Effects of Pressure on Organic Matter Maturation during Confined

Aug 17, 1993 - In order to study the effects of hydrostatic pressure on organic matter ... Pressure also slightly modifiesthe pyrolysis-gas chromatogr...
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Energy & Fuels 1994,8, 741-754

741

Effects of Pressure on Organic Matter Maturation during Confined Pyrolysis of Woodford Kerogen R. Michels and P. Landais' CNRS-CREGU, B.P. 23, 54501 Vandoeuvre lis Nancy Cldex, France

R. P. Philp School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019

B. E. Torkelson AMOCO Production Research, 4502 E. 41st Street, Tulsa, Oklahoma 74135 Received August 17,1993. Revised Manuscript Received February 7, 1994"

In order to study the effects of hydrostatic pressure on organic matter artificial maturation, pure Woodford kerogen was pyrolyzed under confined conditions at 300,700,and 1300bar. This technique allows to pressurize the kerogen and the generated effluents without any contact of inert gases or excess water. Gases, chloroform extract, and solid residues were analyzed. Results derived from the analysis of gases show no effects of pressure while the C7+ hydrocarbons fractions is slightly affected. The amount of chloroform extract at the maximum of oil generation slightly decreases when pressure increases. Correspondingly, the Rock-Eval hydrogen index of the extracted solid residues increases. Pressure also slightly modifies the pyrolysis-gas chromatography fingerprints of the residual kerogen. Increasing pressure leads to the production of extracts depleted in saturates and shift the maximum of saturates generation to higher temperatures. The CIS+n-alkane and aromatic gas chromatography traces do not show clear evidence of pressure effects. Spectroscopic investigation of the total extract gives detailed information on the aromatic substitution pattern: low pressures enhance the concentration of protonated aromatic carbons while high pressures favor the preservation of substituted aromatic carbons and increase the aromatic content of the polars. Gas chromatography and mass spectrometry data indicate that most of the hopane-maturation parameters are already at equilibrium in the temperature range investigated (280-400"C corresponding to the post-diagenesis zone) and no pressure effects are noticed. However, sterane fingerprints show variations related to pressure. Increasing effluents pressure induces a lower conversion of the kerogen and of the polars, and a higher thermal stability of some organic compounds, and favors aromatization reactions in the polars. The pressure effects observed are significantly lower than those observed by other authors. It is proposed that the nature of the pressurizing medium plays an important role in the hydrostatic pressure effect during organic matter maturation.

Introduction Geological studies in high-pressure environments, such as low-temperature high-pressure metamorphic zones,l tectonic zones,2 or deep wells in sedimentary basins3 suggest that hydrostatic pressure has a retarding effect on organic matter maturation. In sedimentary basins, abnormally pressurized compartments are common, and the developed fluid pressures can reach the kilobar levels4 Thus, it is important to know the exact effects of hydrostatic pressure on hydrocarbon generation from source rocks, in order to discuss its effects on the evolution of the oil potential in sedimentary source rocks. Natural sample sets may allow the maturation profile in sedimentary basins to be studied, but the paleopressure is a very variable parameter that cannot be accurately determined. Only estimated values, which often correspond to maxima, are obtained from geothermobarometers like fluid inclusions6 or by phase-diagram model^.^^^ Until recently, only temperature and time were considered Abstract published in Advance ACS Abstracts, March 15, 1994. (1) Goff6, B.; Villey, M. Bull. MinBral. 1984, 107, 81-91. (2) Chiaramonte, M. A.; Novelli, L. Org. Geochem. 1986,10,281-290. (3) Price, L. C.; Clayton, J. L.; Rumen, L. L. Org. Geochem. 1981,3, 59-77. (4) Hunt, M. A.A.P.G. Bull. 1990, 74, 1-12. 0

as major parameters in the modeling of organic matter maturation. In theoretical and applied organic chemistry, however, the role of pressure has been widely investigated and identified as an important parameter. Geometrical disposition and distance between reactants is a critical factor of chemical dynamics and reactivity. The importance of steric disposition in reactions of pure compounds8 and kerogen maturationg has been emphasized. The geometrical relationship between reactants is modified when hydrostatic pressure varies and it is suspected that both the equilibrium concentrations and the conversion rate of hopanes and steranes isomers should be pressure dependent.gJO As shown by Hamann,ll free-radical and molecular dissociation are retarded with pressure and therefore compressed organic materials can be kept stable a t much (5) Narr, W.; Burruss, R. C. A.A.P.G. Bull. 1984,68, 1087-1100. (6) Barker, C. A.A.P.G. Bull. 1972,56, 2068-2071. (7) Plumley, W. J. A.A.P.G. Bull. 1980,64, 414-430. (8) Menger, F. M. Adu. Mol. Model. 1988, I , 189-213. (9) Costa N e b , C. Org. Geochem. 1991,17, 579-584. (10) Costa N e b , C. Advances in Organic Geochemistry 1981; Wiley: New York, 1983; pp 834-838. (11) Hamann, S. D. Chemical Kinetics. In High Pressure Physics and Chemistry; Bradley, R. S., Ed.;Academic Press: London, 1963; Vol. 2, pp 163-207.

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higher temperatures than their normal decomposition temperature. Fabuss e t a1.lZpredicted that, above approximately 400 bar, a pressure increase should favor bimolecular reactions leading to increasing yields of high molecular weight saturates, while an increase in the lower pressure range (P < 400 bar) should favor degradation reactions. Secondary alkylation reactions are also favored and lower the olefin/saturate ratio. Similar results were obtained by Domin613using confined pyrolysis (210-15600 bar) with hexane, 2-4-dimethylpentane, and l-phenylbutane. He proposed two types of reaction mechanisms depending on both temperature and pressure: (a) Under low-temperature-high-pressure conditions the unimolecular decomposition reaction is slow and rate-determining while the H transfers are fast. When this mechanism is predominant, increasing pressure decreases the rate of formation of all products. (b) At high-temperature-lowpressure conditions, the H transfers are slow and ratedetermining while decomposition is fast; rising pressure then increases the overall pyrolysis rate. In geological environments, most of the chemical compounds encountered in oils would be of the low-temperatbre-highpressure type and therefore their decomposition rate should decrease with pressure. Pressure has also a strong effect on the viscosity of organic liquids. Increasing pressure enhances the viscosity; therefore the rates of diffusion-controlled reactions14are lowered and unimolecular cyclization reactions are favored. Such reactions have been observed by DominP during confined pyrolysis of 1-phenylbutane, when 2-phenylbutane and tetralin yields increased steadily with pressure. Relative viscosities might be important when considering a complex mixture of organic moieties including solid, high molecular weight liquid compounds, hydrocarbons, and gases under pressure. The variety of the mechanisms involved in organic material reactions that can be influenced by pressure do not allow the use of the widely mentioned Le Chatelier's law to predict pyrolysis results: this law can only be applied in a state of thermodynamic equilibrium which is not achieved in laboratory experiments. Many experimental studies on source rocks have been carried out in order to test the effects of pressure on hydrocarbon generation. Louis and Tissot15 compared the natural evolution of the Toarcian shale in the Paris basin with samples heated in glass tubes and in pressurized (5 and 150 bar) metallic reaction cells under argon atmosphere. They observed higher chloroformextract and hydrocarbon yields with increasing pressure and concluded that high pressures influence the oil composition. Sajgo et ~ 1 . 1 6heated immature alginite and lignite at 0.06 kbar of vapor pressure (hydrostatic) and 2.5 kbar of pressure load (lithostatic) in the 200-450 "C temperature range for 1 week. They distinguished applied high load pressure that retards coalification, hydrocarbon formation, and the maturation of biomarkers, from low vapor pressure that accelerates maturation when results are compared to those from open pyrolysis systems. On the contrary, Petzoukha (12)Fabuss, B. M.; Smith, J. 0.; Satterfield, C. N. Adu. Pet. Chem. Ref. 1964,3,157-201. (13)Domine, F. Org. Geochem. 1991,17,619-634. (14)Hamann, S. D.Trans. Faraday SOC.1958,54,507-511. (15)Louis, M. C.; Tissot, B. P. 7th World Pet. Congr. Proc. Mexico 1967,2,47-60. (16)Sajgo, Cs.;McEvoy,J.; Wolff, G. A.; Horvath, Z. A. Org. Geochem. 1986,10,331-337.

