Energy & Fuels 1988,2,65-73
65
Thermogravimetric Fourier Transform Infrared Spectroscopy (TG-FTIR) of Petroleum Source Rocks. Initial Results? Jean K. Whelan* Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Peter R. Solomon, Girish V. Deshpande, and Robert M. Carangelo Advanced Fuel Research, Inc., East Hartford, Connecticut 06118 Received M a y 19, 1987. Revised Manuscript Received September 12, 1987
The first results of thermogravimetric Fourier transform infrared analysis (TG-FTIR), as applied to petroleum source rocks, are reported. Sample preparation and running time, roughly equivalent to those used for Rock Eval source rock pyrolysis analyses, are described. Initial results show that the TG-FTIR technique provides considerable additional geochemical information including the following: (a) Simultaneous T,, values for thermal evolution of a number of constituents in addition to hydrocarbons and carbon dioxide-including methane, ethylene, carbon monoxide, ammonia, hydrogen cyanide, sulfur dioxide, and COS are obtained. T,, values of several of these-including methane, ethylene, and ammonia-show progressive increases with maturation state. For one Alaskan North Slope well (Seabee), these T,, values show an excellent correlation with vitrinite reflectance. (b) Data required for calculation of material balances of organic and inorganic sedimentary carbon, nitrogen, and sulfur are obtained. (c) Data for classification of sedimentary organic matter into kerogen types I, 11,and I11 (using either plots of organic H/C versus O/C or H/C versus organic carbon ratios) as well as data for classification according to depositional environment (using thermal evolution patterns of ammonia, HCN, SO2, and COS and possibly COJ are obtained.
Introduction We report here the first results of thermogravimetric Fourier transform infrared analysis (TG-FTIR), as applied to petroleum source rock analysis. Continuous infrared detection has been reported for evolution of carbon dioxide' and carbon monoxide.2 Thermal evolution of sediment nitrogen gases3and sulfur gases4have also been reported. Some work has been carried out demonstrating the utility of other thermogravimetric methods as applied to petroleum source rocks. However, mass spectrometric rather than FTIR detection of evolved products was util i ~ e d . ~The . ~ potential advantages of the FTIR over the mass spectral detector for simultaneous measurement of the thermal evolution profiles of a number of pyrolysis gases are (1) ease of identification and quantitation of evolved compounds, particularly those of low molecular weight, (2) ability to detect the tars and oils that are condensable at room temperature, and (3) possibilities for adaptation to a smaller, cheaper instrument that could be used for routine analysis and might be useful at the well site. Experiments in the current work were aimed at testing the applicability of TG-FTIFt measurements to petroleum exploration as a prelude to development of a commercial instrument. Comparisons of TG-FTIR to the Rock Eval'3 and Chemical Data Systems9J0pyrolysis analyses were carried out. Specific work was aimed at testing the following: (i) applicability to profiling a well covering a wide maturity range from immature (vitrinite reflectance, Ro < 0.5%)to overmature (R, > 2%) in order to characterize 'Presented at the Symposium on Pyrolysis in Petroleum Exploration Geochemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-6, 1987. 0887-0624/88/2502-0065$01.50/0
the gas as well as the oil thermal maturation zones; (ii) applicability of TG-FTIR as a technique to distinguish kerogen types I, 11, and 111, as defined by Tissot," in both isolated kerogens and whole rocks of varying maturities; (iii) applicability of the technique to samples that present difficulties for other methods, such as intervals containing migrated hydrocarbons, organic lean sections, and overmature sediments. Point i is related to defining the depth range in which either sourced or reservoired petroleum hydrocarbons are present (for recent reviews, see ref 11-13). The Rock Eval (1)Leplat, P.; Paulet, J.; Melotte, M. Adu. Org. Geochem., Proc. Znt. Meet., ldth, 1981 1983,613. (2)Daly, A. R.;Peters, K. E. AAPG Bull. 