264
Energy & Fuels 1988,2, 264-266
recovered in the C14+ aromatic fraction. A second improvement to these experiments will be the direct recovery of the C1-C6 fraction and its molecular identification by gas chromatography. The preliminary results presented here allow us to propose the following interpretation of the data. During thermal cracking of asphaltenes (simulating kerogen), dismutation reactions lead to formation of a hydrocarbon fraction with a higher H/C ratio and a coke fraction with a lower H/C ratio than the initial asphaltene H/C ratio. Using asphaltenes instead of kerogen permits us to quantify the amount and elemental composition of the residual coke fraction. Studying the pyrolysis of asphaltenes alone shows that once coke formation occurs, it represents the major constituent in the pyrolysate. The other constituents of the pyrolysate undergo significant secondary reactions with increasing conversion: saturates and unsaturates disappear from the C14+ fraction; aromatics undergo dealkylation and polycondensation leading to a decrease of their H/C ratio. The presence of an hydrogen donor hinders completely coke formation at 390 "C; at higher temperatures there is a competition between polycondensation and hydrogen-transfer reactions. If these results are extended to thermal cracking under geological conditions, the data obtained on the behaviour of tetralin stress the importance of hydroaromatic structures in the
thermal evolution of sedimentary organic matters. These, once liberated in the bitumen by cracking of kerogen, will play the role of hydrogen donors and prevent secondary cracking reactions, resulting in a heavier mean carbon number for the bitumen. Experiments with tetralin have shown that a relative amount of 1/10 (10%) related to the initial organic matter already has an influence. If it is compared to the concentration of the hydroaromatic fraction in type I1 bitumens such as those of the Paris basin, at the onset of hydrocarbon generation, we arrive at 13% of bitumen (weight/weight of insoluble carbon) and 30% of C14+ hydroaromatics in the bitumen to a ratio of 0.30 X 0.13, i.e. around 4 % of hydrogen donors related to the insoluble organic matter. Thus the presence of abundant hydroaromatics in bitumens could be a plausible reason for the high amount of CI4+ extractable compounds in type I1 organic matter. Compared to hydroaromatic molecules, water alone seems to play a negligible role. However, it may enhance the influence of hydrogen-donor molecules, but this point must be verified under conditions nearer to that of natural environments.
Acknowledgment. Fruitful discussions with M. Vandenbroucke and E. Idiz were highly appreciated. We thank C. Leblond for technical assistance. Registry No. HzO, 7732-18-5; tetralin, 119-64-2.
Determining Oil Generation Kinetic Parameters by Using a Fused-Quartz Pyrolysis System? J. E. Zumberge,* C. Sutton, S. J. Martin, and R. D. Worden Ruska Laboratories, Inc., 3601 Dunvale, Houston, Texas 77063 Received May 5, 1987. Revised Manuscript Received March 9, 1988
A number of techniques exist for estimating activation energies and frequency factors for reactions occurring in nonisothermal systems. In the case of the thermal decomposition of kerogen, these kinetic parameters are important for determining the timing of crude oil generation and migration within a particular basin. Regardless of the form of the kinetic equations or the mathematical treatment of the data obtained from laboratory pyrolysis experiments, it is important to obtain both accurate and reproducible temperature measurements during linear programmed heating. The unique properties of fused quartz make it ari ideal material for constructing pyrolysis instruments. A pyrolysis apparatus was constructed entirely from fused quartz, which facilitates direct infrared heating in an inert environment. A three-wire Pt RTD sensor was symmetrically positioned directly below the sample container and was used to both measure the temperature and provide closed-loop computer control to the heating element, resulting in precise linear temperature programmed heating.
Introduction In order to fully integrate organic geochemistry into petroleum exploration strategies beyond the routine source rock evaluation and oil correlation studies, it is necessary to investigate the kinetics of crude oil formation (or kerogen degradation) in situ and in the laborabry.14 Because of the variety of organic source inputs and subsequent 'Presented at the Symposium on Pyrolysis in Petroleum Geochemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-6, 1987.
disparate burial histories, it is often desirable to determine kinetic parameters, such as activation energies, of individual source rock samples obtained from the basin of interest. In laboratory simulation experiments, in which the sample is heated at elevated temperatures (102-103"C) (1)Tissot, B. P.;Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: West Berlin, 1984. ( 2 ) Demaison, G., Murris, R. J., Eds.Petroleum Geochemistry and Basin Eualuation; AAPG Memoir 35; AAPG Tulsa, OK,1984. (3) Sweeney,J. J.;Burnharn, A. K.; Braun, R. L. AAPG Bull. 1987, 71, 967-985.