and Selivanov17suggested that high load pressure at very low temperature would favor oil generation and modify the sterane fingerprint. Experiments in a helium flow system at 0.78 and 7.65 bar between 375 and 500 "C yielded less oil with increasing pressure.18 Price and Wenger'g pyrolyzed a type 11s source rock under helium water vapor conditions (6.36-118 bar pressure) in the 150-450 "C temperature range, in order to obtain a maturation series that could be compared to experiments performed under increasing helium pressure (up to 1077 bar). Increasing helium pressures at 287 and 350 OC strongly retard kerogen decomposition and biomarker evolution compared to helium + water vapor pressure experiments. Michels et dZ0 studied the effects of increasing effluents pressure during confined pyrolysis and liquid water pressure during the hydrous pyrolysis of Woodford shale. They concluded that amounts of expelled oil and total yield (expelled bitumen) are highly dependent on liquid water pressures that delay and hinder the genesis and expulsion of hydrocarbons. However, the pyrolysis of the same sample at identical time-temperature-pressure conditions in gold tubes (confined pyrolysis) without addition of excess liquid waterz0 did not reveal an important pressure effect on the amounts of oil generated or expelled. Monthioux et al.z1-z3used different experimental systems: flushed open pyrolysis devices, glass tube pyrolysis with or without steam, closed autoclaves without liquid water, and confined pyrolysis in gold tubes. The authors observed that increasing the confinement of the system by reducing the dead volume in the reactors leads to better results and that confined pyrolysis in gold tubes properly simulates the various maturation stages (especially organic diagenesis),yielding coal extracts and mature kerogens very similar to those of their natural sample set. In such conditions, the porosity in the sample is filled by the pyrolysis products (high partial pressure of effluents) and not by inert gases as it was the case in the other experiments they had carried out. Additional experiments were performed at various pressures (500-4000 bar) and did not shown any pressure-dependent variations on simple geochemical parameters. Pyrolysis of New Albany shale in the presence of excess liquid water in gold tubes subjected to 300 and 600 bar external pressure24shows very limited effects on the kinetics of production of carboxylic acids and light hydrocarbons. The study of the effects of pressure on the kinetics of kerogen pyrolysis in similar conditionsz5 leads to the same conclusions. Nevertheless, confining hydrostatic pressure influences the viscositiesof coals during pyrolysis and leads to mature coals that have textures close to natural.z6 In opposition

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(17)Petzoukha, Yu.; Selivanov, 0. 15th Meeting E.A.O.G. Org. Geochem. Abstr. 1991,314-319. (18)Noble, R.D.; Tucker, W. F.; Harris, H. G. Fuel 1982,61,482-484. (19)Price, L. C.; Wenger, L. M. Org. Ceochem. 1991,19,141-159. (20)Michels, R.;Landais, P.; Elie, M.; Gerard, L.; Mansuy, L. ACS Prepr. 1992,1588-1594. (21)Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985,8, 275-292. (22)Monthioux, M.; Landais, P.; Durand, B. Org. Geochem. 1986,10, 299-311. (23)Monthioux, M. Maturationsnaturelle et artificielled'une sBrie de charbons homoghes. MBmoire de these #(tat, IFP, April 1986,331 pp. (24)Knauss, K. G.; Copenhaver, S. A.; Braun, R. L.; Burnham, A. K. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1992,1621-1627. (25)Freund, H.; Clouse, J. A.; Otten, G. A. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1992,1628-1635. (26)Goodarzi, F. Fuel 1985,64,156-162.

Effects of Pressure on Organic Matter

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phase) and anhydrous-helium pressurized experiments. to the results of confined pyrolysis presented earlier and In addition, if the helium pressure in experiments perperformed with coal, the pyrolysis in gold tubes of a type formed with water vapor (filling the head space of the I1 source rockz7show that increasing hydrostatic pressure autoclave) is increased to a sufficient value (P > 220 bar), favors the generation and expulsion of polar compounds, we will be in the presence of a disseminated liquid-water but lowers the saturate/aromatic ratio and the absolute phase without vapor phase, and not enough liquid water amount of C13+ hydrocarbons generated. The n-alkane fingerprints (CPI values) also appear less mature when will be present in the reactor to cover the whole sample (heterogeneous pyrolysis medium). At temperatures over pressure is increased. 374 "C, we will be in the presence of a supercritical phase This short literature review shows that the data that fills again the headspace of the reaction vessel. The describing the effects of pressure on hydrocarbon generause of an inert gas to control the pressure in water-vapor tion are often contradictory. In addition to the complex experiments induces many uncontrolled parameters and effects of pressure described in organic chemistry, several uncertainties, because (a) water and inert gas pressures types of pressures are in fact investigated, depending on are not equivalent9 the relationship between organic the experimental device used. According to this short compounds (reactants) and the pressure medium are review, pressure can be defined as follows. different; while inert gas pressure has an influence on 1. Lithostatic pressure. I t has therefore a strong hydrocarbon cracking, pressurized water might have an influence on the source rock structure (porosity, texture). influence on other reactions (i.e., hydrogen transfer, This type of pressure could also affect the structure of solvatation, as a reactant, ...); (b) the cracking rate of the organic solids (coals,refractive part of kerogens),especially kerogen and of the generated hydrocarbons is influenced at high maturation stages (graphitoids) where condensed by both the state of waterz9and the inert gas pressure.28 aromatic sheets could be preferentially oriented. Shear To perfectly control the nature of the pressure, it is stress could have similar effects. necessary to use a single homogeneous pressurizing 2. Fluid pressures. In the common sedimentary basin medium. Pure inert gas is often used to study the pressure conditions (without hydrothermal activity), the fluids effects on organic compounds pyrolysis. These types of involved in hydrostatic pressure are subcritical liquid water investigation are useful on a chemical point of view; as well as the generated hydrocarbons and gases (overhowever, it is known that the pyrolysis of kerogen in an pressure). In laboratory experiments, the fluids are water anhydrous medium, where pure inert gases are the and hydrocarbons (often in vapor state, because they are dominant phase, do not yield oils and geochemical vaporized in the dead volume of the reaction cell) but also parameters evolutions similar to inert gases (which do not exist as pressure medium in the LewanZ9has shown that good simulation results are natural system). The reactants (hydrocarbons) are often obtained when water is present in the reactor. However, in direct contact with the pressurizing medium. when water-vapor conditions exists in low-pressure ex3. An external factor when considering the hydrostatic periments ( P < 220 bar) and if pressure is increased to pressure applied on the gold tube in confined pyrolysis. high values by means of inert gas, the problems described In this case, the pressurizing medium is not in contact earlier appear (water reactivity is strongly modified and with the organic matter, avoiding chemical or physical might hide the pure pressure effect). The best simulation interactions that could be stronger than the pressure effect results are observed when liquid water covers the sample itself. The pressure inside the gold cell is only transmitted during pyrolysis29 (inert gas is not a dominant phase by the effluents generated. The notion of partial pressure anymore). But, usually in hydrous pyrolysis the pressure of effluents (concentration of the reactants) is important is determined by the thermal expansion of the constant in such experimental configuration: this factor must be amount of helium added to the autoclave headspace, the high in order to ensure the confinement (minimum dead water liquid-vapor equilibrium and the generation of oil volume = rock porosity) that leads to accurate simulation and gas. Until recently20this parameter has never been of organic matter maturation in confined p y r o l y s i ~ . ~ l - ~ ~ controlled, while the classical experimental procedure The nature of the pressure in confined pyrolysis will be imposes pressure values always close to the 220-300 bar discussed in the following technical chapter. range (water liquid-vapor equilibrium pressure + oil and In some pyrolysis techniques pressure can exist as a gas pressure helium pressure) and will evolve within combination of the different types previously described. this range depending on the amount and type of organic Therefore, it is important to first analyze what kind of matter and the extent of the reaction. pressure is present in the reaction cell and how precisely Good simulation results are also obtained when the dead it can be controlled. If the pressurizing fluid is in contact volume in the reaction cell is minimum (maximum with the reacting medium, the type and the state of the despite the fact that confinement = rock porosity)21~22~30 fluid can have an influence on the results of the experino excess liquid water is present in the reaction cell. The ments. HirthZ8shows that the pyrolysis rate of ethane gold tube pyrolysistechnique (confined pyrolysis) is a good occurs faster in inert gas (nitrogen) than in supercritical technique to study pressure effects, because the external water. He also demonstrates that the pyrolysis rate of hydrostatic pressure applied on the gold tubes is perfectly ethane is about the same in supercritical water pressurized controlled by means of a pressure pump and is not at 700 bar than in inert gas at 1bar but is highly accelerated temperature dependent (the nature of the pressure in when the inert gas medium is pressurized to 700 bar. this system will be discussed in the Experimental and LewanZ9has shown that bitumen yield in water-vapor Analytical Section). As stated earlier, no contact exists experiments is intermediate between hydrous (liquid-water

