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Energy & Fuels 2002, 16, 1004-1005
Communications The Sudden Release of Oil and Bitumen from Bakken Shale on Heating in Water John W. Larsen* and Koh Kidena Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015-3172 Received October 9, 2001 To explore further the chemistry of hydrous pyrolysis and the process of primary migration, Bakken Shale (17% TOC, 1.43 H/C) has been heated under water at temperatures between 25 °C and 350 °C for 2 h, the short time being used to avoid chemical reactions.1,2 The organic matter (a mixture of oil and bitumen abbreviated OM) liberated from the rock was isolated and weighed. The OM is liberated over a narrow temperature range (Figure 1). Initial release occurs at 275 °C and all of the OM has been released by 300 °C. The release of C1 and C2 as well as the formation of CO2 (see Figure 2) confirm that few chemical reactions occur in 2 h at 300 °C. We believe the rapid release of the OM is due to a physical process, probably the glass-to-rubber phase transition of the kerogen. OM is formed within the kerogen and must be initially dissolved in it. Some part of its release from the source rock must be diffusion through the kerogen. Stainforth and Reinders have argued that diffusion through kerogen is the rate-determining step in primary migration.3 The small amount of information that we have on the physical state of kerogens indicates that those studied are primarily glassy through the oil window.4 In glassy polymeric materials, diffusion is very slow and is very sensitive to the size of the diffusing molecule.5 Any OM dissolved in and diffusionally trapped in a glassy kerogen will be released when the kerogen becomes rubbery, something that will happen when the kerogen is warmed above its glass-to-rubber transition temperature (Tg). One of us has suggested that this situation may be in part responsible for oil release during hydrous pyrolysis.6 Solvent-swelling measurements on kerogens have established the existence of a significant driving force for the expulsion of saturated hydrocarbons from kerogens.7 Samples of Bakken Type II kerogen (0.5 g) were heated under 5.0 mL of water in stainless steel containers using * Corresponding author. (1) Schimmelmann, A.; Lewan, M. D.; Wintsch, R. P. Geochim. Cosmochim. Acta 1999, 63, 3751-3766. (2) England, W. A.; Fleet, A. J. Petroleum Migration; The Geographical Society: London, 1991. (3) Stainforth, J. G.; Reinders, J. E. A. Org. Geochem. 1989, 16, 6174. (4) Parks, T. J.; Lynch, L. J.; Webster, D. S.; Barrett, D. Energy Fuels 1988, 2, 185-190. (5) Diffusion in Polymers; Crank, J., Park, G. S., Eds.; Academic Press: New York, 1968. (6) Larsen, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 393-396. (7) Larsen, J. W.; Li, S. Org. Geochem. 1997, 26, 305-309.
Figure 1. Temperature dependence of organic matter released from Bakken Shale when heated for 2 h under water. “X” is the organic matter obtained by Soxhlet extraction of the shale with CH2Cl2. Each point is the average of at least 4 replicate determinations and the lines denote the maximum and minimum yields obtained.
a fluidized bed sand bath. The gases formed were collected for analysis. The OM was extracted from the water into CH2Cl2 which was dried over anhydrous Na2SO4, evaporated, and the amount of isolated OM was then weighed. The Bakken samples are core chips and contamination from organic-based drilling mud has been ruled out.8 The CO2 dissolved in the water was liberated by acidifying the water with phosphoric acid and measuring the evolved CO2 by gas chromatography. Bakken Shale contains inorganic carbonate that sometimes contributes to evolved CO2. The inorganic carbonate remaining after reaction was determined by measuring the CO2 evolved when the rock was treated with aq. HCl. Heating time was 2 h. to minimize chemical reactions. (See Figure 3.) As shown in Figure 1, only a small amount of OM is released below 275 °C, and the amount is not temperature dependent. Significant OM release begins at 275 °C, is complete at 300 °C, and heating to higher temperatures does not release greater amounts of OM. There are three lines of evidence against the OM formation being due to (8) Price, L. C.; Daws, T.; Pawlewicz, M. J. Pet. Geol. 1986, 9, 125162.