0887-0624/88/2502-0264$01.50/00 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 3, 1988 265
Oil Generation Kinetic Parameters FID
4
POROUS FUSED QUARTZ SAMPLE CONTAINER Pt RTDTEMPERATURE SENSOR FUSED QUARTZSAND HEATING WIRE IN COILED FUSED QUARTZ TUBE FUSED QUARTZ DEWER FOR LIQUID COe COOLING SPACE SHUTTLE TILE INSULATION
cor
COOLINQ
1 Cm
Figure 1. Schematic cross section of fused-quartz pyrolyzer.
for short periods (102-1068) to compensate for lower temperatures that the sample experiences over geologic time, it is important to accurately measure both the temperature of the kerogen sample and the corresponding quantity of evolved organic material. For these reasons, we have developed a fused-quartz pyrolysis apparatus directly interfaced to a fused-quartz flame ionization detector (FID). The system provides the necessary temperature accuracy between 150 and 600 "C during linear temperature program rates of 1-50 OC/min and the inert environment necessary to reproducibly measure the generated hydrocarbons. Experimental Section The unique properties of fused quartz and fused silica make them an ideal material for constructing pyrolysis instruments. Briefly, fused quartz is an amorphous form of natural crystalline quartz s y n t h e a i d during heating (oxyhydrogenflame or electrical current) above the cristobalite melting point (1723 "C), while fused silica can also be formed synthetically from Sic&. The former generally contains more metal impurities (approximate 20-30 ppm) than the latter (4 ppm), although fused silica generally has a greater OH abundance (structural HzO) than fused quartz (approximate 1200 and 180 ppm, respectively). Besides the high temperature stability (>1100 "C) and relatively inert characteristics of fused quartz to organic compounds at high temperatures, clear fused quartz transmits much of the near-infrared wavelengths. Only the small amount of OH content causes slight absorptionat 2 . 7 2 / q . Because of this degree of transmissiveneas, the sample can also be heated directly with infrared radiation rather than only by convection and conduction. A third important property of fused quartz is ita very low coefficient of thermal expansion (0.5 X 104/"C), which permits large thermal gradients without breakage. Therefore, the fused-quartz pyrolyzer can be heated to 600 "C followed by immediate cooling with liquid COa, which returns the system to ambient temperatures in less than 10 min. The low thermal conductivity (0.0033 cal/(cm s "C)) of fused quartz is also important in this regard in that hot (e.g., FID) and cold zones can be closely adjacent. Finally, the high compressive (lo9N/m9 and tensile, torsional, and bending strengths (5 X 10' N/m2), along with the excellent elasticity of fused quartz further enhance its use in pyrolysis equipment. A review of the properties of amorphous silica is given by Bru~kner.'.~ (4) Bruckner, R. J. Non-Cryst. solids (5) Bruckner, R. J.Non-Cryst. Solids
1970,5, 123-175. 1971,5, 177-216.
Table I. Devonian Black Shale" T , Values at Different Heating Rates heating rate (30-600 "C), "C/min 30 15 10 5 Tfllax,"C 461.2 f 1.2 449.9 f 1.5 441.1 f 1.4 427.4 f 1.3 n 7 9 6 7 sample w t 2.5-8.3 3.7-17.0 2.5-17.8 3.3-20.2 range, mg "Sample CU1-1 from Cumberland County, KY: TOC = 13.8%; Ro = 0.62%;Rock-Eva1 2'- = 432 "C (25 OC/min); HI = 416. Figure 1 is a schematic cross section of the fused-quartz pyrolyzer. The helium carrier gas is preheated by conduction as it flows through the hot fused-quartz sand prior to contacting the sample, which is placed in a porous fused-quartz container with a lid. A three-wire Pt RTD temperature sensor (Heraeus Vokert, NY; 1Pt100, K2015) is symmetrically placed within the quartz sand, such that the sample and temperature sensor are separated by no more than 2 mm. A closed-loopcomputer proportional/ integral/derivative control algorithm linked with the dc current power supply (which powers the heating element and is electrically insulated inside a fused-quartz helical tube) provides the heating control. The thermal extracts and pyrolyzates are swept by the He carrier gas to the fused-quartz FID. Surrounding the coiled heater is a fused-quartz Dewar flask, which cools the pyrolyzer as liquid COz is allowed to expand into