Evaluation of Petroleum Generation from Resinites by Hydrous Pyrolysis
M. D. Lewan, J. A. Williams (2)
Hydrous pyrolysis experiments are useful in evaluating the composition, yield, and kinetics of liquid hydrocarbons generated from potential source rocks. Using this technique on a representative set of resinites revealed that fossil resin does not generate a light naphthenic crude oil. Gas chromatograms of liquid hydrocarbons generated by resinites are dominated by aromatic peaks and show no resemblance to commercially produced crude oils. Resinites may make minor contributions to conventionally sourced crude oils, but their limited occurrence inhibits their potential as a prolific source of petroleum on a basinal scale. Data from the natural system and pyrolysis experiments also indicate that resinites do not generate liquid hydrocarbons at abnormally low levels of therm l maturity.
Resinite, the fossil resin preserved in some sedimentary rocks, has been the subject of numerous studies describing its physical and chemical properties (e.g., Selvig, 1945; Murchison and Jones, 1963, 1964; Soos, 1966; Langenheim and Beck, 1968). Some of these properties and specific compounds derived from resins have been used in paleobotanical studies of resin-secreting plants (Langenheim, 1969; Mustoe, 1985) and organic geochemical studies of sediments (Simoneit, 1977; Barrick and Hedges, 1981). A relatively new interest in resinite has developed because of its proposed ability to generate significant quantities of light naphthenic oil at abnormally low levels of thermal maturity (< 0.5% Ro) (Snowdon and Powell, 1982; Nissembaum et al, 1985). This deduction may have a significant influence on petroleum-exploration strategies, particularly in clastic rocks where terrestrial plant debris may be the major source of organic matter.
The method used in this study is called hydrous pyrolysis (Lewan et al, 1979), which involves heating a potential source rock in the presence of liquid water at subcritical temperatures (< 374°C). Although relatively high temperatures (300°-365°C) are used in these experiments to offset the long time requirements of the natural system (106-108 years), the expelled pyrolysates from potential source rocks are similar to natural crude oils (Lewan et al, 1979; Winters et al, 1983). Also, the vitrinite reflectance and atomic H/C ratio of pyrolyzed kerogens change in a manner similar to that observed in the natural system (Lewan, 1983, 1985). In this study, we use hydrous pyrolysis to evaluate the yield, composition, and maturity level of liquid produ ts generated from resinites.
MATERIALS AND METHODS
The resinites used are described in Table 1. According to Jean H. Langenheim (1983, personal communication), who has worked on resinites during the last 20 years, the samples used in this study represent a good global cross section of resinite types. All samples except RSN-9 are greater than 98% by volume resinite and were hand picked. RSN-9 was commercially separated from its encompassing coal by froth flotation and, as a result, it contains approximately 5% by volume coal impurities. The resinites were crushed into granules that ranged in diameter from 62 to 250 µm. The small quantities of sample available for RSN-1 through RSN-8 limited the number of experiments to one per sample; therefore, a small reactor with a 17.5-ml capacity was used. Conversely, larger quantities of RSN 9 were available, so several experiments were performed in a larger reactor with a 250-ml capacity.
Procedure for 250-ml Reactor
General-purpose reactors (T316SS) with a 250-ml capacity were filled with 45 g of resinite RSN-9. Resinite flotation during the experiments was hindered by a nickel-chromium screen that was placed over the resinite prior to loading 100 ml of deionized water (ASTM Type III). The remaining volume in the reactor was purged with helium, then filled with 240 kPa (35 psi) of helium. Three aliquots of RSN-9 were thermally matured to different stages of oil generation by heating them at 300°, 330°, and 360°C for 72 hours. Two additional aliquots were heated at 360°C for 216 and 480 hours to achieve levels of thermal maturity in the post-oil-generation stages.
After each sample had cooled to room temperature, a final gas pressure was recorded and a gas sample was collected in an evacuated 30-ml gas cylinder. Floating oil pyrolysate on the water surface was removed from the reactor with a Pasteur pipet and transferred to a tared glass vial. In the two lower temperature experiments (i.e., 300°C/72 hours and 330°C/72 hours), a floating tar pyrolysate occurred. This pyrolysate was removed from the reactor with a spatula and transferred to a tared glass vial. No attempt was made to collect sorbed pyrolysate by rinsing the residual resinite with solvent, as described by Lewan (1983) for pyrolyzed rock samples. This process was avoided because of the high solubility of the resinite (about 50 wt. % in an azeotropic mixture of benzene and ethanol) and, therefore, we would not be able to distinguish sorbed pyrolysate from soluble resinite. The residual resinite was dried in a vacuum oven for 48 hours at 50°C and submitted for C, H, N, and O elemental analyses. Reflectance measurements were made on the minor amounts of vitrinite associated with the coal impurities within the resinite.
