Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs
B. SHERWOOD LOLLAR, T. D. WESTGATE, J. A. WARD, G. F. SLATER & G. LACRAMPE-COULOUME
Stable Isotope Laboratory, University of Toronto, Toronto, Ontario, Canada M5S 3B1
Correspondence and requests for materials should be addressed to B.S.L. (e-mail: bsl@quartz.geology.utoronto.ca).
Natural hydrocarbons are largely formed by the thermal decomposition of organic matter (thermogenesis) or by microbial processes (bacteriogenesis). But the discovery of methane at an East Pacific Rise hydrothermal vent1 and in other crustal fluids supports the occurrence of an abiogenic source of hydrocarbons2-4. These abiogenic hydrocarbons are generally formed by the reduction of carbon dioxide, a process which is thought to occur during magma cooling5 and—more commonly—in hydrothermal systems during water–rock interactions, for example involving Fischer–Tropsch reactions and the serpentinization of ultramafic rocks6-10. Suggestions that abiogenic hydrocarbons make a significant contribution to economic hydrocarbon reservoirs2 have been difficult to resolve, in part owing to uncertainty in the carbon isotopic signatures for abiogenic versus thermogenic hydrocarbons4, 10. Here, using carbon and hydrogen isotope analyses of abiogenic methane and higher hydrocarbons in crystalline rocks of the Canadian shield, we show a clear distinction between abiogenic and thermogenic hydrocarbons. The progressive isotopic trends for the series of C1–C4 alkanes indicate that hydrocarbon formation occurs by way of polymerization of methane precursors. Given that these trends are not observed in the isotopic signatures of economic gas reservoirs, we can now rule out the presence of a globally significant abiogenic source of hydrocarbons.
Large volumes of methane gas discharge from fractures and exploration boreholes in hard rock mines operating throughout the Canadian and Fennoscandian shields. An abiogenic origin through water–rock interaction was proposed for these gases on the basis of 13C and 2H signatures for methane incompatible with bacterial or thermogenic methane11. Recent experimental studies10, 12, 13 confirm that production of abiogenic methane by water–rock interaction can result in 13C values as depleted as those reported for methane from the above shields (-22.4 to -57.5)11. Nonetheless, the evidence for an abiogenic origin for these shield gases was hitherto largely circumstantial. In this study, the unusual pattern of 13C values among the C1–C4 alkanes provides evidence of abiogenic formation mechanisms (Table 1; Fig. 1a). Thermogenic hydrocarbons have been shown empirically and experimentally to have a characteristic isotope distribution pattern whereby the C1–C4 alkanes become more enriched in 13C (less negative 13C values) with increasing molecular mass. This orderly isotopic distribution results from kinetic fractionation effects, whereby alkyl groups separating from source organic matter cleave preferentially at weaker 12C–12C rather than 12C–13C bonds14. This isotopic distribution pattern is ubiquitous among thermogenically derived gases, and is considered to be diagnostic of gases produced by the thermal decomposition of high-molecular-mass organic matter.
Figure 1 Plot of 13C values of individual n-alkanes against carbon number for gas samples from Kidd Creek mine, and for thermogenic gases from southwest Ontario natural-gas fields29. Full legend
High resolution image and legend (52k)
In contrast, Fig. 1a shows a significant depletion in 13C for C2–C4 with respect to C1 for hydrocarbon gases from the Kidd Creek mine on the Canadian shield. For 13 of the 17 samples in this study, C2 is significantly depleted in 13C (by 2–4) with respect to C1 (Fig. 1a). C3 and C4 are also isotopically depleted in 13C compared to C1 by 1–3 and by 1–6, respectively. No post-genetic alteration of typical thermogenic gas is known that could produce an isotopic enrichment in C1 of the order of 10, and a simultaneous isotopic depletion in C3 and C4 relative to C2, to produce the pattern exhibited by the Kidd Creek gases15-17. Although methanogenic bacteria have been identified in the ground waters of the Kidd Creek mine18, 19, addition of an isotopically depleted bacterial methane to the borehole gases would only reduce the C1 enrichment observed in Fig. 1a and cannot account for the observed isotopic enrichment of C1 relative to the higher-molecular-mass alkanes (Fig. 1b). A bacterial origin overall for the C1–C4 gases is ruled out on the basis of C1/C2+ ratios between 5.10 and 11.51, 2–4 orders of magnitude lower than for typical bacterial gas16.
