PyrolysisâCarbon-Isotope Method - ACS Symposium Series (ACS

Chapter 13

Pyrolysis—Carbon-Isotope Method A l t e r n a t i v e to V i t r i n i t e Reflectance as K e r o g e n M a t u r i t y Indicator

Downloaded by COLUMBIA UNIV on September 8, 2012 | http://pubs.acs.org Publication Date: November 9, 1994 | doi: 10.1021/bk-1994-0570.ch013

William M. Sackett, Zhenxi Li, and John S. Compton Department of Marine Science, University of South Florida, St. Petersburg, F L 33701

The pyrolysis-carbon isotope method for determining the thermal maturity of kerogen is based on the exhaustive pyrolysis of whole rock samples and measurement of the amounts and carbon isotope compositions of the pyrolysis- derived methane and the total organic carbon. Potential problems were 1) the pyrolysis time which has been reduced here to one hour at 700°C 2) possible exchange between methane and carbon dioxide, shown here to be unimportant and 3) the stability of methane, which is also shown to be immaterial for the standard time and temperature. Results for a suite of core samples from the Monterey Formation of the Point Arguello field offshore southern California are consistent with other maturity parameters. The pyrolysis-carbon isotope (PCI) method f o r determining the maturity of kerogen i s based on the exhaustive p y r o l y s i s of whole rock samples and measurement of the amounts and stable carbon isotope compositions of the pyrolysis-derived methane and t o t a l organic carbon i n the rock sample. To insure that o i l s t a i n i n g or migrated o i l does not i n t e r f e r e i t i s best to pre-extract the ground sample with a solvent such as dichloromethane. An amount of ground sample containing about 5 mgs. carbon i s placed i n a 9 mm o.d. Vycor tube with one end previously sealed and followed by a plug of quartz glass wool to prevent loss of a powdered sample during evacuation. The sample i s preheated at 400°C f o r 10 minutes while evacuating to drive o f f most of the water i n the sample. After cooling, the glass tube i s sealed while under vacuum with a natural gas-oxygen torch. The sample tube i s then placed i n oven at 700°C for one hour. Timing i s not started u n t i l the sample i s at 700°C. I n i t i a l l y , a whole suite of gases i s generated at lower temperatures but the only gaseous hydrocarbon that can be found after t h i s treatment i s methane. The procedures f o r combustion and manometric determination of the carbon dioxide from the methane nd kerogen are given i n (1) and (2), respectively.

0097-6156/94/0570-0206$08.00/0 © 1994 American Chemical Society

In Vitrinite Reflectance as a Maturity Parameter; Mukhopadhyay, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Pyrolysis-Carbon-Isotope Method

The model f o r interpreting thé data was presented i n (3) and i s reproduced i n (Figure 1). B a s i c a l l y i t shows that a f t e r organic matter i s deposited i n a basin and i s buried below the zone of microbial a c t i v i t y where only thermal processes become important, carbon-12 r i c h methane i s generated as carbon-12-carbon-12 bonds are broken p r e f e r e n t i a l l y to carbon-12-carbon-13 bonds. A review of carbon isotope e f f e c t s associated with the thermogenic formation of methane i s given i n (4). Referring to (Figure 1), natural processes move the CH /C mole r a t i o from the i n i t i a l to the present-day p o s i t i o n and p y r o l y s i s takes i t from there to the postpyrolysis p o s i t i o n . The same holds f o r A C. When p l o t t e d one against the other,the closer the today p o s i t i o n i s to the i n i t i a l p o s i t i o n , the more immature i s the kerogen and v i c e versa. A c t u a l l y CH /C and A C are independent parameters. One or the other or both may be used as a kerogen maturity indicator. Our past experience suggests that the CH /C mole r a t i o may be somewhat better. As t h i s measurement does not require an isotope r a t i o mass spectrometer but only a vacuum l i n e , Toepler pump, combustion tube and furnace, c a l i b r a t e d manometer, another temperature c o n t r o l l e d furnace f o r the pyrolyses and a n c i l l a r y equipment, workers may prefer t h i s s i m p l i f i e d procedure. Because of safety considerations p y r o l y s i s tubes are shielded i n s t a i n l e s s s t e e l pipes. The pressure inside the glass tubes reaches a maximum of about 4 atmospheres at 700°C. Very few experiments have blown up. A rare problem i s leakage of the seal which may be e a s i l y detected by the absence of black p y r o l y s i s products due to atmospheric oxidation. 4

