Deciphering biodegradation effects on light hydrocarbons in crude oils using their stable carbon isotopic composition: A case study from the Gullfaks oil field, offshore Norway

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Abstract

Compound-specific isotope analysis has become an important tool in environmental studies and is an especially powerful way to evaluate biodegradation of hydrocarbons. Here, carbon isotope ratios of light hydrocarbons were used to characterise in-reservoir biodegradation in the Gullfaks oil field, offshore Norway. Increasing biodegradation, as characterised, for example, by increasing concentration ratios of Pr/n-C17 and Ph/n-C18, and decreasing concentrations of individual light hydrocarbons were correlated to 13C-enrichment of the light hydrocarbons. The δ13C values of C4 to C9 n-alkanes increase by 7–3‰ within the six oil samples from the Brent Group of the Gullfaks oil field, slight changes (1–3‰) being observed for several branched alkanes and benzene, whereas no change (<1‰) in δ13C occurs for cyclohexane, methylcyclohexane, and toluene. Application of the Rayleigh equation demonstrated high to fair correlation of concentration and isotope data of i- and n-pentane, n-hexane, and n-heptane, documenting that biodegradation in reservoirs can be described by the Rayleigh model. Using the appropriate isotope fractionation factor of n-hexane, derived from laboratory experiments, quantification of the loss of this petroleum constituent due to biodegradation is possible. Toluene, which is known to be highly susceptible to biodegradation, is not degraded within the Gullfaks oil field, implying that the local microbial community exhibits rather pronounced substrate specificities. The evaluation of combined molecular and isotopic data expands our understanding of the anaerobic degradation processes within this oil field and provides insight into the degradative capabilities of the microorganisms. Additionally, isotope analysis of unbiodegraded to slightly biodegraded crude oils from several oil fields surrounding Gullfaks illustrates the heterogeneity in isotopic composition of the light hydrocarbons due to source effects. This indicates that both source and also maturity effects have to be well constrained when using compound-specific isotope analysis for the assessment of biodegradation.

Introduction

Anaerobic biodegradation of light hydrocarbons has often been reported and is now widely accepted as a significant process in natural environments (Edwards and Grbić-Galić, 1992, Rabus and Widdel, 1995, Rueter et al., 1994, Widdel and Rabus, 2001). An important example is the biodegradation of hydrocarbons within petroleum reservoirs, leading to substantial change in petroleum composition and an associated decrease in the economic value of the crude oil (Connan, 1984). The recent discovery of anaerobic biodegradation mechanisms (e.g., Rabus et al., 2001, Wilkes et al., 2002, Wilkes et al., 2003) and the detection of anaerobic microorganisms in oil fields suggest that anaerobic degradation of light hydrocarbons is possible within the reservoirs (Aitken et al., 2004). However, to date no microorganisms have been enriched or isolated from petroleum reservoirs that can degrade light hydrocarbons anaerobically.

There is still some debate on the exact order in which different compounds are removed from crude oil during biodegradation, but most studies present similar degradation sequences where short-chain n-alkanes tend to be removed faster than longer-chain n-alkanes, the latter being removed faster than branched and isoprenoid hydrocarbons (Peters and Moldowan, 1993, Volkman et al., 1984, Wenger et al., 2001). The strongest deterioration of petroleum quality occurs from slight to moderate biodegradation (Wenger et al., 2001) or between biodegradation levels 1 and 4 (Peters et al., 2005). To characterise and understand this very important aspect of biodegradation, scientific studies are needed that focus on light hydrocarbons.

Most recently, the determination of compound-specific carbon and hydrogen isotopic compositions has become an effective tool in oil–oil and oil–source rock correlations (Li et al., 2001, Odden et al., 2002, Schimmelmann et al., 2004). Carbon and hydrogen isotopic composition was also used to trace biodegradation of aromatic hydrocarbons or chlorinated compounds in laboratory experiments (Mancini et al., 2003, Meckenstock et al., 1999, Slater et al., 2001, Wilkes et al., 2000) and studies on contaminated aquifers (Hunkeler et al., 1999, Mancini et al., 2002, Meckenstock et al., 2004, Sherwood Lollar et al., 2001). Biodegradation leads to an enrichment of the heavier isotopes in the residual fraction of an organic compound. This change in isotopic composition can be related to the decrease in concentration by the Rayleigh equation, allowing the quantification of the extent of biodegradation in both laboratory and field studies (Meckenstock et al., 1999, Richnow et al., 2003a, Vieth et al., 2005).

