with a quartz wool plug. The loaded quartz tube was introduced into the oven interface under a constant helium gas flow of 30 cm3/min. The program oven was operated under the following standard conditions: ballistic temperature increase from 50 "C to 300 "C; temperature held isothermal a t 300 "C for 6 min (to produce the P1peak); and temperature programmed a t 25 "C/min from 300 to 600 "C, followed by a 6-min isothermal heating a t a final temperature of 600 "C (to produce the P2peak). The pyroproducts volatilized from the sample were swept with helium through a sample splitter and a complex valve system, which were all contained within a valve oven maintained a t 275 "C. Twenty percent of the volatilized pyroproducts were monitored by a flame-ionization detector (FID) while the remaining 80 % were trapped (traps A, B, and D were packed with Tenax). The P2 fraction was analyzed by gas chromatography using a fused-silica capillary column (liquid phase "007" methyl silicone, 25 m X 0.25 mm i.d., 0.25 pm film thickness, Quadrex Corp.). Other conditions for gas chromatography included the following: injection temperature, 240 "C; column program, 6 min a t a constant initial temperature of 10 "C (column cooled cryogenically with C02), then 6 "C/min from 10 to 290 "C, and held for 9 min a t a final temperature of 290 "C; flame-ionization detection. External hydrocarbon standards were analyzed under the same operating conditions. Thus, hydrocarbons generated as pyroproducts from the kerogens were identified by retention time. The run time for PP-GC analysis was about 3 h/sample. The pyrograms obtained from PP-GC provided the T,, and hydrogen index (HI). The T,, represents the pyrolysis oven temperature at which the P2 peak is maximized. The HI of the kerogen, in milligrams of "hydrocarbons" per gram of TOC (total organic carbon), was calculated from
HI = 100(P2)/% TOC With PP-GC data, changes in the capacity of the residual kerogen to generate normal hydrocarbons were also monitored. These values were calculated according to mg of C7-CB n-alkanes and n-alk-1-enes/g TOC = (Carea %)(HI of kerogen)/100
370
0
30
60 90 Kerogen, mg/g Rock
120
100
300
500
700
900
0 Hydrogen Index (mg HC/g TOC)
Figure 2. Comparison of kerogen content in the Green River marl, atomic H/C ratio of the kerogen, and hydrogen index of the kerogen with increasing artificial maturation. These data, plotted versus hydrous-pyrolysis temperature, show that the major decrease in kerogen content occurs prior to the significant reduction in the petroleum-generative potential of the residual kerogen. The capacity of the residual kerogen to generate prist-1-ene, pristane, or phytane was calculated with a similar equation. The absolute quantities calculated from this method are certainly overestimated as discussed below. However, the data can be successfully used in a relative sense to trace thermal-maturation-induced changes in the capacity of the kerogen to generate certain products. Values obtained from the equation above are approximate because (1)numerous small peaks below the minimum integration limit are not included in the total peak area from gas chromatography, (2) C1-C6 hydrocarbons are measured by the pyrogram FID and thus are included in the HI but are lost prior to gas chromatography, and (3) some polar compounds measured by the pyrogram FID are possibly retained during trapping and, therefore, never reach the capillary column. Rock-Eva1 I1 analyses were also performed on the kerogens obtained from the hydrous-pyrolysis experiments. The following standard temperature program was used for the Rock-Eva1 analyses: (1)3 min isothermal at 300 "C (to produce the S1peak) and (2) temperature programmed a t 25 OC/minute from 300 to 550 "C, followed by a 1-min isothermal heating at the final temperature of 550 "C (to produce the S2peak). Each run took about 25 min.
Results and Discussion I. PP-GC Pyrogram Parameters. The hydrogen index (HI) of the isolated kerogen concentrate was calculated from PP-GC and % TOC data to monitor changes in the petroleum-generativepotential of the Green River kerogen after artificial maturation with hydrous pyrolysis. The original, unheated Green River kerogen is highly oil prone (HI = 877 mg of HC/g of TOC). Artificial maturation results in (a) only a slight decrease in the HI of the kerogen for hydrous-pyrolysis experiments up to 330 "C, (b) a substantial reduction in the HI from 330 to 360 "C, and (c) a slight decrease in the HI for experiments over 360 "C (Figure 2). These three segments of the hydrogenindex curve generally correspond to (a) the main stage of bitumen generation in the source rock, (b) the main stage of oil generation (i.e., expulsion of "floating pyrolyzate"), and (c) the zone of oil cracking, respectively (Figure 3). For Green River kerogen, thermal-evolutionary changes in the hydrogen index and atomic H/C ratio are very similar (Figure 2). Significant reduction in the HI of the kerogen does not coincide with the observed changes in kerogen content in the source rock. Most of the kerogen in the source rock is lost as a result of bitumen, oil, and gas generation during
76 Energy & Fuels, Vol.2,No.1, 1988
Huizinga et al.