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E a r l i e r work The basis f o r this maturity indicator was presented i n a paper by Chung and Sackett (5). Their data f o r 19 d i f f e r e n t coal samples and 12 d i f f e r e n t shale samples are reproduced i n (Figure 2). For t h i s diverse suite of samples a systematic trend i s seen from immature shales i n the upper r i g h t through l i g n i t e s i n the center to mature shales and anthracites at the lower l e f t . The exhaustive p y r o l y s i s procedure used by them was to heat samples at 500°C f o r up to eleven days so that comparisons could be made with e a r l i e r work by Sackett and co-workers on model compounds. The most extensive application of the PCI method to date was on a suite of samples of the Bakken shale from the W i l l i s ton Basin i n North Dakota by Conkright et a l (6) . Their procedure was to pyrolyze for f i v e days at 600°C. They showed a good c o r r e l a t i o n between CH /C and A C and other maturity parameters. 4

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Problems and questions Pyrolysis Duration. As mentioned above, Chung and Sackett (5) used eleven days at 500°C and Conkright et a l (6) used f i v e days at 600°C These lengths of time are unacceptable f o r most users who are interested i n a quick and p r a c t i c a l maturity indicator. As there i s no a p r i o r i reason not to go to higher temperatures and shorter


VITRINITE REFLECTANCE AS A MATURITY PARAMETER


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Figure 1. Model f o r the Pyrolysis-Carbon Isotope Method f o r determining kerogen maturity. MPC i s methane precursor carbon. Reproduced with permission from reference 3. Copyright 1984 Elsevier Science Ltd.



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OExtd Shales

-8'

• Whole Coals

Ο ο

ο

• Extd Shales = -60χ + 0.63 (r=0.92) Coals = -83x + 3.42 (r-0.84) Extd Shales + Coals - -74x + 2.55 (r=0.88)

•

Π 0

-

ι 0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

CH /PARENT CARBON (Mole Ratio) 4

Figure 2. The difference between carbon isotope compositions of pyrolyzed-derived methane and kerogen carbon versus the mole r a t i o of methane to kerogen carbon for extracted shales and whole coal sample f o r exhaustive pyrolyses at 500°C. Reproduced with permissionfromreference 5. Copyright 1979 Elsevier Science Ltd.


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times our most recent work uses 700°C f o r one hour. (Table I) presents a time series f o r pyrolysis of a Bakken shale sample at 700°C. The data show that the amount of methane peaks before one hour, does not change s i g n i f i c a n t l y f o r about 40 hours and then may decrease somewhat f o r longer times. As w i l l be discussed l a t e r this decrease i s apparently due to the breakdown of methane. I t appears that one hour p y r o l y s i s at 700°C i s adequate f o r exhaustive p y r o l y s i s without fear of methane decomposition.


TABLE I. Methane produced during the exhaustive p y r o l y s i s at 700°C of a Bakken shale sample as a function of time (adapted from Ref. 1) Pyrolysis Time (hours)

CH

4

(ml/100 mg sample)*

0.25

2.70 2.60 2.70 3.10 3.60 3.40 3.70 3.40 3.60 3.30 3.10 3.10

0.50 1 3 17 20 43 91 138 265

* p r e c i s i o n i s estimated at 0.3 ml/100 mg Carbon isotope exchange between carbon dioxide and methane. In the study on carbon isotope exchange between methane and amorphous carbon (1), the p o t e n t i a l exchange with carbon dioxide was minimized by placing a plug of CaO s p a t i a l l y separated from the sample i n the p y r o l y s i s tubes f o r the purpose of reacting with any C0 to produce i n e r t CaC0 . As the vapor pressure of C0 over CaC0 at 700°C i s about 2 cm (according to the CRC Handbook of Chemistry and Physics) some C0 may be present i n our pyrolysis tubes. In order to confirm that isotope exchange with carbon dioxide i s not a problem, a series of experiments was run with a 50-50 mixture of methane and carbon dioxide (S C-C0 = -3.7 and 5 C-CH 43.7 fc vs PDB ). The gas mixture was sealed i n glass and heated f o r various times and temperatures. I f isotopic exchange takes place, one would expect that both compositions would converge with the equilibrium mixture showing that carbon dioxide i s somewhat enriched i n carbon-13 r e l a t i v e to methane. According to the t h e o r e t i c a l work of Bottinga (7) f o r equilibrium at 700°C, the carbon i n carbon dioxide would be 10 o/oo heavier than the carbon i n methane. The d e f i n i t i v e r e s u l t s shown i n (Table II) indicates that at 600°C there i s l i t t l e or no change over 22 days whereas at 800°C there i s a s i g n i f i c a n t change i n methane but only a s l i g h t change i n carbon dioxide over a two day period. This work 2

3

2

3

2

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2

4


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i s continuing but strongly suggests minimal carbon isotope exchange over one hour at 700°C and that methane i s breaking down over longer time periods with a large f r a c t i o n a t i o n between i t and i t s product(s). TABLE I I . Data f o r carbon isotope exchange between methane and carbon dioxide at 600 and 800°C

Experiment 1.

Carbon Isotope Exchange at 600°C Chemical Species Time (hours) 5 C* 13

C0 CH C0 CH C0 CH C0 CH C0 CH C0 CH C0 CH Carbon isotope Exchange at 2

0

2

0

2

120

2

122

2

524


4

2.