Natural occurrences of hydrocarbon gas exhibiting isotopic depletion in C2–C4 compared to C1 are rare. Reported occurrences in the field include: C1–C3 hydrocarbons in two gas samples from fluid inclusions associated with the Khibiny massif on the Kola peninsula, Russia20; and C1–C2 hydrocarbons from six gas samples from fluid inclusions associated with the Ilimaussaq complex, South Greenland21, 22. Unfortunately, none of these studies were able to obtain a significant number of 13C values for homologues higher than C2. As apparent isotopic depletion of C2 versus C1 can occur owing to bacterial oxidation of methane, conclusive evidence of isotopic depletion of higher hydrocarbons with respect to C1 requires comparison to a larger range of higher hydrocarbons, particularly C3 and C4. Only one isotopic data set exists for C1–C4 alkanes of undisputed abiogenic origin. Values of 13C obtained for C2–C4 n-alkanes for the Murchison meteorite show an isotopic depletion of 5–8 with respect to C1 (Fig. 2). The absolute 13C values of the hydrocarbons from this meteorite are significantly more enriched in 13C than the Kidd Creek samples, reflecting their extraterrestrial origin. Nonetheless, the Murchison and Kidd Creek samples show remarkable similarities in the isotopic distribution pattern between the C1–C4 n-alkanes (Fig. 2). Although C3 in the Murchison sample is more depleted relative to C2 and C4 than for the Kidd Creek samples, both sample sets show (1) significant depletion in 13C values of C2–C4 versus C1, and (2) an isotopic pattern for C1–C4 alkanes distinctly different from the typical pattern for thermogenic gases.
Figure 2 Plot of 13C values of individual n-alkanes against carbon number. Full legend
High resolution image and legend (37k)
Such depletion patterns in C2–C4 alkanes relative to C1 have been shown to be produced by polymerization reactions, such as production of higher hydrocarbons from methane in a spark discharge experiment14 (Fig. 2). This pattern results during kinetically controlled synthesis of higher-molecular-mass homologues from lower ones owing to the fact that 12CH4 reacts faster than 13CH4 to form chains, so that 12C is more likely to be incorporated into larger hydrocarbon chains14, 23. In other abiogenic polymerization reactions such as the Fischer–Tropsch synthesis, C2–C4 hydrocarbons produced from CO and H2 are also isotopically depleted with respect to C1, but the pattern of isotopic reversal (13CC1 > 13CC2 > 13CC3=13CC4) is not always as consistent as for the spark discharge experiment12, 13. Experimental results to date certainly show that whether by spark discharge or Fischer–Tropsch synthesis, hydrocarbons produced by abiogenic polymerization reactions do not show the isotopic enrichment trends for C1–C4 typical of thermogenic gases. So although the 13C pattern between C3 and C4 in the Murchison sample is not identical to the spark discharge experimental results, a similar abiogenic polymerization reaction has been invoked23 to explain the overall C2–C4 isotopic depletion pattern for the Murchison meteorite n-alkanes. For the Kidd Creek samples, the pronounced isotopic depletion of C2–C4, and the lack of a thermogenic-type enrichment trend for C1–C4 are as consistent with kinetically controlled polymerization reactions as the Murchison n-alkanes. Potential mechanisms for abiogenic gas synthesis in this geologic environment include: surface-catalysed polymerization from reduction of CO in the Fischer–Tropsch synthesis24; heating or metamorphism of graphite–carbonate-bearing rocks25, 26; or other vapour–water–rock alteration reactions in the presence of catalytically active metals27. Confirmation of the exact mechanisms in natural systems will be dependent on constraining possible reactions with additional experimental data. Regardless of the specific mechanism, the Kidd Creek gases are (to our knowledge) the first reported terrestrial samples to demonstrate this consistent depletion of C2–C4 with respect to C1 indicative of abiogenic formation reactions.
The 2H values for the Kidd Creek hydrocarbon gases provide additional support for an abiogenic origin. Figure 3 illustrates the positive correlation of 13C and 2H values that is an established pattern for thermogenic gas reservoirs worldwide, due to increasing isotopic enrichment in both 13C and 2H with increasing molecular mass for the C1–C4 homologues16. In contrast, both the isotopically depleted absolute 2H values, and the relationship between 13C and 2H values for the Kidd Creek C1–C4 alkanes, are distinctly different from those of thermogenic gas. In particular, the inverse correlation of 13C and 2H values between C1 and C2 supports the participation of these compounds in an abiogenic polymerization reaction. It has been proposed14 that in a kinetically controlled synthesis of higher-molecular-mass homologues from lower ones, the lighter isotope (12C) will react faster than the heavy isotope (13C) to form a 2-carbon chain. 12C would be more likely to be incorporated into the product of the carbon addition reaction, and the resulting ethane would be depleted in 13C versus the methane precursor. By analogy, owing to preferential cleavage of the weaker 12C–1H bond versus the 12C–2H bond, the light (1H) isotope would be preferentially eliminated in a polymerization reaction. Hence, the resulting ethane would be expected to be isotopically enriched in 2H, and isotopically depleted in 13C, with respect to the methane precursor. This inverse relationship of 13C isotope depletion and 2H isotope enrichment between C1 and C2 for the Kidd Creek samples (Fig. 3) supports a polymerization reaction as the first step in the formation of the Kidd Creek higher hydrocarbons from a methane precursor.