Unheated
P2
290 300
P $
al
310
C
3
B
320
E
a 330 340 350 360 370 0
10
20
30 40 Product, mg/g Rock
50
60
70
Figure 3. Plot of the quantities (mg/g of rock) of generated bitumen and expelled oil versus hydrous-pyrolysis temperature. Hydrous pyrolysis of Green River marl shows that bitumen generation precedes oil expulsion. The zone of oil cracking occurs a t hydrous-pyrolysis temperatures greater than 350 "C.
lime
-
"I
Figure 5. P2 chromatogram produced from programmed pyrolysis-gas chromatography of unheated Green River kerogen.
Unheated
I f 5
2 260 280
320
300
Green River Kerogen PP-GC 0 Rock-Eva1 II
320
Miocene Type II Kerogen A Rock-Eva1 II
al
E
I
310
al
f
::I 330
360
I
340 370
"
0
360
'
5
I
1
10
"
15
mg Prist-I-enelg TOC 410
430
450
470
Tmax,OC
490
510
Figure 4. Plot of T,, of kerogens versus hydrous-pyrolysis temperature. T,, values from programmed pyrolysis-gas chromatography (PP-GC) and Rock-Eval I1 pyrolysis are different due to differences in instrumental design and operating conditions.
hydrous pyrolysis from 290 to 330 "C (Figure 2). Therefore, the major decrease in kerogen content occurs prior to significant reduction in the HI. A smaller decrease in kerogen content occurs from 330 to 350 "C, where a minimum concentration of 23 mg/g of rock is attained. For experiments at temperatures greater than 350 "C, the concentration of residual insoluble organic matter increases slightly, suggesting that pyrobitumen is being formed. T,, values, obtained from PP-GC of artificially matured Green River kerogen, show no significant change with increasing hydrous-pyrolysis temperature (Figure 4). Although the TmBx values from Rock-Eval pyrolysis are lower than those from PP-GC due to differences in instrumental design and operating conditions, the Tmaxvalues from Rock-Eval pyrolysis of Green River kerogen also show no significant change with increasing maturation (Figure 4). In contrast, hydrous pyrolysis of a type-I1 kerogen from the Miocene Monterey Formation produces a systematic increase in Rock-Eva1 T,,, with progressive thermal maturation (Figure 4). Therefore, the lack of an increase in the T,, of artificially matured Green River kerogen is not an artifact of the hydrous-pyrolysis technique. Other researchers have also observed a restricted range of variation in Tmax for highly oil-prone, aliphatic, lacustrine kerogens; thus, TmaX is not a useful maturation parameter for many type-I kerogens.21*22 (21) Espitali6, J.; Deroo, G.; Marquis, F. Reu. Inst. Fr. Pet. 1986,41,
73-89. (22) Tissot, B. P.; Pelet, R.; Ungerer, P. AAPG Bull., in press.
I
&
100
I
I
300
I
I
I
500
mg n-Alkanes + n-Alk-1-enes (C,C,)/g
I
J
700 TOC
Figure 6. Plot showing changes in the capacity of artificially matured kerogen to produce prist-1-ene and C7-C29normal hydrocarbons during programmed pyrolysis-gas chromatography. These data, plotted versus hydrous-pyrolysis temperature, reveal that isoprenoid precursors are eliminated from the kerogen at a lower maturity than are n-alkane precursors.