Procedure for 17.5-ml Reactors
Tube reactors with a 17.5-ml capacity (OD = 15.875 mm, ID = 12.70 mm, T316SS and Swagelok end caps) were filled with 0.30 to 1.00 g of resinite depending on the amount available. Resinite flotation during the experiments was prevented by capping the sample with coarse stainless-steel gauze before adding 6.0 ml of deionized water (ASTM Type III). The reactors were heated in a fluidized sand bath (Tecam, SBL-2D) for 72 hours at 360°C. This condition was used because results from the series of experiments with RSN-9 indicated it was within the range of maximum oil-pyrolysate generation. After the reactors cooled to room temperature, the amount of gas generated was determined by weighing the reactors before and after venting their gases. Floating oil pyrolysate was collected from the reactors with a Pasteur pipet and transferred to a tared glass vial. Unlike the experiments with the 250-ml reactors, the yields obtained from these smaller scale experiments should only be considered semiquantitative due to the small sample size.
Floating oil pyrolysates were run on a Hewlett-Packard 5830A gas chromatograph with a flame ionization detector. Samples were injected in the split mode at 360°C with hydrogen as the carrier gas. The oven was programmed for an initial temperature of 20°C for 5 min, followed by heating at 6°C/min to a final temperature of 350°C for 20 min. The chromatographic column was a 50-m bonded phase, fused silica, 007 methyl silicone Quadrex column with an ID of 0.25 mm and a film thickness of 0.5 µm.
Gas Chromatography/Mass Spectrometry
Floating oil pyrolysates were run on a Hewlett-Packard 5996A gas chromatograph/mass spectrometer (GC/MS) in the electron impact mode at 70 eV. Samples were injected in the split mode at 330°C with helium as the carrier gas. Source and analyzer temperatures were held at 250°C with the transfer line set at 320°C. The oven was programmed for a heating rate of 5°C/min from an initial temperature of 30°C to a final temperature of 330°C, which was held for 20 min. Except for its 25-m length, the column was the same as the one described for gas chromatography. Mass spectra were generated for prominent peaks in the total ion chromatograms. Compounds were tentatively identified on the basis of comparisons with mass spectra published by Mass Spectrometry Data Centr (1974) and Heller and Milne (1978).
RESULTS AND DISCUSSION
Oil Pyrolysate Yields
Results from the hydrous pyrolysis experiments on resinite RSN-9 are shown in Figure 1. The floating tar generated in the early part of the experimental series may be considered analogous to the bitumen developed during the incipient oil-generation stage of amorphous Type II kerogens (Lewan, 1983, 1985). Expulsion of this viscid tar from sedimentary rocks bearing resinite is not likely, and it is more likely to be retained in its host rock as an extractable bitumen. As the floating tar decomposes with increasing thermal
Table 1. Description of Resinite Samples Used in Hydrous Pyrolysis Experiments stress, an oil pyrolysate is generated. This stage is analogous in part to the start of the primary oil-generation stage of amorphous Type II kerogens, where bitumen decomposes as the generation of expelled oil commences (Lewan, 1983, 1985). The oil-pyrolysate maximum of 38 wt. % at 360°C for 72 hours may be considered the end of primary oil generation and the start of the post-oil-generation stage. Oil-pyrolysate yields for the other resinite samples at the same experimental conditions range from 34 to 64 wt. % (Table 2). Although these convertibilities are encouraging from a source rock standpoint, the potential of resinites as a significant source of petroleum on a basinal scale is weakened by their limited and localized occurrences. Admittedly, resinites may locally occur in igh concentrations (e.g., 2-5 wt. %), but the lateral extent of these anomalous occurrences within sedimentary basins has not been adequately documented.
Composition of Oil Pyrolysates
The major compounds identified in the gas chromatograms of the oil pyrolysates are given in Figure 2. Although many of these compounds occur in all of the oil pyrolysates, variations in their relative concentrations permit the results to be divided into four groups (Figures 3-6). Group A includes resinite samples RSN-8 and RSN-9, which generate oil pyrolysates with a high concentration of a dimethyl naphthalene (Figure 3, peak N-2). Group B includes resinite samples RSN-1, RSN-3, RSN-7, and RSN-10. The oil-pyrolysates generated from these resinites have high concentrations of a methyl ethyl naphthalene, a xylene, and a trimethyl benzene (Figure 4, peaks N-6, B-2, and B-6, respectively). Group C includes resinite samples RSN-4, RSN-5, and RSN-6, which generate oil pyrolysates with high concentrations of a pentamethyl indan and a methyl ethyl naphthalene (Figure 5, peaks I-7 and N-6, respectively). Group D includes only resinite sample RSN-2, which generates an oil pyrolysate (Figure 6) that appears to be a combination of groups B and C. No attempt is made to attribute particular compounds to specific resin precursors, but the generation of basic structural components from various precursory compounds reported in resins and resinites (Simonsen and Ross, 1957; Frondel, 1967; Thomas, 1969; Cunningham et al, 1983; Brackman et al, 1984) is suggested diagrammatically in Figure 7.