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(27) Blanc, Ph.; Connan, J. Energy Fuels 1992, 6, 666-677. (28) Hirth, T. Pyrolyse, hydropyrolyse und oxidation kohlenstoffhaltiger Verbindungen in iiberkritischem Wasser bei Driicken bis 1000 bar. Dissertation, Universitiit Karlsruhe, June 1992; 170 pp.

(29) Lewan, M. D. Laboratory simulation of petroleum formation: hydrous pyrolysis. In Organic Geochemistry; Engel, M. H., Macko, S. A., Eds.; Plenum Press: New York, 1993; Chapter 18, pp 419-442. (30) Sweeney, J. J.; Burham, A. K.; Braun, R. L. A.A.P.G. Bull. 1987, 71,967-985.

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Procedure and pyrolysis apparatus are described in detail by between the pressurizing medium and the reactants; thus Landais et al.37 the hydrostatic pressure in the system is only transmitted In confined pyrolysis, two types of pressures can be distinby the generated effluents (when no additional phase is guished. One is the pressure applied on the outside of the gold added into the gold tube). When water is added to the cell by means of pressurized water. During the heating process, sample,24~25only the effect of pressurized water-effluents the gold tube is increasingly pressurized by water expansion and mixture is investigated without interference of inert gas the pressure is adjusted to the final value with a pump as soon pressure (that does not exist in the natural system). In the isothermal stage is reached (about 1hour after heating has such configuration, pure hydrostatic pressure can be started). The second is developed by the decomposition with studied as the only variable. Hence, confined pyrolysis is temperature of the organic matter inside the gold cell. In a simulation system adapted to study the effects of experiments performed at low isothermal temperature (Le,, 260 increasing hydrostatic pressure on oil generation. In "C), the decomposition of kerogen is limited and the internal pressure developed is not sufficient to counterbalance the applied addition, it is a simulation technique that has been extensively compared to a natural maturation set.21,23*31-34 pressure. The internal volume of the gold cell remains minimal (kerogen volume + porosity). At higher temperatures, when The use of pure dry kerogen instead of whole source rock kerogen cracking is very efficient, the internal pressure rises. As allows the effects of pressure to be tested alone, without soon its value becomes greater than the applied external pressure, interference of minerals catalysis and reactivity of added the volume of the gold cell increaaes, The swelling of the gold water (the only water available in the system results from tube is a function of reaction progress and nature of the organic kerogen deoxygenation reactions). material (oil or gas prone). However, the hydrostatic pressure Monthioux et a1.21p22have shown that pressure has no inside the gold cell is always adjusted and close to the controlled external pressure, but the concentration of the effluents (high significant effect on oil generation from a Mahakam coal. partial pressure) is maintained (no dilution by a pressurizing However, coals are known to have a microstructure that medium, i.e., inert gas or water). influencestheir evolution with maturation (especiallytheir Gas Analysis. After confined pyrolysis, one set of gold cells expulsion a b i l i t i e ~ ) . Therefore, ~ ~ , ~ ~ it is possible to assume was pierced at 250 "C in a thermodesorption-multidimensional that the absence of pressure effects observed could be a gas chromatograph system38 in order to analyze in a single consequence of a structural effect. In addition, coals are injection the c1-C~hydrocarbons H20, C02, and H2S (TCD not oil-prone kerogens. The use of an oil-prone type I1 detector) and hydrocarbons in the C d m r a n g e (flame ionization kerogen (with a less-developedmicrostructure) is therefore detector). The gold tubes are carefully cleaned with chloroform suitable to test this hypothesis. In addition, results can and placed into the thermostated thermodesorption chamber. be directly compared to those of Domin@ who tested the The air in the chamber is evacuated (the vacuum is controlled pure hydrostatic pressure effects on chemical compounds by a pressure gauge). When the temperature is stabilized at 250 pyrolyzed in confined conditions. Only a few s t ~ d i e s ~ ~ p"C, ~ ~the gold tube is pierced. After 30 min of thermodesorption, 0.5 mL of effluents is injected via a Valco valve sampling loop have been performed with a system were pure perfectly and a capillary silica transfer line a t 280 "C into a Siemens controlled hydrostatic pressure is the only variable. In Sichromat 2-8 multidimensional gas chromatograph. Inside the regard to the extremely different behaviors of organic gas chromatograph, two individual thermostated ovens are both matter reactivity with the various types of pressures equiped with different columns (oven1 OV1, 50 m X 0.32 mm; (sometimes combined in one single experimental setup), oven2 paraplot Q, 23 m X 0.32 mm) connected by a pneumatic it is expected that confined pyrolysis results will help to line switching system. After injection of the sample, a selected explain combined pressure effects. portion of the effluents in column 1containing partially resolved