In summary, the bulk of the generation of bitumen from the Green River kerogen appears to occur without a major change in the residual kerogen structure. Although the kerogen content of the Green River marl decreases substantially with increasing hydrous-pyrolysis temperature up to about 330 "C, the residual kerogen shows only a slight decrease in HI and no significant change in Tma. During this stage of maturation, thermal degradation of the Green River kerogen forms bitumen largely consisting of resins and asphaltenes (K.E.P., unpublished data). With subsequent maturation, the Tmax of the residual kerogen continues to show no significant change; however, the HI of the kerogen displays a sharp decrease with increasing hydrous-pyrolysis temperature above 330 O C. Thus, the kerogen becomes progressively more refractory with increasing thermal maturation and displays reduced petroleum-generative potential. 11. Gas Chromatography of Pyroproducts. A. Normal Hydrocarbons. Gas chromatograms of the P2 fraction, obtained from PP-GC of Green River kerogen concentrates, are dominated by an extensive range of nalkane and n-alk-1-ene doublets (Figure 5 ) . Therefore, this kerogen is highly aliphatic. Although the hydrogen index and 7'- data suggest that no major structural changes take place within the residual kerogen during hydrous-pyrolysis experiments up to 330 "C (Figures 2 and 4),some compositional changes in the kerogen do occur. With increasing artificial maturation, the capacity of the residual kerogen to yield n-alkanes and n-alk-1-enes during PP-GC increases slightly up to a
Programmed Pyrolysis-Gas Chromatography
Energy & Fuels, Vol. 2, No. 1, 1988 77
Table I. Comparison of Elemental Analysis, Programmed Pyrolysis-Gas Chromatography, and Rock-Eva1 11 Pyrolysis Results for Kerogens from Artificially Matured Green River Marl PP-GC Rock-Eva1 I1 pyrolysis hydrouspyrolysis atomic Tmam PI Tmax PI temD. "C % TOC H/C P," P," "C HIb P,/(P, + P,) S," S," S', "C HIb OId S,/(S, + S,) 69.31 1.59 17.5 608 499 877 0.03 19.9 608 5.31 448 877 8 0.03 unheated 0.02 3.70 607 2.18 446 846 3 0.01 71.70 1.59 15.5 606 506 845 290 2.51 591 2.44 450 820 3 0.01 72.14 1.58 300 5.49 448 866 0.04 3.21 648 23.8 625 497 835 7 0.01 74.86 1.54 310 4.40 450 818 0.03 4.04 596 6 0.01 15.2 555 497 803 72.84 1.48 320 3.09 448 728 0.05 6.90 526 4 0.01 26.1 501 505 694 72.26 1.42 330 2.15 449 699 9.26 497 3 0.02 71.06 1.37 330' 0.09 11.8 304 3.17 446 453 1.15 28.6 287 508 427 5 0.04 67.11 340 7.26 277 1.19 3.03 445 463 5 0.03 59.78 340' 233 5.11 443 352 10.8 8 0.04 1.09 66.26 345 4.85 442 294 24.0 183 497 341 0.12 5.32 158 9 0.03 1.00 53.81 350 0.10 3.89 117 3.00 443 195 5 0.03 17.1 158 500 263 0.90 60.26 350' 5.47 444 191 10 4.01 108 0.04 0.94 56.38 355 0.21 3.25 68.7 4.67 445 126 9 0.05 21.2 81.6 503 149 0.94 54.78 360 0.18 1.68 57.4 5.98 445 92 10 0.03 18.3 83.0 500 133 0.80 62.42 365 "Units: mg of HC/g of rock. bunits: mg of HC/g of TOC. cUnits: mg of COz/g of rock. dunits: mg of COz/g of TOC.
Table 11. Normal Hydrocarbon Parameters Obtained from Programmed Pyrolysis-Gas Chromatography of Artificially Matured Green River Kerogens area of C7-CB mp" of C7-Cn, hydrousn-alkanes + n-alkanes + OEP' of pyrolysis n-alk-1-enes/ n-alk-1-enes/ c15-c19 OEP" Of c15-c19 temp, "C total peak area g of TOC n-alk-l-enes/n-alkanesb n-alkanes n-alk-1-enes 0.82 1.16 0.94 unheated 0.61 530 0.61 520 0.90 1.13 0.93 290 0.70 580 0.90 1.08 0.95 310 0.69 550 0.92 1.05 0.96 320 0.74 510 0.92 1.04 0.98 330 0.80 340 0.83 1.02 0.99 340 0.79 270 0.82 0.99 0.99 350 0.89 230 0.86 1.00 0.99 350' 0.89 130 1.4 1.00 1.00 360 0.97 130 1.4 1.00 0.98 365 n-alk-1-enesln-alkanes = CC7-Czo n-alk-l-enes/~C7-Czon-alkanes. Odd-even a mg of compounds/g of TOC = (Carea % /100)(HI). predominan~e:~~ OEP15_19= (CIS+ 6C17 + C19)/(4c16 + 4Cls).