Gas chromatograms of oil pyrolysates generated from the hydrous pyrolysis experiments on resinite sample RSN-9 are shown in Figure 8. Although the relative concentrations of a few compounds change to some degree with thermal maturity (e.g., peak N-1 vs. peak N-2), the overall character of the chromatograms does not change significantly. Also, in the natural system, inorganic and organic matrices may influence the composition of liquid hydrocarbons generated from resinites. Therefore, hydrous pyrolysis experiments
Fig. 1. Variations in amounts of floating tar pyrolysate, floating oil pyrolysate, and generated gas collected from hydrous pyrolysis experiments on RSN-9 resinite.
Table 2. Amount of Starting Material and Resinite Yields from Hydrous Pyrolysis Experiments at 360°C for 72 Hours
Fig. 2. Tentative mass-spectrometric identification of chromatographic peaks in Figures 3-6 (Me = :CH3, Et = :CH2-CH3, iPr = CH3-CH2-CH3). were performed with smectite and lignite to evaluate their effect on the composition of liquid pyrolysates. One aliquot of RSN-10 was mixed with an equal weight of smectite (Wyoming bentonite), and a second aliquot of RSN-10 was mixed with an equal weight of lignite (Fairfield, Texas). Two grams of each mixture were loaded into 34-ml reactors with 12 g of deionized water (ASTM Type III) and heated for 72 hours at 360°C. Gas chromatograms of the oil pyrolysates generated from these two mixtures (Figure 9) are essentially the same as that for the liquid pyrolysate generated from the matrix-free resinite (RSN-10, Figure 4).
The predominance of aromatic peaks and lack of an unresolved hump in the gas chromatograms indicate that resinites do not generate light naphthenic oils. Furthermore, the lack of any resemblance between liquid hydrocarbons generated from resinites and commercially produced crude oils suggests that resinites have not been a significant source of petroleum. Philp et al (1983) also arrived at this
Fig. 3. Gas chromatograms of oil pyrolysates generated from group A resinites (RSN-8 and RSN-9) by hydrous pyrolysis at 360°C for 72 hours. FID = flame ionization detector.
Fig. 4. Gas chromatograms of oil pyrolysates generated from group B resinites (RSN-1, RSN-3, RSN-7, and RSN-10) by hydrous pyrolysis at 360°C for 72 hours. FID = flame ionization detector.
Fig. 5. Gas chromatograms of oil pyrolysates generated from group C resinites (RSN-4, RSN-5, and RSN-6) by hydrous pyrolysis at 360°C for 72 hours. FID = flame ionization detector.
Fig. 6. Gas chromatogram of oil pyrolysate generated from group D resinite (RSN-2) by hydrous pyrolysis at 360°C for 72 hours. FID = flame ionization detector.
conclusion for the Gippsland basin, which contains notable resinite-bearing coals near its oil-bearing strata. Although not a significant source of petroleum, resinites may contribute minor amounts of hydrocarbons to conventionally sourced crude oils. These minor contributions may be in the form of thermal or soluble products. Thermal products contributing to conventionally sourced crude oils would be primarily naphthalenes or indans derived from the breakdown of diterpenoids and triterpenoids in the resinites. Bendoraitis (1974) provided an example of this type of contribution from resinites in the crude oils of the Eocene Jackson Formation of south Texas. Soluble products would be primarily diterpenoids and triterpenoids, which would result from conventionally sourced crude oils mig ating through resinite-bearing rocks. This has been suggested by Philp et al (1983) to explain the diterpenoids in the crude oils of the Gippsland basin, and may also explain the diterpenoids observed by Snowdon and Powell (1982) in crude oils from the Beaufort-Mackenzie basin.
Maturity Level of Oil Pyrolysate Generation
Hydrous pyrolysis experiments by Lewan (1985) indicate that the kinetics for oil generation may vary significantly,
Fig. 7. Diagram suggesting possible precursory resin compounds as source of basic structural components identified in oil pyrolysates generated from resinites.
Fig. 8. Gas chromatograms of oil pyrolysates generated from resinite RSN-9 by hydrous pyrolysis at various temperatures and times. FID = flame ionization detector.