Figure 3 Plot of 13C versus 2H values for C1–C4 for the Kidd Creek samples, and for thermogenic gases from southwest Ontario natural-gas fields29. Full legend
High resolution image and legend (32k)
The distinct carbon isotopic depletion trends observed for the Kidd Creek gases, and their similarity to the trends observed for the Murchison meteorite n-alkanes, provides evidence for production of these Canadian shield gases by abiogenic synthesis rather than by conventional thermogenic or bacteriogenic processes (Fig. 2). The pattern of 13C isotopic depletion and 2H enrichment between C1 and C2 in the Kidd Creek gases (Fig. 3) fits a model of isotopic fractionation involving hydrocarbon formation by kinetically controlled polymerization reactions. This study demonstrates that abiogenic processes that may have generated prebiotic organic molecules in the early Earth continued to play a significant role in the production of hydrocarbons in the crystalline rocks of the Canadian shield. The contrasting range of 2H values, and the markedly different relationship of 13C and 2H values demonstrated for abiogenic C1–C4 compared to thermogenic natural-gas reservoirs provide important criteria for distinguishing between these two sources. Based on the isotopic characteristics of abiogenic gases identified in this study, the ubiquitous positive correlation of 13C and 2H values for C1–C4 hydrocarbons in economic gas reservoirs world-wide is not consistent with any significant contribution from abiogenic gas.
Methods
Kidd Creek mine, situated in the southern volcanic zone of the Abitibi greenstone belt (approximately 2,700 Myr old), 24 km north of Timmins, Ontario, is one of the world's largest volcanogenic massive sulphide deposits. All gas samples were collected from the 6,800- and 6,900-foot levels (2,072–2,100 m below land surface), from boreholes drilled approximately perpendicular to the steeply dipping felsic, mafic and ultramafic units28. Gases occur in association with saline ground waters and brines in pressurized 'pockets' formed by sealed fracture systems within the rocks. During exploration drilling, these systems are ruptured, triggering depressurization and gas release at rates as high as 30 l min-1 per borehole. Samples were collected from freely discharging boreholes in evacuated glass sample vessels. Boreholes were sealed with packers, and samples collected from the packered interval to minimize any air contamination.
Gases are composed of methane (C1, 69.3–78.1%), ethane (C2, 5.55–11.7%), H2 (0.40–12.7%) and N2 (3.88–12.7%), along with minor concentrations of helium (1.83–2.45%), propane (C3, 0.71–2.42%) and butane (C4, 0.21–0.77%). CO2 concentrations are all below detection. Compositional data are available as Supplementary Information. Compositional analyses were carried out as described in ref. 11. On the basis of crustal 3He/4He ratios for the shield gases, no mantle-derived component for the hydrocarbon gases is suggested11.
Carbon isotopes were measured by compound specific isotope analysis using a Finnigan MAT 252 mass spectrometer interfaced with a Varian 3400 GC. Accuracy and reproducibility of 13C values are 0.5 with respect to VPDB standard. 13CC4 values include both n-butane and isobutane for all samples. Hydrogen isotopes were measured by compound specific isotope analysis using a Finnigan MAT delta+XL mass spectrometer interfaced with an HP6890 gas chromatograph. Accuracy and reproducibility of 2H values are 5 with respect to VSMOW standard. 6000 series boreholes were sampled from the 6,800-foot level (2,072 m below land surface) of Kidd Creek mine in 1996–97. As this predates the availability of continuous flow compound specific hydrogen isotope analysis, only 13C values could be analysed. 7,000 and 8,000 series boreholes were sampled from the 6,900-foot level (2,100 m below land surface) of Kidd Creek mine in 2000. Both 13C and 2H values are reported for these samples, and 2HC4 values include both n-butane and iso-butane.
Supplementary information accompanies this paper.
Received 22 November 2001;accepted 18 February 2002
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Acknowledgements. This study was supported in part by Falconbridge Mining Ltd and by the Natural Sciences and Engineering Research Council of Canada. We thank N. Arner and N. VanStone, and the Geology Office at Kidd Creek mine (P. Olson, A. Coutts, R. Cook) for providing geological information and assistance with underground field work.
Competing interests statement. The authors declare that they have no competing financial interests.
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