maximum value for the kerogen recovered from the 310 "C hydrous-pyrolysis experiment (Figure 6). In contrast, the hydrogen index shows a very slight decrease up to 310 "C (Figure 2). This comparison reveals that, relative to n-alkane precursors, some organic constituents are preferentially eliminated from the kerogen during the early stages of thermal maturation. Following peak bitumen generation (Figure 3), hydrous-pyrolysis heating from 330 to 365 "C causes a sharp decrease in the potential of the residual kerogen to produce normal hydrocarbons (Figure 6). However, the capacity of the kerogen to generate other volatile pyrolysis products decreases even more rapidly, since the ratio of the summed peak area of C7-Czs normal hydrocarbons to the summed area of all PP-GC products continuously increases with increasing artificial maturation (Table II). A similar trend has been observed in naturally matured kerogens from the Toarcian Shale.lZ Therefore, relative to n-alkane precursors, other structural units continue to be preferentially eliminated or are more susceptible to carbonization reactions within the kerogen. The normal hydrocarbons formed during PP-GC of Green River kerogen show several distinct features. These characteristics, which are observed in kerogens recovered from hydrous-pyrolysis experiments conducted at 350 "C or less, are listed below. 1. The n-alkanes, produced from kerogen during PPGC, display an odd-to-even carbon number predominance (Figure 5). This observation is consistent with previous studies of Green River k e r ~ g e n . ' ~ The ~ * ~odd-to-even ~~*~
I
Y
f
1
290
P
-
310 -
300
320
-
330
-
340
-
n-Alkanea n-Alk-I-ener
380 350
I
I
OEP (ClS-ClS)
Figure 7. Plot of the odd-even predominance (OEP) of C1&9 n-alkanes and n-alk-1-enes,produced during programmed pyrolysis-gas chromatography of the residual kerogen, versus hydrous-pyrolysis temperature.
carbon number predominance in the n-alkanes diminishes rapidly with increasing artificial maturation of the kerogen, 89 shown by the odd-even predominance (OEP) parameter calculated for the C15-C19n-alkanes produced from PP-GC (23) Douglas, A. G.; Eglinton, G.; Henderson, W. In Aduances in Organic Geochemistry, 1966; Hobson, G. D., Speers, G. C., Eds.; Pergamon: Oxford, England, 1970; pp 369-388. (24) Larter, S. R.; Solli, H.; Douglas, A. G. J . Chromatogr. 1978,167, 421-431.
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78 Energy & Fuels, Vol. 2, No, 1, 1988 Unheated[
Huizinga et al.
290
1.o
0.5 e
Floating Pyrolyzete
,
350
,
I
I
+ I
2.0 350”
I
z
3611 370
i 7
I 1.0
@ 1.1
E
1.2
1.3
1.4
1.5
365” I
1.5
1.6
OEP (c15-clO)
Figure 8. Plot of the odd-even predominance (OEP) of C15-C19 free n-alkanes in bitumen and floating pyrolyzate versus hyd-
rous-pyrolysis temperature.
of the residual kerogen (Figure 7). This trend is similar to the decrease in the OEPlklg of free n-alkanes in the bitumen or floating pyrolyzate with increasing maturation (Figure 8). 2. The n-alk-1-enes, generated from kerogen during PP-GC, show an even-to-odd carbon number predominance (Figure 5). This observation is also consistent with previous studies of Green River kerogen.114se*23.24 The odd-even predominance (OEP) was also calculated for the C15-Clg n-alk-1-enes from PP-GC of the kerogen. The immature kerogen yields an OEPlSl9 value of 0.94, reflecting the even predominance of n-alk-1-enes in this carbon number range. This parameter displays only a small increase up to about 310 OC (Figure 7). However, above 310 OC, the OEP,,,, of n-alk-1-enes increases more rapidly in residual kerogens subjected to increasingly higher hydrous-pyrolysis temperature and thus approaches a value of 1 with progressive maturation. 3. From PP-GC of the residual kerogens, the n-alk-leneln-alkane ratios measured a t individual carbon numbers show higher values for even carbon numbers than for adjacent odd carbon numbers in the C13+ range, a bimodal distribution with maxima a t Cll and C18, and a general decrease in the ratio with increasing carbon number (Figure 9). The differences between n-alk-1-eneln-alkane ratios for even versus odd carbon numbers are reduced with increasing maturation of the kerogen. Alkenes (e.g., n-alk-1-enes, prist-1-ene) are an artifact of kerogen degradation during open-system pyrolytic techniques, such as PP-GC, and are not indicative of the products generated during natural catagenesis or artificial maturation with hydrous pyrolysis.’”15 In the latter case, n-alkanes are produced under confined, high-pressure conditions. During analysis by PP-GC, the kerogens yield n-alkanes and n-alk-1-enes; however, the distribution of these normal hydrocarbons can be used to provide information on maturation-dependent changes in the kerogen structure and composition. The alkene/alkane ratios obtained by using open-system pyrolytic techniques can be altered by changing the heating rate during pyrolysis6 or by changing the flow rate of the purge gas used to volatilize pyroproducta. Care was taken in this study to operate the CDS 820GS pyroanalyzer under fixed conditions so that maturation-dependent changes, induced by hydrous pyrolysis of the kerogen, could be observed. During open-system pyrolysis of kerogen, the generation of n-alkanes with an odd-to-even carbon number predominance and n-alkenes with an even-to-odd preference has been commonly attributed to the degradation of ester-
1.0
0.5
a 10
15
20
25
10
15
20
25
Carbon Number
Figure 9. Distribution of n-alk-1-eneln-alkaneratios from programmed pyrolysis-gas chromatography of Green River kerogen that w a artificially ~ matured by hydrous pyrolysis at 290, 330,350, and 365 “C.