Fig. 9. Gas chromatograms of oil pyrolysates generated from resinite RSN-10 mixed with smectite and lignite. Hydrous pyrolysis conditions were 360°C for 72 hours. FID = flame ionization detector.
and some kerogens may generate oil at abnormally low levels of thermal stress. Figure 10 shows that oil pyrolysate from Phosphoria Retort Shale was generated at a lower thermal stress than that required for Woodford Shale. Lewan (1985) showed that the differences in the determined kinetic parameters for the kerogens in these two rock units may result in significantly different predictions of the maturity level for oil generation within sedimentary basins. Hydrous pyrolysis data on the resinite RSN-9 are not sufficient for accurately determining the kinetic parameters for oil generation, but an approximation may be made by plotting the available data on Figure 10. The close association of the resinite data in Figure 10 with the Woodford Shale trend suggests oil generation from resinite is not likely to occur at abnormally low levels of thermal stress, as demonstrated by the Phosphoria Retort Shale (Lewan, 1985).
These experimental observations are also supported by earlier studies that showed resinites to be more resistant to thermal alteration than their encompassing coals (White, 1914; Berl and Schmidt, 1937; van Krevelen, 1961, p. 120; Murchison, 1966). White (1914) and van Krevelen (1961, p. 120) reported that chemical alteration of resinites is first observed in high-rank bituminous coals, which may be equated to a vitrinite reflectance of approximately 1.2% Ro (Teichmuller, 1975, p. 42). Rock-Eval pyrolysis data on the positively identified resinites in the study by Mukhopadhyay and Gormly (1984, their Figure 4) showed no significant differences in the hydrogen index (i.e., milligrams of generated hydrocarbons per gram of organic carbon) between resinites associated with 0.50 to 0.61% Ro and those associated with 0.30 to 0.41% Ro. Reflectance measurements made on the vitrinite impurities in resinite RSN-9 showed the generation of oil pyrolysate to occur at values greater than 0.96% Ro (Figure 11). A consensus of these different studies on the vitrinite reflectance value at which resinites generate oil is not possible, but collectively they do indicate oil generation from resinites is not likely to occur at
Fig. 10. Percent of oil pyrolysate generated at different temperatures by hydrous pyrolysis of separate aliquots of Phosphoria Retort Shale, Woodford Shale, and resinite RSN-9. Data and kinetic parameters (EA = activation energy and Ao = pre-exponential factor) for the Phosphoria Retort Shale and Woodford Shale are from Lewan (1985). Data for the resinite RSN-9 are from Figure 1.
Fig. 11. Relationship between percent of oil pyrolysate generated from resinite RSN-9 by hydrous pyrolysis and the corresponding vitrinite reflectance (mean of 50 readings) of coal impurities inherent to sample. Parenthetic numbers adjacent to data points refer to temperature and time (°C/hours) conditions imposed on each pyrolyzed aliquot. Coal rank scale is from Heroux et al (1979).
abnormally low vitrinite reflectance values (< 0.6% Ro).
It may be argued that because resinite RSN-9 was derived from a high volatile C bituminous coal (0.40-0.54% Ro), an early pulse of oil may have been expelled from the resinite at a lower rank (subbituminous or lignite). Elemental analysis indicates that this is unlikely. Figure 12 shows that the atomic H/C ratio of resinite RSN-9 (1.50) is essentially the same as resinites derived from lignites (1.50 ± 0.03); therefore, the notable loss of hydrocarbons at coal ranks below high volatile C bituminous is not plausible. The thermal maturation trend of resinite is similar to that for type I kerogens as defined by Tissot et al (1974). Hydrous pyrolysis data indicate that oil generation begins and ends at atomic H/C ratios of 1.35 and 0.65, respectively.
Hydrous pyrolysis experiments show that liquid hydrocarbons may be generated from resinites, but their compositional character is not that of light naphthenic crude oils. Gas chromatograms of these resinite-derived liquids show no resemblance to commercially produced natural crude oils. Although resinites may contribute minor components to more conventionally sourced crude oils, their limited occurrences and localized concentrations appear to have rendered them as unlikely sources of commercial oil on a basinal scale. Natural resinite occurrences and hydrous pyrolysis experiments indicate that the generation of these compositionally peculiar liquid hydrocarbons does not occur at abnormally low levels of thermal maturity (< 0.6% Ro).
Fig. 12. Van Krevelen diagram showing elemental analyses of RSN-9 in comparison with resinites from lignite and subbituminous coal, and residual resinite after hydrous pyrolysis of RSN-9.
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(2) Amoco Production Company Research Center, P.O. Box 3385, Tulsa, Oklahoma 74102.
We thank the management of Amoco Production Company Research Center for support of this research and permission to publish. The broad base of this study was made possible with the wide variety of resinites provided by Jean Langenheim at the University of California, Santa Cruz, California, and W. Bruce Saunders at Bryn Mawr College, Bryn Mawr, Pennsylvania. GC/MS data were provided by David Dolcater, and our interpretations of these data were reviewed and improved upon by Sy Meyerson. Gas chromatography was performed by Frank Vu. Jim McDonald and Davis Redding provided helpful assistance and suggestions in the laboratory. Critical reviews of the manuscript by R. R. Thompson and J. C. Winters were helpful and appreciated.
AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.