4

3.

4

4.

4

5.

4

6.

2

4

7.

2

4

1.

co CH C0 CH C0 CH C0 CH

2

800°C 0

4

2.

2

18.5

2

43

2

44.5

4

3.

4

4.

4

-4.0 -44.1 -3.6 -43.5 -4.4 -45.4 -3.8 -44.8 -4.2 -45.2 -3.9 -45.2 -3.8 -44.6 -3.7 -43.8 -2.6 -28.4 -4.8 -23.2 -4.7 -26.6

* i n % versus PDB. Methane s t a b i l i t y at high temperatures. Several of the experiments discussed above suggest that methane may decompose at temperatures above 600°C. To determine the threshold temperature f o r t h i s to happen, pure methane was sealed i n Vycor tubes and placed i n an oven at 700, 800, 900 and 1000°C. Data are given i n (Figure 3). They suggest that at temperatures of about 700°C methane begins to show some decomposition over a period of several days. At higher temperatures decomposition precedes more rapidly. For our standard time and temperature of one hour at 700°C, l i t t l e methane decomposition should occur. Recent Application The Miocene Monterey Formation i n C a l i f o r n i a i s an organic-rich s i l i c e o u s shale that i s the source and reservoir rock of major


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Ο 700 °C

0

• 800 °C

50

100

1000°C

Δ 900 °C

τ 200

150

r 250

300

Time (hours) Figure 3. Methane s t a b i l i t y at 700, 800, 900 and, 1000°C.

-6800 -

0LU Û LU

< CO

-7800 -

-8000

Figure 4. samples.

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A C versus core depth

(feet) f o r Monterey


rock

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hydrocarbon accumulations. The organic matter i n the rocks i s generally considered to be Type II marine a l g a l material, having good o i l - b e a r i n g p o t e n t i a l . Hydrocarbons from the Monterey Formation are generally heavy with an API gravity less than 20° and are considered unusual because they appear to have been generated from kerogens of low maturity as indicated by kerogen v i t r i n i t e reflectance (Ro) values of 0.3 to 0.6 % and Rock-Eval Tmax values of 400 to 440°C (8). The PCI method was applied to a suite of samples from the Chevron B-2 well d r i l l e d from the offshore Platform Hermosa located i n the southeastern part of the Point Aguello F i e l d approximately 10 km west of Pt. Conception. The core material was made up of fractured chert, p o r c e l l a n i t e s and dolostones with from 0.3 to 3.4% t o t a l organic carbon. The CH /C r a t i o ranged randomly between 0.05 to 0.2. As shown i n (Figure 4), however, A C values show an o v e r a l l general increase with b u r i a l depth as predicted. Similar to Ro and Tmax, as discussed by Walker et a l . (9) i t appears that the PCI method, on the basis of these i n i t i a l r e s u l t s , i s not a p a r t i c u l a r l y u s e f u l maturity indicator for Monterey rocks.


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Conclusions By decreasing p y r o l y s i s time to one hour at 700°C and showing that isotopic exchange between methane and carbon dioxide and the thermal decomposition of methane do not occur for t h i s time and temperature, the pyrolysis-carbon isotope method should become an even more a t t r a c t i v e method for the determination of the maturity of kerogen i n whole rock samples. Acknowledgments P a r t i a l support f o r this study was provided by the following grants: NSF #OCE-9015580, DOE #DE-FG 05-92 ER 1430 and the donors of the Petroleum Research Fund, administered by the ACS. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

9.

Sackett, W.M.; Org. Geochem. 1993, 20, 43-45. Sackett, W.M.; Org. Geochem. 1986, 9, 63-68. Sackett, W.M.; Org. Geochem. 1984, 6, 359-363. Sackett, W.M. ; Geochim. et Cosmochim. Acta 1978, 42, 571-580. Chung, H.M.; Sackett, W.M.; Geochim. et Cosmochim. Acta 1979, 43, 1979-1988. Conkright, M.E.; Sackett, W.M.; Peters, K.E.; Org. Geochem. 1986, 10, 1113-1117. Bottinga, Y.; Geochim. et Cosmochim. Acta 1969, 33, 49-64. Peterson, N.F.; Hickey, P.J.; In Exploration f o r Heavy Crude O i l and Natural Bitumen; Studies i n Geology 23; American Association of Petroleum Geologists: Tulsa, OK, 1987. Walker, A.L.; McCulloh, T.H.; Petersen, N.F.; Stewart, R.J.; i n Petroleum Generation and Occurrence i n the Miocene Formation C a l i f o r n i a ; (Eds.; CM. Isaacs and R.E. Garrison), P a c i f i c sec. SEPM: Tulsa, OK, 1983, 185-190.

RECEIVED May 13,1994


PyrolysisâCarbon-Isotope Method - ACS Symposium Series (ACS

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