bound fatty acids and alcohols, respectively>p,5a During the thermal degradation of esters, the alcohol moiety yields n-alk-1-enes that are the same chain length as the parent alcohol. However, ester-bound fatty acids produce mainly n - h e s that are one carbon shorter than the parent fatty acid. Therefore, the pyrolytic generation of n-alk-1-enes with an even carbon number predominance and n-alkanes with an odd carbon number preference may reflect an original even carbon number predominance for both long-chain alcohols and fatty acids. Due to their functional-group chemistry, long-chain alcohols and fatty acids may be readily incorporated into the macromolecular structure of p r o t o k e r ~ g e n s . ~ ~ During thermal decomposition of Green River kerogen by PP-GC, n-alkanes and n-alk-1-enes may also form by C-C bond scission and primary free-radical interactions. However, this process would not impart a significant odd or even carbon number predominance to the resulting normal hydrocarbons. Although virtually all of the n-alk-1-enes and most of the n-alkanes produced by PP-GC of the residual kerogen probably result from thermal cracking of chemically bound n-alkyl chains linked to the kerogen, some unbound nalkanes may be physically adsorbed within the kerogen and released upon thermal d e g r a d a t i ~ n .Other ~ researchers have also suggested that physically entrapped compounds may occur within the Green River k e r o g e r ~ . ~ ~ rThe ~l presence of an unbound n-alkane fraction, occluded and (25) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1969,33, 1183-1194. (26) Connan, J. In Aduances in Organic Geochemistry, 1973; Tissot, B., Bienner, F., Eds.; Editions Technip: Paris, 1974; pp 73-95. (27) Douglas, A. G.;Coates, R. C.; Bowler, B. F. J.; Hall, K. In Advances In Organic Geochemistry, 1975; Campos, R., Goni, J., Eds.; ENA-DIMSA Servicio de Publicaciones: Madrid, 1977; pp 357-374. (28) Harrison, W.E. Chem. GeoE. 1978,21, 315-334. (29) Larter, S.R.;Douglas, A. G. Ceochim. Cosmochim. Acta 1980,44, 2087-2095. (30) Burlingame, A. L.;Haug, P. A.; Schnoes, H. K.; Simoneit, B. R. In Aduances in Organic Geochemistry, 1968; Schenck, P. A., Havenaar, I., Eds.; Pergamon: Oxford, England, 1969; pp 85-129. (31) Yen, T. F. In Oil Shale; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: New York, 1976; pp 129-148.
Energy & Fuels, Vol. 2, No. 1, 1988 79
Programmed Pyrolysis-Gas Chromatography
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Carbon Number 10
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15 20 25 Carbon Number
30
20
Figure 10. Correlation of the distribution of free n-alkanes in the bitumen (dashed bars) with n-alk-1-eneln-alkaneratios from programmed pyrolysis-gas chromatography of the coexisting kerogen (solid bars). Green River marl from the hydrous-pyrolysis experiment at 290 "C is used for this correlation. Free n-alkanes in the immature extract are presumed to be similar in distribution to potentially adsorbed n-alkanes in the immature kerogen. Relatively high quantities of free n-alkanes in the extract match with possible n-alk-1-eneln-alkaneratio suppression. weakly adsorbed within the interstices of the kerogen matrix, may partially explain some of our observed trends. n-Alkanes adsorbed within the immature kerogen would be expected to possess a distribution similar to that of free n-alkanes within the immature bitumen. For this reason, the distribution of free n-alkanes in the immature bitumen has been correlated with n-alk-1-eneln-alkane ratios from PP-GC of the coexisting kerogen (Figure 10). These correlations show the following points. 1. With increasing carbon number, a general increase in the n-alkane concentration in the bitumen correlates with a general decrease in n-alk-1-eneln-alkane ratios from PP-GC of the kerogen. A greater content of high versus low molecular weight n-alkanes adsorbed in the kerogen would help explain the observed decrease of n-alk-leneln-alkane ratios with increasing carbon number. 