1982,66, 2672. (3)Rohrback. B. G.:Peters. K. E.: Sweenev. R. E.: Kaulan. . I. R. Ado. Org. Geochem., hoc. Int. Meet., loth, 1981-1983,819. (4)Madec, M.; Espitalie, J. J. Anal. Appl. Pyrolysis 1985,8, 201. (5) Durand-Souron, C.In Kerogen; Durand, B., Ed.; Editions Technip: Paris, 1980; Chapter 5. (6) Durand-Souron, C.; Boulet, R.; Durand, B. Geochim. Cosmochim. Acta 1982,46,1193. (7)Espitalie, J.; Madec, M.; Tissot, B.; Mennig, J. J.; Le plat, P. Proceedings of the 9th Annual Offshore Technology Conference; Offshore Technolom Conference: Dallas. TX. 1977.Vol. 3.u 439. (8) Espitali6;"J.; Marquis, F.; Barsony, I. In Analytilal Pyrolysis; Voorhees, K. J., Ed.; Butterworths: Boston, MA, 1984;p 276. (9)Whelan, J. K.;Hunt, J. M.; Huc, A. Y. J.Anal. Appl. Pyrolysis 1980,2,79. (10)Huc, A. Y.;Hunt, J. M.; Whelan, J. K. J. Geochem.Explor. 1981, 15, 671. (11)Tissot, B. AAPG Bull. 1984,68, 545. (12)Tissot, B. P.;Welte, D. Petroleum Formation and Occurrence. Springer-Verlag: New York, 1978. (13)Hunt, J. M. Petroleum Geochemistry and Geology; W. H. Freeman: San Francisco, CA, 1979. (14)Magoon, L. B.; Claypool, G. E. Adu. Org. Geochem., Proc. Int. Meet., IOth,1981 1983,28. (15)Magoon, L. B.; Claypool, G. E. Org. Geochem. 1984,6, 533. 0 1988 American Chemical Society
Whelan et al.
66 Energy & Fuels, Vol. 2, No. 1, 1988 800
Exhaust
-. 9
600
\
~~
5s ? !?t
400
P,
- ...- - ..-.. Heater Coils
200
0 0
10
20
30
40
50
60
Time (mlilufes)
Heater Coils
Figure 2. Typical temperature profile.
Figure 1. Schematic diagram of TG-FTIR apparatus.
unit is now commonly used for this purpose, sometimes at the drill site. Point ii is important because different kerogen types produce petroleum under different time/ temperature conditions. In addition, certain kerogen types (such as I) are capable of producing more petroleum per given carbon content than others (e.g. type 111). Point iii relates to better definition of the gas (as compared to the oil) generation zone and to better definition of basin-wide thermal maturities where current pyrolysis methods are limited. Results concerning points i and iii are reported in this paper, including a TG-FTIR profile of the Seabee well from the Alaskan North Slope. Initial work regarding point ii is in progress and will be reported in future publications. Experimental Section Samples. The Alaskan North Slope samples, including a few samples from the Ikpikpuk well and a detailed suite from the Seabee well, have been studied extensively at Woods Hole.lg Both wells contain predominantly type I11 kerogen and cover a very wide maturation range going completely through the oil and gas generating zones (vitrinite reflectance, Ro = 0.5-2.5%). Extensive ancillary geological and geochemical data, including vitrinite reflectance; kerogen elemental analysis, thermal alteration index (TAI), and vitrinite reflectance; pyrolysis data including Rock Eval; light hydrocarbon data; and a detailed log of well contaminants, were kindly provided by Magoon and co-workers a t the U.S. Geological Survey (summarized in ref 14-16) and made available in more comprehensive form via the Geological Analysis Systems (GAS) computer program and North Slope Well File, Petroleum Information Co., Denver, CO, run on a VAX 780 computer system, Digital Equipment, Corp. An organic rich marine sediment from the Southwest Atlantic (from the Deep Sea Drilling Project, DSDP, Leg 75) was also run for c~mparison.'~The Cretaceous black shale (DSDP sample 530A-88-3,31-33, recovered from a subbottom depth of 952.3 m) was immature (maximum geothermal exposure less than 30 "C) ~
(16) Magoon, L. B.; Bird, K. J. In Alaskan North Slope Oil-Rock Correlation Study; Magoon, L. B., ClaypooI, G. E., Eds.; AAPG Studies in Geology 20; AAPG Tulsa OK, 1985; p 31. (17) Jasper, J. P.; Whelan, J. K.; Hunt, J. M. Initial Rep. Deep Sea Drill. Proj. 1984, 75, 1001. (18) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M. In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E. L. Jr., Ed.; ACS Symposium Series 205; American Chemical Society: Washington, DC, 1982, p 77. (19) Whelan, J. K.; Farrington, J. W.; Tarafa, M. E. In Advances in Organic Geochemistry 1985; Leythaeuser, D., Rullkotter, J., Eds.; Pergamon: Oxford, U.K., 1986; p 207.