Carbon isotope fractionation of the residual substrate occurs in the initial reaction step, which mechanistically is localised at one specific carbon atom of the substrate in most known cases. Therefore, the overall isotope effect will become less with increasing number of carbon atoms in the molecule (Boreham et al., 1995). For biodegradation of, for example, aromatic hydrocarbons, starting with an enzyme reaction of the benzylsuccinate-type, it was calculated that hydrocarbons with more than 13 carbon atoms will not show a detectable change in the carbon isotopic composition, even when more than 90% of the substrate was degraded (Morasch et al., 2004). Thus, it can be assumed that biodegradation of hydrocarbons with more than 15 carbon atoms will not lead to a significant change in their isotopic composition. In support of this, Boreham et al. (1995) observed no enrichments in 13C in n-C12+ alkanes from biodegraded oils. Therefore, the carbon isotopic compositions of such hydrocarbons are not compromised and can still be used as specific indicators of the origin and maturity of the oil sample.

It is also known that other in-reservoir processes such as water washing and evaporation have little or no effect on the isotopic composition of petroleum hydrocarbons (Harrington et al., 1999, Mansuy et al., 1997, Smallwood et al., 2002). Thus, it can be expected that the carbon isotopic composition of petroleum hydrocarbons within a certain reservoir, regardless of differences in isotopic composition due to source and/or maturity, might only be influenced by the kinetic isotope effect caused by biodegradation processes. This is important with respect to the differentiation of various alteration processes, that all may influence the concentration of the petroleum components.

To date, only few studies have used carbon isotope ratios of light hydrocarbons in crude oils as an indicator of biodegradation. In general, the enrichment of 13C of certain hydrocarbons in the residual oil served as a qualitative indicator of the biological process (George et al., 2002, Masterson et al., 2001, Rooney et al., 1998). The present study pays particular attention to the quantitative evaluation of the biodegradation process in reservoired oils using carbon isotope fractionation of light hydrocarbons. As described in more detail below, crude oils from the Gullfaks field, offshore Norway, provide a unique opportunity to study the effects of biodegradation for two reasons. First, as has been shown previously, the oil column in the Brent Group reservoir at Gullfaks is vertically homogeneous (Horstad et al., 1990a). Therefore, relatively few oil samples from different wells representing the lateral heterogeneity in the petroleum composition are sufficient for characterisation of the principal biodegradation effects. Second, the oils are characterised by a high similarity with respect to source facies and maturity, giving rise to the assumption that changes in the molecular and isotopic composition are predominantly due to in-reservoir biodegradation.

Section snippets

Geological background

In this study, 12 crude oil samples from the Gullfaks and surrounding oil fields were investigated. The location of all sampled wells within the Tampen Spur area is given in Fig. 1 and a detailed description of each sample can be obtained from data in Table 1. The main oil–source rock in the Tampen Spur area is the Upper Jurassic Draupne Formation (Gormly et al., 1994). Also the underlying Heather Formation locally represents a source rock with high oil potential (Gormly et al., 1994, Horstad

Abbreviations

The abbreviations used in the text, figures, and tables for the different hydrocarbons are listed in Table 2 with their respective compound names.

Whole oil analysis (GC)

The amount of individual light hydrocarbons of the oil samples was quantified by gas chromatography (GC)-FID-measurements. The crude oil (0.5 μl) was injected into the GC (6890 Series, Agilent Technology, USA), equipped with a programmable temperature vaporisation inlet (PTV, Agilent Technology, USA) with a septumless head, working in split/splitless

Origin of the investigated oils

Crude oils in the North Viking Graben are either sourced from the Draupne or the Heather Formation, or represent mixed oils originating in variable proportions from both of these two source rocks (Gormly et al., 1994, Horstad et al., 1995). Gormly et al. (1994) demonstrated that pristane/phytane (Pr/Ph) ratios and whole oil carbon isotopic signatures are diagnostic for oil–oil and oil–source correlations in the North Viking Graben. Draupne-sourced oils were shown to have lower Pr/Ph ratios

Conclusions

Compound-specific carbon isotope analysis of light hydrocarbons has been shown to be a versatile tool for evaluating biodegradation processes in an oil reservoir. Importantly, our results document that the Rayleigh model is applicable to the biodegradation of light hydrocarbons in such reservoirs. Therefore, carbon isotope signatures can be used to quantify the mass loss of individual light hydrocarbons due to biodegradation, provided valid fractionation factors from laboratory studies are

Acknowledgments

We are grateful to Rouven Elias, Ann-Kathrin Scherf, and Kristin Günther for whole oil measurements and Michael Gabriel for his assistance in isotope analysis. We thank Dr. Arnd Wilhelms for oil samples and background information about the study area. Drs. Simon C. George, Christopher J. Boreham, and Roger E. Summons provided valuable comments on an earlier draft which helped to improve this publication.

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