Experimental and Analytical Section Sample. The type I1 kerogen (H/C = 1.07 O/C = 0.075 HI = 534 mgHC/gTOC) used in the confined pyrolysis experiments was isolated by acid treatment (HF-HC1 in inert atmosphere) from an immature Woodford Shale collected in the Anadarko basin (OK).M This Upper Devonian source rock was sampled from a single homogenous 10 cm thick shale layer (labeled WD26 and provided by Amoco Production) belonging to a rythmic sequence of black shale and chert. The shale collected is rich in quartz, illite, contains some kaolinite and some pyrite, and is very rich in organic matter (22% total organic carbon). Confined Pyrolysis. Aliquots (150mg) of extracted, vacuumdried Woodford kerogen were sealed under argon atmosphere in gold cells (length = 5 cm, diameter = 0.5 cm). Two sets of samples were heated isothermally for 72 h at 260,300,330,350,365, and 400 O C at hydrostatic pressures of 300,700, and 1300 bar. The temperature was controlled by an internal thermocouple in contact with the gold cells. For every experiment, two gold cells (each of every set) were placed in the same autoclave, so they experienced the exact same time-temperature-pressure history.

components (Ar, C02, H2S, H20, C1-Ce hydrocarbons) is transferred to column 2 for complete separation and detection by a TCD (thermal conductivity detector). The other components (C7-C20 hydrocarbons) are separated by column 1 and directly transferred to a FID (flame ionization detector). The weight loss was measured after thermodesorption. The gold tubes of the second set were pierced and held a t ambient temperature for 24 h in order to measure the weight loss related to the highly volatile fraction (CI to about CIOhydrocarbons, C02, H2S, and to a certain extent HzO). The gold tubes were then opened and the organic matter pulverized and extracted with a large excess of hot chloroform for ll/z h. Calculations are based on peak integration corrected by the response factor of every compound (determined by injection of calibration mixtures). Analysis of the Solid Residue and Chloroform Extract. The extracted solid residues were characterized by elemental analysis, Rock-Eval pyr0lysis,3~and flash pyro1ysis.a Flash pyrolysis was performed using the CDS pyroprobe system. The kerogen samples were pyrolyzed a t 610 "C for 20 s while the effluents were flushed into the injector system of the Varian 3300 GC. The oven was held at -25 'C during pyrolysis, then heated a t 4 OC/min up to 300 "C and kept at 300 "C for 10 min.

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(31)Monthioux, M.; Landais, P. Energy Fuels 1988,2, 794-801. (32)Landais, P. Org. Geochem. 1991,17,705-710. (33)Benkhedda, Z.;Landais, P.; Kister, J.;Dereppe, J. M.; Monthioux, M . Energy Fuels 1992,6,166-172. (34) Monthioux, M.; Landais, P. Fuel 1987,66, 1703-1708. (35)Pickel, W.;Gtitz, G.K. E. Org. Geochem. 1991,17,695-704. (36)Kirkland, D. W.;Denison, R. E.; Summers, D. M.; Gormly, J. R. Okl.Geol. Sur. Circ. 93,1992,38-69.

(37)Landais, P.; Michels, R.; Poty, B. J . Appl. Pyrol. 1989,16, 103115. (38)Landais, P.; GBrard, L.; Michels, R. 15th Meeting E.A.0.G Manchester, Org. Geochem. Abstr. 1991,637-640. (39)EspitaliB, 3.;Deroo, G.; Marquis, F. Rev. Jnst. Fr. Pet. 1985,40, 563-579. (40)Philp, R.P.; Bakel, A. Energy Fuels 1988,2, 59-64.

Effects of Pressure on Organic Matter The fused silica WCOT column (30 m X 0.32 mm i.d.) was coated by a DB5 phase (1 pm film). Effluents were analyzed by a flame ionization detector and a flame-photometric detector (FPD). Aliquots of the chloroform-soluble organic matter were analyzed using Iatroscan (thin layer chromatography coupled with an FID") in order to determine the saturate, aromatic, and polar contents. The remainingbitumen was fractionated by thinlayer chromatography in order to recover the saturate and aromatic hydrocarbon fractions. The saturate fractions were analyzed by gas chromatography (HP5890Agas chromatograph; on-column injector; fused silica WCOT column (30 m X 0.32 mm i.d,), 0.25 pm DB5 film; temperature program 40 to 130 "C at 15 "C/min, followed by 130 to 300 "C at 3 "C/min, and kept isothermal at 300 "C for 10 min FID detector) and the aromatics by gas chromatography-FID detection (Varian 3300 GC with cooled (-25 "C) injector; fused silica WCOT column 30 m X 0.32 mm i.d. 0.25 pm DB5 film; temperature program 40 to 300 OC at 3 "C/min plus 300 "C isothermal for 10 min; effluents split and detection by FID and FPD) and gas chromatography-mass spectrometry'2 (same GC conditions as previously described, performed with a Varian 3300 connected to a Finnigan (TSQ7O) mass spectrometer used in full-scan mode) in order to identify the chemical compounds present. The branched and cyclic fractions were subsequently isolated by molecular sieving and analyzed by gas chromatographymass spectrometry in MID mode (Varian3300 connected to a Finnigan (TSQ70)mass spectrometer; temperature program 40 to 300 "C at 3 OC/min followed by an isothermal stage at 300 "C for 10 min) in order to obtain the hopanes (m/z = 191) and steranes (m/z = 217 and 218) fingerprints. SpectroscopicCharacterizationof Chloroform Extracts. In order to investigate the structural changes of the whole chloroformextract, global spectroscopictechniques were used in addition to the gas chromatography-mass spectrometry studies. Fourier Transform Infrared (FTZR). FTIR spectra of dry, whole extract aliquots were recorded on a Nicolet 20 SXB Fourier transform spectrometer. Procedure, deconvolutionmethod, and assignment of the different FTIR bands are given in Benkhedda et a1.,33 Bartle et al.,'3 and Meldrum and Roschester."

Carbon and Proton Magnetic Resonance (lSCNMR and 'H NMR). NMR spectra of total extract aliquots (diluted in CDCls for 1H NMR) have been obtained on a high-resolution Brucker spectrometer 500 MHz AM series. The assignments of 13Cand 1H NMR bands are given in Benkhedda et al.,33 Snape et a1.,46 and Attalla et al.*

Results Weight Loss and Gas Analysis. Weight loss values increase from 2 to 19% with maturation after piercing the gold tube at ambient temperature, and from 10 to 34% after thermodesorption at 250 "C (Figure l)\ Novariations with increasing pressure are observed when piercing the gold cells at ambient, suggesting that the total amount of highly volatiles compounds (C1to about Clo hydrocarbons, COz, HzS, and HzO to some extent) are not modified. However, thermodesorption yields are slightly higher at 300 bar for 350 and 365 "C experiments, suggesting some (41) Berrut, J. B.;Jonathan, D. Characterization of Heavy Crude Oils and Petroleum Residues; Technip: Paris, 1984; pp 400-405. (42) Philp, R. P.; Oung, J.;Lewis, C. A. J . Chromatogr. 1988,446,3-16. (43) Bartle, K. D.; Martin, T. G.; Williams, D. F. Fuel 1975,54,226235. (44) Meldrum, B. J.; Roschester, C. H. Fuel 1991, 70,57-63. (45) Snape, C. E.; Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Gerstein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989, 68,547-560. (46) Attalla, M. I.; Vassallo, A. M.; Wilson, M. A. Nuclear Magnetic Resonance studies of coals. In Spectroscopic Analysis of Coal Liquids; Kershaw, J. R., Ed.; Elsevier, 1989, 195-241.