2. The free n-alkanes show an odd over even predominance that coincides with relatively lower n-alk-l-enelnalkane ratios a t odd versus even carbon numbers. The existence of a physically adsorbed n-alkane fraction within the kerogen would help suppress n-alk-1-eneln-alkane ratios primarily a t odd carbon numbers, although to different degrees depending on the specific distribution of unbound n-alkanes adsorbed within the kerogen. Also, for hydrous-pyrolysis experiments up to 310 "C, the OEP15-19of the n-alkanes, produced during PP-GC of the kerogen, decreases rapidly while only a small change occurs in the OEP,,,, of the n-alk-1-enes (Figure 7). These differences may be partially explained by the preferential loss of physically adsorbed n-alkanes from the kerogen during the early stages of thermal maturation. On the other hand, the slight net concentration of normal hydrocarbon precursors in the kerogen during the main stage of bitumen generation (Figure 6) suggests that the concentration of physically adsorbed n-alkanes must be quantitatively small, relative to kerogen-bound n-alkyl chains. Overmature kerogens (including pyrobitumen), recovered from hydrous-pyrolysis experiments over 350 "C, are significantly different from the less mature kerogens. These overmature kerogens show extremely poor potential for generating additional quantities of normal hydrocarbons (Figure 6). Also, the n-alkanes and n-alk-1-enes that are generated during PP-GC display no distinguishable odd or even predominance (Figure 7 ) and show chain lengths distinctly shorter (Figure 11) than those of less
Time
-
Figure 11. Pzchromatogram produced from programmed pyrolysis-gas chromatography of kerogen that was obtained from the hydrous-pyrolysis experiment at 365 OC. "P" marks the retention time for prist-1-ene. r
Unheated
t
e e
290
310
0
E
c"
330
e 0.6
0.8
1.0
1.2
1.4
1.6
(Total C7-C20 n-Alk-I-enes) / (Total C7-C20 n-Alkanes)
Figure 12. Total C d , n-alk-l-enes/totalC&, n-alkanes ratio, produced from programmed pyrolysis-gas chromatography of residual kerogen, versus hydrous-pyrolysis temperature. mature samples (Figure 5). Compared to PP-GC analyses of the less mature kerogens, PP-GC analyses of the overmature kerogens yield substantially larger n-alk-l-enelnalkane ratios a t all carbon numbers (e.g., compare the distribution from the 365 "C experiment to those from the 290-350 "C experiments in Figure 9). The sudden increase in the proportion of C7-Czon-alkenes to n-alkanes, produced from PP-GC of kerogens that were obtained from hydrous-pyrolysis experiments over 350 "C, is readily apparent from Figure 12. The enhanced production of nalkenes relative to n-alkanes is probably due to the decrease in hydrogen-donor concentration in the kerogen. B. Acyclic Isoprenoids. In addition to normal hydrocarbons, some acyclic isoprenoids were also identified in pyrolyzates generated by PP-GC of Green River kerogen. Prist-1-ene, which was originally identified by Larter and his c o - w ~ r k e r sis, ~a~particularly abundant product during PP-GC of the original unheated kerogen (Figure 5). This observation is consistent with previous studies (32) Larter, S. R.; Solli, H.; Douglas, A. G.; de Lange, F.; de Leeuw,
J. W. Nature (London) 1979,279,405-407.
Huizinga et al.
80 Energy &Fuels, Vol. 2, No. 1, 1988
Table 111. Acyclic Isoprenoid Parameters Obtained from Programmed Pyrolysis-Gas Chromatography of Artificially Matured Green River Kerogens hydrousmg' of m g of m g of pyrolysis temp, "C unheated 290 310 320 330 340 350 350' 360 365
prist-1-ene/g of TOC 18 9.6 3.8 1.8
1.4 0.71 0.40 0.31
b b
pristanelg of TOC
phytane/g of
1.2 0.90
2.1 1.9
b b b b b b b b
phytane/
prist-1-ene/
0.92 0.82 0.45 0.36 0.28
0.74 0.39 0.17 0.10 0.069 0.052 0.029 0.026
0.095 0.084 0.050 0.055 0.042 0.035 0.029 0.026
phytane 8.6 5.1 3.5 2.0 1.7 1.6 1.1 1.1
b b
b b
b b
TOC
prist-1-ene/
(n-C1,
1.1
+ n-cl,.,)
(n-C,a + n-cla.1)
'mg of compound/g of TOC = (area %/100)(HI). *Trace. CDuplicaterun.