and contained 9.2% organic carbon and little or no carbonate. Methodology. The apparatus,25illustrated in Figure 1, consists of a sample suspended from a balance (Mettler, Model AE160) in a gas stream within a furnace. The furnace employs a helically wound nichrome ribbon surrounding the sample. As the sample is heated, the evolving tars and gases are carried out of the furnace directly into a 5-cm-diameter gas cell (heated to 150 "C) for analysis by FTIR (IBM Instruments, Model IR-85). The apparatus is designed to carry evolved products from the furnace to the gas cell over a short path to minimize secondary reactions or condensation on cell walls. In addition, the furnace geometry, the sample size, the sweep gas, and flow rate have been chosen to insure that the condensable products form a submicron aerosol mist. This size aerosol has two advantages: (1)the particles follow the gas stream lines, thus minimizing condensation; (2) the particles produce little scattering in the mid-IR region, so the condensable products can be analyzed on-line in the FTIR cell. The FTIR can obtain spectra every 0.2 s to quantitatively determine the evolution rate and composition of most pyrolysis products. The system allows the sample to be heated in a gas flow of selected composition on a preprogrammed temperature profile a t rates between 3 "C/min and 100 "C/s up to a temperature between 20 and 1000 "C and held for a specified time. The system continuously monitors (i) the time-dependent evolution of the gases (including specific identification of the individual species such as CO, COz,H20, CH4,C2&, C2H4, C2H2,C3Hs, benzene, heavy paraffins, heavy olefins, HCN, HCl, NH3, SO2, CS2, COS, CH30H, CH3COOH, and CH3COCH3),(ii) the tar evolution rate and its infrared spectrum with identifiable bands from the functional groups, and (iii) the weight of the nonvolatile material (char plus mineral components). An analysis of C, H, N, and S in the residue a t the end of the pyrolysis experiment can be obtained by introducing oxygen to burn the residue and analyzing the combustion products. For the work described below, the system was operated as follows: the sediment or source rock under analysis (containing a minimum of about 3 mg of organic carbon, equivalent to about 100 mg to 1 g of dry sediment, depending on organic carbon content) was continuously weighed while it was heated according to the temperature profile shown in Figure 2. The heating program consisted of three stages where (i) the sample was dried (at a temperature of 150 "C) and (ii) pyrolyzed in an inert gas (T = 150-900 "C a t 30 "C/min) and (iii) the char was combusted by the addition of oxygen (T = 200-900 "C a t 30 "C/min). The volatiles were carried into an FTIR cell in a helium stream where the rate and amount of the volatile species including those listed above were measured. For this initial work, the specific gases chosen for analysis and the extinction coefficients used in quantitation were estimated from previous work on coals.la
Results and Discussion General Description of Results. Figures 3 and 4 show some typical three-dimensional plots of the TG-FTIR results. Fourier transform infrared spectra of the evolved products (x axis) are recorded continuously as a function of temperature (y axis) throughout the heating cycle so
Energy & Fuels, Vol. 2, No. 1, 1988 67
TG-FTIR of Petroleum Source Rocks
R
I
4000
I
I
I
3200
I
I
2400
I
1600
I
I
I
800
Wavenumbers cm-f
."" a
r 4000
3200
Figure 3. Spectra from TG-FTIR of DSDP Leg 75 sample (immature Cretaceous black shale containing 9.2% sediment organic carbon). Identity of unlabeled peaks same as in ref 18.
that the z axis gives a qualitative idea of the relative intensities of the different characteristic infrared bands as a function of temperature. For example, the DSDP Leg 75 sample is a very organic rich (9.2% organic carbon) immature sediment taken from the Walvis ridge off the southwest African coast. The intense C-fr bands in the frequency range of 2900-3000 cm-l are characteristic of P2 (S,)hydrocarbons that have been thermally evolved from the sediment organic matrix in the temperature range of about 400-500 "C.Parts a and b of Figure 4 show similar plots for two samples taken from the Ikpikpuk well of the Alaskan North Slope. The Szpeaks are again strong in the 790-ft sample, which, at a vitrinite reflectance of OS%, is just entering the petroleum maturation zone. However, the 8900-ft sample from the same well shown in Figure 4b shows that the C-H bands have become much weaker. The deeper sample shows a much higher maturity (R,= 0.9%) and a much reduced P2peak. Because the samples are roughly comparable in kerogen type IIP9and organic richness, these differences can be largely attributed to the deeper sample having passed further into the petroleum generation zone. However, the 8900-ft sample also shows C-H bands at a lower temperature as well, labeled S1 in Figure 4b, because of the oil that has already been generated. The infrared peaks typical of thermal evolution of methane are also apparent in Figure 4a,b, so that pyrolytic methane evolution can be followed independently from heavier hydrocarbons. A number of other characteristicla infrared bands are also apparent in Figures 3 and 4. The thermal evolution of any of these as a function of temperature could be followed, if desired. For example, the peaks typical of ammonia evolution are indicated in Figure 4a. These peaks are not typically observed in coal pyrolysisla and could be arising either from clay minerals or from sediment organic matter, as will be discussed later. One noticeable characteristic of most of the bands in Figures 3 and 4 is the high degree of resolution and fine structure, which is typical of gas-phase infrared spectra, as compared to those of liquids or solids. This ability to delineate fine structure is a distinguishing characteristic of the FTIR-technique, which allows pyrolysis products
2400
800
1600
Wavenumbers cm-'
b
I
4obo
I
1
3200
1
2400
I
1600
I
0
l
800
Wavenumbers cm-f
Figure 4. Spectra from TG-FTIR of Ikpikpuk samples: (a) 790 ft; (b) 8900 ft.