Energy & Fuels, Vol. 8, No. 3, 1994 745 0

e e

*

I

A

I

t

P

I

A

300b (25°C) 700b(250C) 1300b(25°C) 300b(250°C) 700b(25OoC) 1300b(250°C)

P 0

H

240 260 280 300 320 340 360 380 400

PYROLYSIS TEMPERATURE ("C) Figure 1. Weight loss (in weight percent of initial kerogen) obtained after confined pyrolysis of two sets of Woodford kerogen aliquots at various hydrostatic pressures (300,700,and 1300bar). The weight loss is determined ( a ) in the first set of aliquots by mass balance before and after piercingof the gold cella at ambient (release of the effluents volatile at 25 "C: C1 to about C ~ O hydrocarbons, COz, HzS, HzO); (b) in the second set of aliquots by mass balance before and after thermodesorption of the gold cells at 250 "C (TD-MDGC) (release of C1-Cm hydrocarbons,

Cl300b Cl700b A Cl13oob 0-C6300b O-C6700b 4 O-C61300b 0

; !

; a

; 0

B P

240 260 280 300 320 340 360 380 400

PYROLYSIS TEMPERATURE PC)

Figure 2. Percentage of methane and CZto C6 hydrocarbons generated after confined pyrolysis at various pressures, relative to the total amount of gases (Ci-Ce hydrocarbons, HzO, COz, HzS) thermodesorbed at 250 "C from the gold cells.

pressure related changes concerning the higher molecular weight compounds (Clo+ hydrocarbons fraction). The relative amount of methane steadily increases with maturation. The contribution of the c2-C~hydrocarbons reaches a maximum at 330 "C and starts to decrease at 365 "C (Figure 2). The relative abundances (calculated on the TCD traces) of COz and HzO decrease linearly with maturation. The COz/methane ratio (Figure 3) decreases with increasing maturation, and no significant pressure effects are noticeable at temperatures greater than 260 "C. Figure 4 shows the evolution of the C,+ fraction (FID traces) obtained after confined pyrolysis at 300 bar and analyzed by thermodesorption-gas chromatography. At 260 "C, the chromatogram is composed of n-alkanes dominated by branched and cyclic compounds. At higher temperatures (300,330,350"C) the n-alkanes become more abundant and are enriched in Clo+ compounds (maximum of saturates generation, 350 "C). At 365 "C, aromatic compounds (like toluene and xylenes) become more important, while the n-alkanes distribution becomes lighter (beginning of n-alkanes thermal breakdown). At 400 "C, the n-alkanes contribution is very low and the aromatic hydrocarbons are strongly dominant. No major pressure effects are noticeable in the FID traces obtained after pyrolysis between 260 and 330 "C. At 350

Michels et al.

746 Energy & Fuels, Vol. 8, No. 3, 1994

w

c

14 12

'i

B

2

O

I

~

.

I

'

I

.

8

.

I

.

I

'

I

I

'

I

'

l

240 260 280 300 320 340 360 380 400

PYROLYSIS TEMPERATURE("C) Figure 3. Evolution of the Codmethane ratio at various pressures as a function of pyrolysis temperature. Calculations are based on FPD (detection of c1-C~hydrocarbons, Cop, HsO, H2S) peak areas (response factor corrected) obtained after thermodesorption-gaschromatography analysis of the effluents in the gold cells. (b = bar).

and 365 "C, pressure slightly changes the distribution of the C1o-C15 n-alkanes (Figure 5a). The monitoring of the C15+/(C~-C15+)saturates reaches a maximum at 350 "C (Figure 6). Pressure significantly decreases this ratio at 350 "C. This feature as well as the changes in the saturates and aromatic compounds distributions in the bitumen will be discussed later ("chloroform extract" and "hydrocarbons distribution" sections). As far as the general parameters that have been discussed are concerned, the effects of pressure on the evolution of the thermodesorbed products (C1-C20 hydrocarbons, H20, COZ, and H2S) relative amounts and distribution are limited. Only the methane contributions at 400 "C, the Cz-Cs contribution between 330 and 400 "C and the c15+/ (C7-C15+) ratio evolution suggest a pressure-dependent mechanism concerning the effluents generation or thermal degradation. Characterization of t h e Extracted Solid Residue. The H/C = 1.07 and O/C = 0.075 atomic ratios of the starting sample are typical of an immature type I1kerogen. During maturation, the O/C atomic ratio slightly decreases while a drastic H/C decrease is observed between the raw sample and 350 "C (Table 1). The percentageof corrected organic carbon, COC = % C/(% C 5% H + % 01, defined as a maturation parameter for coals21 increases linearly with maturation, but no significant differences are observed with pressure. The Rock-Eva1hydrogen index (Table 1)decreases from 534 mg of HC/g of TOC down to 10 mg of HC/g of TOC with temperature and T,, increases from 422 to 540 "C (Table 1). On basis of the Rock-Eva1 data, two main maturation stages can be distinguished: the oil window that is characterized by a sharp decrease of the hydrogen index and minor variations of T,, followed by the postcatagenesis stage when Tmaxstrongly increases. When pressure is increased from 300 to 700 bar a slight increase of hydrogen index is observed. At 300 "C especially but also at 330 and 350 "C, the 1300 bar samples show significantly higher values (Table 1). T,, evolution is not affected by pressure during the confined pyrolysis of Woodford kerogen. Pyrolysis-gas chromatography provides some interesting features concerning the structure of the extracted solid residue. The original kerogen shows a fingerprint similar to those obtained at 260 and 300 OC with the alkene-alkane doublets up to C25 predominating along with toluene and xylenes. At 330 "C (Figure 7a) and 350 "C, the C19+alkane-

+

alkene doublets become less important, whereas benzene and toluene continue to dominate the traces. The chromatograms at 365 "C are drastically modified: aromatic compounds in addition to benzene and toluene start to appear and become predominant over the alkene-alkane doublets. At 400 "C (Figure 7b) alkene-alkane doublets could not be unequivocally identified and aromatic components heavier than benzene dominate the chromatogram. The pyrolysis-gas chromatography fingerprints do not strongly change with pressure up to 300 "C. However, single peaks, like benzene and unidentified component X, show irregular variations with pressure. At 330 "C (Figure 7a) the 1300 bar pyrogram shows some differences with the 300 and 700 bar traces: the benzene peak is dominant (and coelutes partially with an unidentified peak) and peak X is absent. At 350 "C, benzene is dominant at 1300 and 300 bar, while the relative abundance of peak X and Y (a second unidentified peak) decrease between 300 and 700 bar and are not detected a t 1300 bar. Because of these variations, the fingerprint is slightly affected by pressure. A t 365 "C other differences are noticeable: at 300 bar, compound X and Y are present while they are already absent a t 700 and 1300 bar. The strong dominance of benzene that appeared at 330 "C for the samples pyrolyzed at 1300 bar now appears also at 365 "C 700 bar. At 400 "C (Figure 7b) the same aromatic components are present in the pyrograms at all pressures (besides of one unidentified compound in the 1300 bar chromatogram that elutes between toluene and the xylenes). The distributions of these compounds are fairly similar; hence, at 300 and 700 bar they exhibit slightly more low molecular weight aromatics: the peaks that dominate the pyrograms seem to be shifted from alkylbenzenes at 300 and 700 bar (onering aromatics) to two- and three-ring aromatic compounds (Le., naphthalenes and phenanthrenes) at 1300 bar. Chloroform Extracts. The chloroform extract yield (Figure 8) increases from about 5 % (weight percent of raw kerogen) at 260 "C to reach a maximum value of 27 % between 300 and 330 "C and decreases down to a few percent at 400 "C. The error bars in Figure 8 show the maximum scattering obtained with four different experiments for every pressure at the maximum of production (300 "C). The absolute error at 300 "C is *1.25%. Increasing pressures decrease the extract yield. In regard to the scattering of the values, the maximum decrease that can be estimated ranges between 27 % (300 bar) and 20% (1300 bar). If mean values are considered, the decrease observed is somewhat smaller. However, these variations are significant, whereas at other temperatures the scattering of the different values remains within the range of analytical error (*1.25% ) and can not be directly refered to pressure effects. With increasing maturation, the relative amount of polars in the total extract (Table 2) maximizes between 260 and 300 "C and starts to decrease as soon as the maximum of bitumen production is passed. Over the same maturation range the contribution of aromatic hydrocarbons to the total extracts decreases first from 30 % at 260 "C to an average of 24% at 300 "C, and then increases steadily, up to 60% at 365 "C. The relative amounts of saturates in the extracts increases all along the maturation profile investigated (260-365 "C) but always remain below 15% (Table 2). At 400 "C the saturates content of the bitumen is too low to be accurately determined.