of Green River kerogen.4~6J1*24~32 Much smaller amounts of phytane and pristane are also generated (Table 111). The acyclic isoprenoids produced during pyrolysis-gas chromatography of isolated kerogen may originate from different sources. Prist-1-ene may be derived from ester-bound phytanic acid%or kerogen-bound tocopherols.34 In contrast, most of the phytane, as well as the pristane, may originate from the release of physically adsorbed alkanes contained within the kerogen structure.1° Acyclic isoprenoids are preferentially lost from kerogen during the early stages of thermal maturation. For example, the capacity of the kerogen to generate prist-1-ene during PP-GC rapidly declines with increasing hydrouspyrolysis temperature up to 320 OC (Figure 6). In contrast, n-alkane precursors increase slightly within the kerogen during this early stage of artificial maturation. Other researchers have also observed a strong reduction in the prist-1-ene-generating capacity of kerogen with increasing thermal maturation.10p12 Possible explanations for this trend are listed as follows: 1. Prist-1-ene may be primarily eliminated during early maturation due to the preferential cleavage of the labile chemical bonds that link this structure to the kerogen. This is probably the most important factor. 2. The lower degree of thermal stability of branched aliphatic chains relative to straight chains may assist in the rapid decrease in the capacity of the kerogen to generate prist-1-ene. Branched chains are more susceptible to intrachain scission, resulting in lower molecular weight isoprenoids, or are more likely to be involved in carbonization reactions within the residual kerogen. In addition, the capacity of the Green River kerogen to produce phytane decreases with progressive thermal maturation (Table 111). This also appears to occur for pristane; however, the amounts of pristane produced during PP-GC are too low to quantify for all kerogens obtained from hydrous-pyrolysis experiments over 290 "C. C. Thermal-Maturation-SensitivePP-GC Parameters. Certain compound ratios from PP-GC of Green River kerogen are sensitive to changes in thermal maturation. These parameters, which follow the relative changes in kerogen pyroproducts formed during PP-GC, include the following: (a) prist-1-ene/(n-heptadecane+ n-heptadec-1-ene), (b) phytane/ (n-octadecane + n-octadec-1-ene), ( c ) the odd-even predominance of CI5-Cl9 n-alkanes (OEPIkl9 I l.O), and (d) the odd-even pre(33) Van de Meent, D.; de Leeuw, J. W.; Schenck, P. A. In Aduances in Organic Geochemistry, 1979; Douglas, A. G., Maxwell, J. R., Eds.; Pergamon: Oxford, England, 1980; pp 469-474. (34) Goossens, H.; de Leeuw, J. W.; Schenck, P. A.; Brassell, S. C . Nature (London)1984, 312,440-442. (35)Scalan, R. S.; Smith, J. E. Geochim. Cosmochim. Acta 1970,34, 611-620.
Pr181-l-ene/(n-C1,
+
n-C17:,)
Phyiand(n-C18 + n-Cl&.)
Figure 13. Prist-1-ene/(n-heptadecane+ n-heptadec-1-ene)ratio and phytane/(n-octadecane+ n-octadec-1-ene)ratio, produced from programmed pyrolysis-gas chromatography of residual kerogen, versus hydrous-pyrolysis temperature.
dominance of C15-Clg n-alk-1-enes (OEPlkIg I1.0). Because of the relatively low abundances of pristane and phytenes from PP-GC of Green River kerogen, these compounds were excluded from the calculation of parameters a and b, respectively. With increasing artificial maturation of Green River kerogen, parameters a and b systematically decrease (Figure 13) while parameters c and d approach unity (Figure 7). The source dependence of the parameters listed above may limit their applicability as thermal-maturation indicators. The ratio of an isoprenoid alkane plus alkene to the adjacent n-alkane plus n-alkene may show a strong source variation, even within the Green River Formation.6 Also, comparisons of various kerogens subjected to similar levels of catagenesis reveal significant differences in the relative abundance of prist- 1-ene to n-heptadecane plus n-heptadec-1-ene.ll Although previous research shows source-dependent differences in kerogens, our study demonstrates that the level of maturation must also be considered when pyrolysis-gas chromatograms of kerogens are compared.
Conclusions The sample of Green River marl used in this study contains highly oil-prone, aliphatic kerogen. Programmed pyrolysis-gas chromatography (PP-GC) of the unheated kerogen concentrate yields abundant n-alkanes with a distinct odd carbon number predominance and n-alk-lenes with an even carbon number preference. These distributions may result from the degradation of esterbound fatty acids and alcohols, respectively. During thermal decomposition of the kerogen by PP-GC, a portion
Energy & Fuels 1988,2, 81-88
81
drogen index of the residual kerogen decreases slightly, n-alkane precursors are concentrated in the kerogen, and certain acyclic isoprenoids (especially prist-1-ene) are preferentially lost. With increasing hydrous-pyrolysis temperature above 330 “C, the hydrogen index and concentration of n-alkane precursors within the residual kerogen progressively decrease. The T,, of the Green River kerogen does not change significantly during artificial maturation. However, several PP-GC ratios from the kerogens, such as prist-1-ene/(nheptadecane n-heptadec-1-ene) and phytane/(n-octadecane + n-octadec-1-ene), and the odd-even predominance (OEP) of n-alkanes or n-alk-1-enes,show systematic changes with increasing maturity. The source dependence of these parameters may limit their widespread use as thermal-maturation indicators. However, our study clearly shows that the level of maturation must also be considered in kerogen studies using pyrolysis-gas chromatography.