10.1021/ef0102465 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/27/2002
Communications
Figure 2. Temperature dependence of the release of C1 and C2 hydrocarbon gases from Bakken Shale when heated for 2 h under water. Each point is the average of at least 4 replicate experiments and the lines denote the maximum and minimum yields obtained.
Figure 3. Figure 3. Yields of CO2 released from Bakken Shale on heating for 2 h under water (#) and by treating the shale with aqueous HCl (9).
bond-breaking chemical processes. First, the fact that the amount of OM released is constant above 300 °C is inconsistent with a chemical reaction origin of the OM. In 2 h, the chemical reactions would not be completed, so higher temperatures would yield greater amounts of OM. The second is the short reaction time: 2 h. The normal reaction time used in hydrous pyrolysis is 72 h and only a small fraction of the reactions are expected to occur in 2 h. The third line of evidence comes from light gas formation. This just becomes significant in 2 h at 350 °C. We conclude that bond cleavage cannot explain the release of OM over the observed 25 °C range. That some CO2 is formed from kerogen at 325 °C demonstrated that some reactions are occurring. It seems unlikely that the OM, which amounts to about one-third of the organic material present, would be formed in a 25 °C temperature range by the CO2 forming reactions. These data require a physical rather than a chemical explanation.
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There is a physical explanation for these data that fits all of our observations: OM diffusionally trapped in glassy kerogen is released when the kerogen undergoes a phase change and becomes rubbery. What little evidence there is, the NMR studies of Lynch et al., has the glass-to-rubber transition of isolated kerogens occurring over a broad temperature range.4 Green River kerogen is a glass at room temperature, about 50% rubbery at 225 °C and fully rubbery only above 430 °C.4 Rundle kerogen also has a very broad transition from glassy to rubbery. It is 50% rubbery at 160 °C and fully rubbery at about 400 °C.9 There is no published information about Tg for other kerogens and we have begun a study. That there is a thermodynamic driving force for expulsion, especially of saturated hydrocarbons, is demonstrated by the Regular Solution Theory treatment of kerogen solvent swelling.7 If a kerogen containing diffusionally trapped OM is warmed until it becomes rubbery, the OM will be released. This model, based on physical and not chemical structure changes, fits all of the data that we have, including those presented here. We have not presented proof of this model, but enough data to demonstrate that it must be considered and tested. It is not the only possible explanation. Hydration of clays or disruption of mineral-organic interactions are other possibilities. Thermal desorption is ruled out by the fact that the composition of the OM is independent of the temperature at which it is liberated. If thermal desorption were occurring, the weakly interacting hydrocarbons would be liberated first, before the more strongly interacting bitumens. This is not observed. Bakken Shale is unusual: it is both source rock and reservoir rock. Its release of oil is most probably a physical process, different from the kerogen-water reactions that occur during hydrous pyrolysis. The data presented here suggest that the oil in Bakken Shale my be diffusionally trapped and requires a change of state of the kerogen for its release. This study deals with a physical process that should not be confused with the higher-temperature chemical processes that occur during hydrous pyrolyses that are being studied by several groups.10 Acknowledgment. Grateful acknowledgment is made to the Petroleum Research Foundation, administered by the American Chemical Society, for partial support of this work. Support by the National Science Foundation is also gratefully acknowledged. Conversations with Raymond Michels (Universite Henri Poincaire) were important in developing the ideas contained in this paper. The Bakken Shale was a gift from Leigh Price: we are grateful and we miss him. EF0102465 (9) Parks, T. J.; Lynch, L. J.; Webster, D. S. Fuel 1987, 66, 338-344. (10) Lewan, M. D. Geochem. Cosmochim. Acta 1997, 61, 3691-3723 and references therein.