Effects of Pressure on Organic Matter

,

Energy & Fuels, Vol. 8, No. 3, 1994 747

l7 /B

7~

IO

,,

12 ,3

r!

350°C 300 bars

400T300 bars

Figure 4. Evolution of the C,+ hydrocarbons distribution as a function of increasing pyrolysis temperature at 300 bar hydrostatic pressure. The chromatograms were obtained after thermodesorption-gaschromatography analysis of the effluents in the gold cells. Numbers from 7 to 19correspondto the n-alkanecarbon numbers. Tol, toluene;Xyl, xylenes; TMB,trimethylbenzenes;N, naphthalene; MN, methylnaphthalene; DMN, dimethylnaphthalene. 17

1

9

10

,/

,,

3XPcmb.n

+

ln

x. !-

U -.

+

ln

G 0,oo 240

260

280

300

320

340

360

380

PYROLYSIS TEMPERATURE("C)

Figure 5. (a, left) FID chromatograms of the effluents thermodesorbed at 250 "C after pyrolysis in confined conditions (350 "C at 300,700, and 1300 bar) of Woodford kerogen. Numbers from 7 to 19 correspond to the n-alkane carbon numbers. Tol, toluene; Xyl, xylenes; Pr, pristane; Ph, phytane. The detector signalis saturated for the C&O n-alkanespeaks. (b,right) FID chromatograms of the effluents thermodesorbed at 250 "C after pyrolysis in confined conditions (400 "C at 300, 700, and 1300 bar) of Woodford kerogen. Numbers from 7 to 11correspondto n-alkane carbon numbers. Tol, toluene; Xyl, xylenes; TMB, trimethylbenzenes;N, naphthalene; MN, methylnaphthalene; DMN, dimethylnaphthalene. The detector signal for Cy-alkane (300 bar sample only), toluene, and xylenes are saturated. The extracts obtained after pyrolysis at various pressures do not show significant differences concerning their polar contents. The extracts from the 300 bar experiments at 330 and 350 "C are slightly depleted in aromatics, but enriched in saturates. In fact, at 350 and 365 OC, increasing the pressure from 300 to 1300 bar lowers the relative amounts of saturates in the bitumen by about 4%. These results are also in agreement with those of Blanc and Connan,27with the Cis+/(CrCla+)ratio evolution (Figure 6) and explains the higher weight loss obtained after thermodesorption of the 300b samples pyrolyzed at 350

Figure 6. Evolution of the Cla+/(C&s+) saturate ratio with increasingtemperature at various pressures. The hydrocarbons were analyzed after thermodesorption-gas chromatography of the effluents in the gold cells. Calculations are based on peak integrations. and 365 "C (Figure 1). As a matter of fact, the absolute amount of C15+ saturates produced (in mg/g of initial kerogen) decreases from 300to 1300bar and the maximum of saturates generation is shifted to higher temperatures with increasing pressures (Table 3). Furthermore, at 300 bar the saturates production reaches a maximum at 350 "C, followed by a decrease at 365 "C while the saturates generation still increases at 700 and 1300 bar. This observation is consistent with the decrease of the CIS+/ (C&15+) saturates ratio (thermodesorbed fraction, Figure 6) that occurs at a lower rate at 1300 bar. All the results concerning the saturates indicate that high pressures decrease the amount of c16+saturates generated at 350 "C, shift the maximum to higher temperatures, and lower the thermal degradation rate of these compounds. When the kerogen is pyrolyzed at 300 bar, the maximum of aromatics generation is obtained at 330 OC and therefore occurs earlier than at higher pressures (the maximum of generation appears between 350 and 365 "C for 700 and 1300bar experiments; Table 3). The 700 bar experiments

Michels et al.

748 Energy & Fuels, Vol. 8, No. 3, 1994

Table 1. H/C Atomic Ratio and Rook-Eva1 Pyrolysis Parameters Measured on Extracted Solid Residue Obtained after Pyrolysis in Confined Conditions of Woodford Kerogen Aliquots at Various Temperatures and Pressures (300,700,1300 bar)' 300bar 1.02 0.80 0.65 0.58 0.55 0.51

260 "C 300 "C 330 "C 350 "C 365 "C 400 "C

H/C atomic 700bar 1.02 0.82 0.64 0.59 0.58 0.51

1300bar 1.00 0.85 0.68 0.57 0.57 0.52

300bar 317 154 55 26 24 13

T- ("C)

HI mg/g of TOC 700bar 1300bar 317 308 167 203 50 75 31 41 27 29 19 24

300bar 430 436 445 501 515 540

700bar 432 436 445 496 521 541

1300bar 429 434 447 495 523 543

a HI = hydrogen index in mg/g of total organic carbon; T-, temperature at the maximum of 52 peak in "C. The characteristics of the starting kerogen (raw sample) is given in the bottom part of the table. Hydrogen index values are determined in a 10 mg/g of TOC error range. Raw sample H/C at. = 1.07; HI = 534 mg/g of TOC. -'2 = 422 "C.

,

To1

I' I

330°C 300 bars

400°C 300 bars

I

I' I

I

400°C 700 bars

33OoC 700 bars

, 400°C 1300 bars 330°C 1300 bars

-Increasing Retention Time

-

-

Increasing Retention

T n "

Figure 7. (a, left) Flash pyrolysis-gas chromatograms from extracted solid residues after confined pyrolysis at 330 OC and various pressures. Alkenealkane doublets are labeled with their respective carbon numbers. Benz, benzene; Tol, toluene; Xyl, xylenes; N, naphthalene. The C ~alkene-alkane O doublet is coeluting with trimethylbenzenes. The CISalkene-alkane doublet is coeluting with methylnaphthalenes. (b, right) Flash pyrolysis-gas chromatograms from extracted solid residues after confined pyrolysis at 400 O C and various pressures. 1,benzene; 2, toluene; 3, xylenes; 4, trimethylbenzenes; 5, C1-benzenes; 6, naphthalene; 7, methylnaphthalene; 8, dimethylnaphthalene; 9-10, C& alkylnaphthalenes + alkylthiophenes; 11,dibenzothiophene; 12, phenanthrene; 13,methyldibenzothiophenes; 14, methylphenanthrenes; 15, alkyldibenzothiophenes + alkylnaphthalenes; ?, unidentified peak.