of the n-alkanes and n-alk-1-enes may also form by C-C bond cleavage and primary free-radical interactions. In addition, a small fraction of the n-alkanes may possibly originate from the release of physically adsorbed compounds in the kerogen. Prist-1-ene, which is an abundant product from PP-GC of the unheated Green River kerogen, is derived from kerogen-bound sources. However, the small amounts of phytane and pristane may possibly arise from the release of physically adsorbed alkanes contained within the kerogen structure. Green River marl was subjected to laboratory-simulated maturation with hydrous pyrolysis. The residual kerogens were subsequently isolated and studied, by using programmed pyrolysis-gas chromatography (PP-GC) and Rock-Eva1 I1 pyrolysis, to elucidate any structural and compositional modifications that were induced by artificial maturation. Hydrous pyrolysis of Green River marl up to 330 “C results in the thermal degradation of most of the kerogen to form bitumen that is enriched in asphaltenes and resins. During this main stage of bitumen generation, the hy-
+
Registry No. Pristene, 2140-82-1; pristane, 1921-70-6;phytane, 638-36-8; heptadecane, 629-78-7; 1-heptadecene,6765-39-5.
Quantitative Study of Biomarker Hydrocarbons Released from Kerogens during Hydrous Pyrolysis+ T. I. Eglinton and A. G. Douglas* Organic Geochemistry Unit, Department of Geology, The University, Newcastle upon Tyne NE1 7RU, U.K. Received J u n e 23, 1987. Revised Manuscript Received September 22, 1987
Several hydrous pyrolysis experiments using immature kerogens have been completed in which the absolute amounts of steranes and hopanes released were determined. Steranes and hopanes exhibited parallel, but displaced, release curves with maximum hopane yields being obtained a t a higher temperature than those of steranes. Under the experimental conditions used, there was a large variation in the potential of the different kerogens to liberate steranes and hopanes. Comparison of the concentration of steranes and hopanes in bitumen extracts of unheated rocks and in the pyrolyzates of the associated kerogens allowed predictions, taking into account the bitumen and kerogen concentrations in the rock, of the nature of the biomarker fingerprint of an oil that might result from maturation of the rock. Some kerogens would appear to be major, and others negligible, contributors to such a biomarker fingerprint. Support for these conclusions was provided by comparison with whole rock pyrolyzates. Kerogen-derivedbiomarkers from Monterey shale dominated the obtained from the hydrous pyrolysis of the whole rock, while in the case of the Green River shale, the bitumen appeared to be the major contributor of biomarkers. These results may have significance in oil-source rock correlation studies where immature source rocks are related to oils sourced deeper in the basin. To obtain more accurate correlations with biomarkers, the potential of the source rock kerogen to contribute biomarkers and the biomarker distributions of the source rock kerogen should be examined.
Introduction The production of sterane and hopane “biomarker” hydrocarbons from kerogens in heating experiments has been well documented (Galleg,os,l Seifert,2 H ~ e r i n g , ~ Tannenbaum et a1.,4 Philp and Gilbert,5 and van Graasa). The biomarker hydrocarbons so produced have been shown to possess stereochemistry similar to those in the
corresponding soluble (bitumen) fractions and in related crude oils (Seifert2). As a result, biomarker fingerprints from kerogen and asphaltene pyrolysis have been suggested as tools for oil-source rock correlation studies (Philp and
‘Presented at the Symposium on Pyrolysis in Petroleum Exploration Geochemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-6, 1987.
Acta 1986,50, 805-812. (5) Philp, R. P.; Gilbert, T. D. Org. Geochem. 1984, 6 , 489-501. (6) Van Graas, G. Org. Geochem. 1986, 10, 1127-1135.
0887-0624/88/2502-0081$01.50/0
(1) (2) (3) (4)
Gallegos, E. J. Anal. Chem. 1975, 47, 1524-1528. Seifert, W. K. Geochim. Cosmochim. Acta 1978,42,473-484. Hoering, T. C. Org. Geochem. 1984,5, 267-278. Tannenbaum, E.; Ruth, E.; Kaplan, I. R. Geochim. Cosmochim.
0 1988 American Chemical Society