to be routinely identified with a high degree of confidence (see, for example, ref 18). Broader bands for the condensable products (which form an aerosol mist) can also be observed. Data from a typical run are presented in a different way in Figure 5 where the intensity of infrared bands characteristic of specific components are plotted as a function of temperature along the x axis. Thus, the y axis is showing the loss of a specific gas (or, in the case of part a, total weight loss) as a percentage of total sample weight. The temperature at which the maximum rate of evolution of each pyrolysis product occurs, T,,, can also be read for each component by superimposing curves from Figure 5 on the heating profile (Figure 2). Total weight loss (Figure 5a) can also be balanced against total percent of weight represented as the sum of all products (Figure 5b) so that a material balance for specific elements can be calculated for the total pyrolysis plus combustion processes.
Whelan et al.
68 Energy &Fuels, Vol. 2, No. 1, 1988
Table I. T-.- and Yields of Hydrocarbon Pyrolysis Products from TG-FTIR ~~
Seabee deDth, ft % Rn
% C tot
% C org
paraffin T,,, "C yield, % Nanushuk
methane T,,, " C yield, %
ethylene T,,, " C yield, %
1.21 1.35
555-590 540-590
0.014 0.014
510-590 510-590
565-570 590-640 560-600 575 570-620 600-640 610-660 630-660
0.011
540
b
0.17 0 0 0.028 0 0 0.056
0.01 0.011 0.0084 0.017 0.015 0.019
560 520-580 530-610 580 580-640 580-640
0 0 0.084 0.011 0.022 0.013
600-640
0.025
560
0.017
1.38
600-640
0.017
600
0.014
a
610-650 640 650
a
595 605 600
a
390 690
0.44
1.82 2.1
1.19 1.64
510 490
1890 1890 6300 8400 10020 10920 11610 12330
0.58 0.58 0.73 0.85 1.12 1.24 1.48 1.68
1.18 1.18 1.56 2.3 2.58 2 2.4 2.7
1.02 1.02 1.56 1.11 1.02 1.44 1.54 1.41
200-500
12930
1.85
3.6
1.75
100-850
3.6
2.12
100-900
.014 .02
Torok 510 510 560 300; 380
Torok/Fortress Mountain 0.365
Pebble Shale 13170
Kineak 0
13230 14550 15510 a
3.3 6 5
2.02 2.36 2.35
2.09 3.67 3.91
Not determined. b Negligible.
Weak. TG-FTIR
Temperature l"Cl pLlo/ys,s Canhstlon
Pyro&sso Cmhst!on
@ro&ss,s Combustm
c-----c----.(-r
C
4
,{
.I
Ammonia
1
1
..
A. I
550
94
Sum
88
Weight Loss
85
I
1
,
T I
Paraffin
800 400
9997 99 96 99 95
I
(2M
0
2440
3660
0
I
I
I
1220
I
I
I
2440
trrrrrrrrl
,
3660
0
2440
12s
3660
TmelsJ
Figure 5. TGFTIR analysis of Seabee source rock sample (12 930 ft): (a) measured weight loss; (b) sum of weight loss from individual gases; ( e i ) weight loss from individual gas species. Heating rate = 0.5 "CIS.
Paraffin ,,T , 100
790 f t
8900ft
P
$
Tm,,=520"C
70
' .
6 0 1 T m 0 x = f'. 4 8 4 $ L
1
,,