8 A

R 250

275

300

325

350

375

400

PYROLYSIS TEMPERATURE ("0 Figure 8. Amounts of bitumen (in weight percent of initial kerogen) obtained after chloroform extraction of the kerogen pyrolyzed in confined conditions at various temperatures and pressures. The maximum scattering of the data is obtained at 300 "C with a 1.25% error range (calculations based on four different experiments at every pressure).

yield slightly more aromatics. A t the maximum of bitumen generation (300 "C), the amounts of polars produced from the kerogen decrease when pressure increases (Table 3).

However, the differences observed a t 300 "C are quickly reduced (at 350 "C the concentrations of polars are again similar at all pressures), suggestingthat polars are degraded faster a t lower pressures (300 bar) than at 700 and 1300 bar. Hydrocarbons Distribution. The saturated fraction is chiefly composed of alkanes lighter than C19 with relatively lower concentrations of branched and cyclic compounds. At a pyrolysis temperature of 260 "C the C15-C19 n-alkanes become strongly dominant and the GC fingerprint does not undergo any drastic changes with increasing temperature (CPI values remain at 1.1, the same as the original shale). The Pr/C1, and the Ph/C18 ratios decrease progressively with increasing maturation. The global aspects of the chromatograms (Figure 9) and the derived parameters are not affected by pressure. However, in Figure 9, the 1300bar chromatogram shows an increased naphthenic envelope. Price and Wengerlg demonstrated that high pressures can be responsible for the relative increase of the naphthenic envelope in n-alkane traces. In our case, this particularity appears every time the com-

Effects of Pressure on Organic Matter

Energy & Fuels, Vol. 8, No. 3, 1994 749

Table 2. Relative Amounts of Saturates, Aromatics, and Polars in Weight Percent of Bitumen Obtained after Extraction of Woodford Kerogen Aliquots Pyrolyzed in Confined Conditions at 300,700,1300 bar Hydrostatic Pressure and Various Temperatures. 300 bar

T ("C) 260 300 330 350 365 0

% SAT 1.45 2.63 4.43 9.88 13.51

% ARO 29.73 25.80 34.91 48.71 62.14

700 bar % Pol 68.82 71.57 60.66 41.41 24.35

The composition of the bitumens obtained a t 400 O

% SAT 1.29 1.62 4.31 7.77 11.60 C

% ARO 30.35 22.48 37.48 52.58 60.10

1300 bar

% POL 68.36 75.90 58.21 39.65 28.30

% SAT 1.35 2.73 3.20 6.00 10.50

% ARO 30.98 23.79 37.60 53.90 62.14

%

POL

67.67 73.48 59.20 40.10 27.36

could not be accurately determined because of their very low saturates content.

Table 3. Yields of Saturate, Aromatic, and Polar Fractions in mg/g of Initial Kerogen Obtained after Confined Pyrolysis of Woodford Kerogen at Various Temperatures and Pressures. 300 bar

T (OC) 260 300 330 350 365

SAT 0.64 6.71 9.75 14.82 13.51

ARO 13.08 65.79 76.80 73.07 62.14

700 bar

POL 30.28 184.54 133.45 63.32 24.35

SAT 0.49 3.48 9.27 12.04 13.92

ARO 11.53 48.35 80.58 81.50 72.12

1300 bar

POL

SAT

ARO

POL

25.98 163.19 125.15 61.46 33.96

0.57 5.46 6.40 8.40 10.50

13.17 47.58 75.20 75.46 62.14

28.76 146.96 118.40 56.14 27.36

The yields a t 400 "C could not be accurately determined (see Iatroscan data).

pounds lighter than C15 are affected by evaporation during and alkylbenzothiophenes (Figure 10)than those obtained the sample preparation and seems to be independent of from the experiments performed at 300 and 1300bar which pressure. Concerning this point, the chromatograms are very similar to each other. This last observation seems obtained by thermodesorption-gas chromatography give to be in contradiction with the absence of pressure effects more reliable data, and no increase with pressure of the on the aromatic fingerprint in the thermodesorbed chronaphthenic envelope is observed (Figure 5a). However, matograms. However, it is not impossible that in some as stated earlier, the distribution of the n-alkanes in the cases the compounds up to C3-naphthalenes have been C1o-C15 range is slightly modified when pressure increases affected by evaporation during the fractionation procedure from 300 to 1300 bar (Figure 5a). Blanc and C ~ n n a n ~ ~of the bitumens. described an increase of the odd-even predominance index Spectroscopic Studies. Spectroscopic Analysis of the with increasing pressure at a given temperature during Total Extract. FT-IR and 13Cand lH NMR analyses of confined pyrolysis of a Paris Basin shale. This disparity the total extracts allow the structure and the functionwith the current results can be related to the nature of alization of the bitumens to be investigated. This type of their original sample whose CPI is not 1. data cannot be strictly related to the composition of isolated fractions (saturated,aromatic or polar); however, The aromatic compounds in the chromatograms obpolars represent at least 24% (and up to 76% at 300 "C) tained after thermodesorption of the gold cells (Cl-CZo of the total bitumen, and the saturate proportion does not hydrocarbons) are alkylbenzenes and naphthalenes up to exceed 15% (at 365 "C). dimethylnaphthalenes (Figure 5b). With increasing maturation, the aromatic compounds become relatively more The 13C NMR aromaticity factor (Fa = percentage of important and at 400 "C they strongly dominate the aromatic carbons; Figure 11)of the bitumen increases with chromatograms. Increasing the pressure from 300 to 1300 maturation from 50% at 300 "C to 60% at 365 "C, and bar does not change the aromatic fingerprint of the correlatively, the FT-IR v(CHdiph)/r(CH,om) ratio dethermodesorbed effluents. However, some variations can creases. Both parameters indicate an enrichment in be observed in the 260-365 "C experiments (but might be aromatic compounds of the extract. Whatever the pylinked to coeluting branched and cyclic compounds) and rolysis temperature, the aromaticity factor (Fa)increases at 400 "C variations appear between peaks in the low from 300 to 1300 bar which is in agreement with data from molecular weight range (alkylbenzenes and associated Hamann14 and D o m i n P (aromatization reactions are peaks) but are difficult to elucidate (Figure 5b). favored by increasing pressure). This result can be related to the FT-IR V(CHdiph)/y(CH,om) ratio that show higher The aromatic fractions that could be isolated from the aliphatic contents of the bitumen extracted from the bitumens only contain compounds heavier than dimethsamples pyrolyzed at 300 bar, and to the Iatroscan data ylnaphthalenes (because the lighter compounds have from the chloroform extracts (Table 1). The variations in evaporated during sample preparation) and this completes aromaticity induced by pressure are stronger at 300 "C the information obtained after thermodesorption-gas and smaller at 400 "C. I t is not clear if the reduced intensity chromatography (Figure 10). The chromatograms are of the pressure effect with increasing temperature is due dominated by series of alkylnaphthalenes, alkylphenanto the structural changes of the bitumen with on-going threnes, alkylthiophenes, alkylbenzothiophenes,and alkylmaturation or to the higher pyrolysis temperatures: dibenzothiophenes (Figure 10). Pressure has little effect D o m i n P suggests that high temperatures lower the on the aromatic fingerprints: only the relative abundance pressure effect by reducing the hydrocarbon viscosity. of single compounds seem to vary, but the coelution of Detailed information can be extracted from the subunresolved peaks eliminates the possibility of quantitation. Nevertheless, at 350 and 365 "C, aromatic components stitution pattern of the aromatic carbons in the bitumen (aromatics in polar and hydrocarbon fractions). With from the experiments performed at 700 bar contain relatively higher concentrations of the Cz-C3 naphthalenes increasing maturation, the percentage of protonated

Michels et al.