the Dewar. A silica and alumina fibrous composite (Space Shuttle Tile, Lockheed) provides insulation for the system. In the present experiments, powdered Green River Formation rock samples3were heated from 30 to 600 "C at 5,10,15, and 30 "C/min. In addition, a series of experiments at the same heating rates using a Devonian Black Shale outcrop from Kentucky were performed to determine precision and the effect of sample size. Sample weights ranged from 0.4 to 20.2 mg. Instrumental error wa~ observed to be *0.5 "C during the 5 "C/min heating rate and f l "C during the 30 "C/min rate in the 150-600 "C temperature range. This was determined by comparing the RTD resistance values (and correspondingtemperatures based on DIN standard 43760, CY = 0.00385) with the set or desired temperatures. He carrier gas flow was set at 60.0 mL/min by using a mass flow controller (Unit Instruments, Orange CA.); FID fuel gases were set at 550 mL/min for air and 55 ml/min for Hz, and the FID temperature was maintained at 390 "C. It is estimated that the time required for the pyrolyzate to ionize in the FID flame subsequent to release from the sample is less than 2 s. FID response peak areas were integrated and compared to a n - C a s 2standard for quantification. Incremental areas were converted into fraction
266 Energy & Fuels, Vol. 2, No. 3, 1988 Table 11. Fused-Quartz versus Rock-Eva1 T , Values T-, 'C Rockfused source rock samples Eval quaitz AT Devonian Black shales (USA) 461.2 f 1.2 29 CU1-1(KY) 432 428 9-11(KY) 455 27 420 450 30 7-4(KY) 424 456 32 7-11(KY) 437 459 22 1-1 (KY) 26 1-4(KY) 429 455 27 429 456 4-5(KY) 426 456 30 7B-2(KY) 426 455 29 9-9(KY) 430 456 26 9-7(KY) 434 452 18 CA-5 (KY) 426 453 27 4-10(KY) 435 455 20 6-1(KY) 433 454 21 BG-1 (TN) 436 469 33 CHATT (OK) 435 465 30 EXCELLO (OK) Kimmeridge clay (North Sea) STD (well) 438 472 34 MF (well) 445 467 22 425 445 20 D4341 (outcrop) 422 445 23 D4343 (outcrop) Colombian outcrops (Cretaceous) La Luna FM 443 466 23 Simiti FM 485 26 459 515 47 468 Paja FM British Columbian cores 444 Doig FM (Triassic) 472 28 442 459 17 Nordeg FM (Jurassic) miscellaneous samples Monterey FM outcrop (CA) 400 405 5 Green River coal (WY) 427 467 40 Sweden outcrop (Ordovician) 440 464 24 460 485 25 Australian Torbanite (Devonian) 26 av
of total area and related to corresponding temperatures, including the temperature at maximum generation (i.e., 5"-).
Results and Discussion The results of the linear temperature programmed ex-
Zumberge et al. Table 111. Green River Pyrolysis Kinetics" rate, 'C/min 30 15 10 5
l/Trate,K/s T - , T 0.500 466.5 0.250 449.9 0.167 444.2 0.083 432.1
T-,K 739.6 723.1 717.4 705.2
X
1.352 1.383 1.394 1.418
In (rate/ Tm2) -13.9055 -14.5528 -14.9403 -15.6080
"E, = 51.4 kcal/mol; r2 = 0.9899;In (rate/Tmm2)= In (A/(E,/
R)) - E,/RT-.'
periments are listed in Table I for the Devonian Black Shale samples. Little variation in T,, values was observed in the sample weight range given in Table I. However, T,, temperatures are routinely lower (up to 6 "C) when sample weights for this particular sample were below about 2 mg. This phenomenon also holds for most other samples, although the degree of T,, reduction and the sample weight threshold likely depends upon organic richness, kerogen type, and host rock lithology. The RTD measured T,, values determined with a heating rate of 30 OC/min in the ,, fused-quartz pyrolyzer are almost 30 "C higher than T values determined from a Rock-Eva1 instrument with heating rate of ca. 25 OC/min (432 "C). Where other comparisons were available between pyrolysis in fused quartz and Rock-Eval T,, values (Table 11),pyrolysis in fused quartz a t 30 OC/min gave T,, values that ranged between 5 and 47 "C higher and averaged 26 OC higher than Rock-Eval T,, values for many types of organic-rich rocks. Table I11 gives the calculated activation energy for the type I Green River sample based on the Juntgen and van Heek equation! This activation energy of 51.4 kcal/mole is similar to that reported by Sweeney et aL3 Other Green River Formation samples, however, can yield different activation energies due to widespread occurrence and variations in kerogen composition and thermal maturity. (6) Juntgen, H.; van Heek, K. Fortschr. Chem. Forsch. 1970,12,601.