750 Energy & Fuels, Vol. 8, No. 3, 1994

350°C 300 bars

I

20

80

40 60 Retention time (minutes)

350°C 700 bars

.I

become more abundant (breakdown of high molecular weight compounds in the bitumen). Another 13C NMR parameter based on the A4 13C NMR band33 measures the percentage of substituted aromatic carbons relative to total aromatic carbons, which evolves with increasing maturation through a maximum a t 350 "C before starting to decrease (generation followed by thermal breakdown). Both I3C NMR (A1 and A4 bands) parameters are related and show significant pressure effects: low pressures enhance the concentration of protonated aromatic carbons whereas high pressures favor the preservation of substituted aromatic carbons. The percentage of aromatic protons can also be deduced from lH NMR measurement@ [H, = aromatic protons/(aromatic protons + aliphatic protons)]. This ratio increases with pyrolysis temperature and values are significantly higher for the samples obtained from experiments performed under a pressure of 300 bar at 350 and 365 "C, thus confirming l3C NMR observations. Mass Spectrometry-Gas Chromatography Analysis of the Aromatic Hydrocarbons Fraction. Because some slight variations were observed in the chromatograms of the aromatic fraction in the bitumens, the alkylphenanthrene compounds were analyzed by gas chromatography and mass spectrometry, in order to test if the commonly used methylphenanthrene indexes (MPI)47are affected by pressure. The MPIl = 1.5(2MP 3MP)/(P + 1MP 9MP), MPI2 = 3(2MP)/(P 1MP + 9MP), and MPI3 = (2MP + 3MP)/(lMP 9MP) ratios were calculated by integrating the peaks obtained on the m/z = 178 (phenanthrene) and m / z = 192 (methylphenanthrenes) traces. The MPI indexes increase first slightly between 260 and 350 "C, the main evolution taking place between 350 and 400 "C. Between 260 and 350 "C, the trends of MPIl and MPI2 show a slight decrease when the experimental pressure increases from 300 to 1300 bar. At higher temperatures, these differences disappear. The MPI3 index, which does not take phenanthrene into account, showed no pressure-dependent variations. We can assume that the thermal degradation of the phenanthrenes does not take place before 350 "C (maximum of aromatic compounds production). Therefore, the differences observed in the methylphenanthrene indexes between 260 and 330 "C values could be related to different generation rates of phenanthrene. Mass Spectrometry-Gas Chromatography S t u d y of Hopanes. The hopane fingerprint of the pyrolyzed Woodford kerogen based on the m l z = 191 mass chromatogram is dominated by T,, T m , 17a(H),21p(H)-30norhopane, 17a(H),Blp(H)hopane, and the ( S ) and (R) epimers of the 17a(H),2lp(H)-homohopanes series. A t a pyrolysis temperature of 260 "C,the T$(T, + Tm) maturity parameter48value is 0.5, and the 22S/(22S + 22R) epimer ratio of the 17a(H),2lp(H)-homohopanes series is already close to its proposed equilibrium value, Le., 0.55.49 These two parameters do not undergo additional changes at higher pyrolysis temperatures. The j3a/@a+ a@)hopane r a t i 0 ~ 9 decreases ~l with increasing maturation from 0.15 at 260 "C to 0.09 at 365 "C. These results are consistent with literature data49 and together with parameters such

+

c__

20

80

60

40

Retention time (minutes)

350°C 1300 bars

.1 20

Ii"

~~

40

60

80 Retention time (minutes) Figure 9. CIS+ gas chromatograms of the saturate fraction isolated from the bitumens obtained after confined pyrolysis of Woodford kerogen at 350 O C and 300, 700, and 1300 bar hydrostatic pressure. Pr, pristane; Ph, phytane. Some of the n-alkanes are labeled by their carbon number. Evaporation during sample preparation has affected the chromatogram at 1300 bar up to the CIS n-alkane.

aromatic carbons increases (A1 13CNMR bandz7)(Figure 12) together with the FT-IR-&H,om)/y(C=Carom) ratio, suggesting that smaller, less substituted aromatic units

+

+

+

(47) Radke, M. Organic Geochemistry of Aromatic Hydrocarbons. In Advances in Organic Geochemistry;Brooks, J., Welte,D.,Eda.; Academic Press: London, 1987; Vol2, pp 141-207. (48) Seifert, W. K.; Moldowan, J. M. Geochim. Cosmochim. Acta 1978, 42, 77-95. (49) Peters, K. E.; Moldowan, J. M. The Biomarker Guide; PrenticeHall:Englewood Cliffs, NJ, 1993; 363 pp.

Energy & Fuels, Vol. 8, No. 3, 1994 751

Effects of Pressure on Organic Matter 7

365OC 300 bars

n

365OC 700 bars 't I

365OC 1300 bars

-

21

-

-

25

r

-

28

-

32

~

-

36

-

~

40

-

44

-

-

47

r

-

51

-

-

55

~

59

~

-

Minutes Figure 10. Gas chromatograms of the aromatic hydrocarbons fraction isolated from the bitumen obtained after confined pyrolysis of Woodford kerogen at 365 "C and various pressures. 1, Dimethylnaphthalenes; 2, trimethylnaphthalenes; 3, Cd-naphthalenes benzothiophenes; 4,dibenzothiophene; 5, phenanthrene; 6, methyldibenzothiophenes; 7, methylphenanthrenes; 8, dimethyldibenzothiophenes; 9, dimethylphenanthrenes; 10, trimethyldibenzothiophenes.

+

as H/C ratio, Rock-Eval HI, and T,, extract yield confirm that the initial kerogen already reached the end of the diagenetic stage in the source rock where it was collected. The hopanes evolve mostly in the early stage of oil generation, which might explain why their fingerprint is not significantly modified during pyrolysis. However, the distributions of hopanes and related parameters are not affected by pressure in the time-temperature conditions investigated. Mass Spectrometry-Gas Chromatography S t u d y of Steranes. The sterane fingerprint is determined from the mlz = 217 mass chromatogram. When OB-steranes are involved in the calculations, the m / z = 218 chromatogram (60)Mackenzie, A. S.; Patience, R. L.; Maxwell, J. R. Geochim. Cosmochim. Acta 1980,44, 1709-1721. (51) Seifert, W. K.; Moldowan,J. M. Adu. Org. Geochem.,Phys. Chem. Earth 1980,12, 229-237.

9

Y

52t

2"h 50 280

300 320 340 360 380 PYROLYSIS TEMPERATURE PC)

Figure 11. 13C NMR aromaticity factor (Fa= percentage of aromatic carbon) measured on whole bitumen extracted from kerogen aliquota pyrolyzed in confined conditions at various temperatures and 300, 700, and 1300 bar pressure.

Michels et al.

752 Energy & Fuels, Vol. 8, No. 3, 1994